Improving the thermal management of the RX-888 (Mk2)

Figure 1:
The RX888 showing the "top" and RF connectors.  While
the heat sinks attached to the sides are visible, the large one
on the "bottom" plate are not.
Click on the image for a larger version.

Note:

If you are considering buying an RX-888 - or have already bought one, be sure to read the following BEFORE you power it up!

* * *

The RX-888 Mk2 SDR is a USB3-based software-defined receiver that, unlike many others, is JUST and analog-to-digital converter (with a bit a low-pass filtering and adjustable attenuation and amplification) coupled to a USB 3 PHY chip.  With a programmable sample rate and a 65-ish MHz low-pass filter, it is capable of simultaneously inhaling the entire spectrum from a few 10s of kHz to about 60 MHz when run at 130 Msps - a rate which pretty much "maxes out" the USB 3 interface.

(Note:  There is also a frequency converter on board which will take up to a 10 MHz swath of spectrum between about 30 and 1800 MHz and shift it to a lower frequency within range of the A/D converter - but that's not part of this discussions.)

The purpose of this post is to discuss the thermal management of the RX-888 Mk2 which, in two words, can be described as  "marginal" and "inconsistent".

Other RX-888 articles:

After posting this entry I produced another article about understanding the gain and properties of the HF signal path on the RX-888 (Mk2) - including information that can also be applied to other direct-sampling "all band HF" Software Defined Radios like the KiwiSDR, Red Pitaya and others.  You may read that article here:  Measuring signal dynamics of the RX-888 (Mk2) - Link.

Using an external clock with the RX-888:  The 27 MHz external clock input to the RX-888 is both fragile and fickle.  To learn a bit more about how to reliably and safely clock an RX-888 from an external source, read THIS article.

Please note:

Despite the impression that the reader might get about the RX-888 (Mk2)'s thermal design and potential reliability, I would still consider it to be an excellent device at a good price - warts and all.

Its performance is quite good and especially since it lacks the FPGA that many other direct-sampling SDRs use, it is quite "future proof" in the sense that support of this receiver - and others like it that will no doubt appear soon - will be based on code running on the host computer (typically a PC or SBC) rather than on an FPGA contained within that requires specialized tools and knowledge for development and is limited by its own capacity.

If you think that an FPGA is needed, consider this:  For a few "virtual" receivers using "conventional" DSP techniques (e.g. HDSDR, SDR-Radio, etc.) a moderate Intel i7 is sufficient:  If using an optimized signal processing program like ka9q-radio along with a modest Intel i5, hundreds of virtual receivers covering the entire HF spectrum can be managed - but these are topics for another discussion.

In other words:  If you need a fairly simple, modestly-priced device to receive multiple RF channels it is well worth getting an RX-888 (Mk2) and performing some simple modification to it to improve its durability.  We can hope that future versions of this - and similar devices - will take these observations into account and produce even better hardware.

What's the problem?

There are scattered anecdotal reports of RX-888 (both the original and Mk2) simply "dying" after some period of time.  For most of these reports there are few details other than comments to this effect in various forums (e.g. little detailed analysis) but this was apparently enough of a problem with the original version of the RX-888 that with the Mk2, "improved" thermal management is one of the new features noted by its sellers.  (I do not have an original RX-888, but I would expect that the same general techniques could be applied to it as well.)

In short, here are a few comments regarding the thermal management of the RX-888 Mk2:

• DO NOT run it outside its case.  There is a compressible thermal pad that goes between the exposed metal pad below the A/D converter that is intended to transfer heat to the case and without this in place the A/D converter and surrounding components can exceed 100C at moderate ambient temperatures.  If you plan to shuck the case, you should be aware of this and make appropriate arrangements to draw away heat via the same method. 

Figure 2:
Showing the paper double-sided "sticky tape" used to mount
the heat sinks.  Despite improper materials, these work "less
badly" than expected, but it's best to re-attach them properly.
Click on the image for a larger version.

• The heat sinks are held on by double-sided tape.  The heat sink on the A/D converter appears to be some sort of thermal table like that seen on Raspberry Pi heat sink kits, but  those on the exterior of the case (one on each side, another the top) are held on with standard, paper-based double-sided tape:  People have reported these falling off with handling.  Additionally, because both the case and heat sinks are extruded their surfaces are not flat and all of the RX-888 (Mk2) units that I had a gap between the heat sink and the case through which a sheet of paper can be slid meaning that the heat sinks should be flattened a bit and/or attached using a material that will work as a thermally-conductive void filler.
  

• The thermal pad may not be adequate.  Unless the small-ish thermal pad is placed precisely in its correct location, it will not be effective in its thermal transfer.  Additionally, these pads require a bit of compression between the board and the heat sink to be effective and it seems that the spacing between the board and the case is somewhat "loose" in the slot into which the PCB slides and that thermal contact may be inconsistent - more on this shortly.
  

• Other components get very hot.  Next to the A/D converter are the 3.3 and 1.8 volt linear regulators which run very hot.  While this may be OK, they are next to (what appear to be) electrolytic capacitors which - if run very warm - can have rather short lifetimes.  While it is unknown if this is the case here, many regulators will become unstable (oscillate) if their associated capacitors degrade with lower capacitance and/or increased ESR (Equivalent Series Resistance) and if oscillation occurs due to capacitor degradation, this is likely to make the device unusable until the components are replaced.
  

Figure 3:
The top of the RX888 board.  The ADC's heat sink was
removed for the photo, but glued in place later to improve
its thermal transfer.
Click on the image for a larger version.

• The FX3 USB interface chip can get very warm.  This chip is right next to the A/D converter.  There are anecdotal reports (again, nothing confirmed) that this particular chip can suffer reliability problems when running near its maximum rated temperature:  Whether this is due to a failure of silicon or (more likely) a mechanical failure of a solder connection on its BGA (ball grid array) as a result for thermal cycling remains to be seen, but either one could explain one of the RX-888's reported failure modes of no longer appearing to be the expected type of USB device, making the unit non-functional even though it seems to enumerate - albeit improperly.

Several different people have made spot measurements of the temperatures within an RX-888 and come up with different results, further indicating inconsistency in the efficacy of the passive cooling and showing the inherent difficulty in making such measurements - but here are a few comments that are likely relevant:

• Unless you need coverage >30 MHz, do NOT run a sample rate higher than 65-70 Msps.  As with most devices, more current (and higher heat dissipation) will occur at a higher sample rate so keeping it well below its maximum (around 130 Msps) will reduce heating and potentially improve the lifetime.  

If you do run at a sample rate 64-70 Msps, it is recommended that a 30 MHz low-pass filter be installed as this will prevent aliasing due to this lower rate and the fact that the RX-888 (Mk2) has only a 60 MHz low-pass filter internally.

• At normal "room" temperatures (68F/20C) the thermal properties of the RX-888 Mk2 are likely "Okay" if run at just 65-70 Msps - but increasingly marginal above this.  On several RX-888s, the temperature of the A/D converter and other components was fairly high, but not alarmingly so, although this seemed to vary among samples (e.g. some seemed worse than others.)  Since thermal resistance can be characterized by a temperature rise, it makes sense that as the ambient temperature increases, so will the components by the same amount meaning that if the unit is in a hot location - or placed such that it will become warm (convective air movement across the heat sinks is restrictive or in/near the hot air flow of other equipment) then thermal stresses of the components also increase.
  

Again, the reader should be cautioned that the reported inconsistency between units (e.g. the efficacy of the thermal pad) may mean that the above advice may not apply to all units as some may have, say, a misplaced thermal pad or extra "slop" in the spacing between the board and the case which reduces the compression of the pad causing extra thermal resistance.

"Board slop"doesn't help: 

Figure 4:
Measuring the "board slop" in the mounting rails.  As noted
in the text, the board's looseness was nearly 1 mm - the far
extent of which exceeding the 5mm thickness of the pad.
Click on the image for a larger version.

On this latter point (e.g. "slop" in the board position) with the covers removed I measured a variance of 0.170-0.205" (4.32-5.207mm) from the board to the case due to looseness in the board fitting in the rail on one of my RX-888.  Of the three units that I have to measure, this was the worst - but not by much as the the photo (figure 4) from another unit shows.

Considering that the thermal pad is nominally 5.0mm thick, this means that the board MAY not be effectively conducting heat to the case if the gap is closer to 5.2mm.  Also considering the fact that the thermal pad will work better when it is compressed it would be a very good idea - if possible - to reduce this gap - more on this later.

I also observed that with the USB end plate fitted, it happened to push the board "down" (e.g. reduced the gap between the board and the case) by about 0.02" (0.5mm) and since this is the end of the board closest to the A/D converter chip, it likely reduces the gap by about 0.015" (0.38mm) owing to geometry (e.g. the fact that the A/D converter is located away from the edge.)  If desired, this fact could be exploited by adding a shim to the top of the USB connector and filing the bottom a bit to allow the end plate to push "down" on the board a bit, better-compressing the thermal pad and potentially reducing its thermal resistance. 

Figure 5:
The screwdriver tip points to where the end plate is pushing
down on the connector and board to reduce board-to-case
distance to better-compress the pad.
Click on the image for a larger version.

On the opposite end of the board, the RF connectors fit rather loosely in their mounting holes meaning that one could, in theory, move the connectors to the "bottom" of their holes and tighten the nuts on the SMA connectors.  This would not be advisable without adding a washer of appropriate thickness between the plate and the SMA connector as the connectors themselves are not right at the edge of the circuit board and firmly tightening the nuts would likely bend/break them loose.

Before getting out the file, however, I suggest considering the methods/modifications mentioned below to improve the thermal performance of the RX-888 (Mk2) in several other ways.

Ways to improve the thermal performance:

There are two ways to improve the thermal performance and reduce the temperature of the onboard components.

Add another heat sink and a fan

A "brute force" approach to this would be to move more air through and around the unit. using a small fan.  If you do this I would recommend two minor modifications:

• Glue the heat sink to the A/D converter.  As noted earlier, the heat sink the A/D converter is held on by tape, but I would recommend that this be removed from the heat sink and the chip itself (using a bit of paint thinner or alcohol to remove residue) and it be reattached using thermally conductive epoxy rather than conventional "clear" epoxy.  This epoxy is readily available at the usual places (Amazon, etc.) but it should be noted that the gray (not clear!) "JB Weld" epoxy (available at auto-parts and "big box" stores) also has reasonable thermal conductivity and works quite well in this application.   Do NOT use an adhesive like "super glue" as it is not void-filling by its nature and it is unlikely to endure the heat.
  

• Add a heat sink to the FX3 chip.  This chip - next to the A/D converter - should also be cooled and a small heat sink - such as that which comes with a Raspberry Pi heat sink kit - may be attached.  Again, I would recommend thermally-conductive epoxy rather than supplied double-sided sticky tape.

As for the fan mounting, several people have simply removed both side plates and fabricated the attachment for a small fan (say, 20x20mm to 30x30mm) on the side with the USB connector to blow air through the case on both sides of the board.  Others have temporarily removed the board from the case and put holes in "top" of the case (on the side with the labels) into which a fan is mounted.

Either of these will be quite effective - but since these are not passive cooling, the failure of a fan could result in excess heat if other methods are not also employed.

Improve passive cooling by using a much larger thermal pad

This is likely the favored approach as it does not depend on a fan which will have a defined useful lifetime, and the failure of which could result in immediate overheating in certain circumstances.  There are two parts to this approach:

Replace the thermal pad. 

At reasonable ambient temperatures I believe that the area of the external heat sinks on the RX-888 are of adequate size, provided that they are open for air flow and not placed in the heat exhaust of equipment and properly attached to the case - more on that shortly.

As noted, the thermal pad is seemingly marginal and it is only as large enough to draw heat away from the area immediately proximate to the A/D converter - an issue that may be exacerbated by the inconsistent board-to-case spacing mentioned above.  Improper placement of this pad will prevent it from conducting heat from the A/D converter - the major heat producer - to the case - and subsequent heating of adjacent components.

Figure 6:
A piece of 45mm x 65mm thermal pad on the bottom of the
board.  This piece is large enough to cover all heat-
generating components.
Click on the image for a larger version.

It is also likely that the thermal pad material supplied with the unit is of lower thermal conductivity than other materials that are available (to save cost!) so the use of better thermal material and a larger pad will draw more heat away from all of the heat-producing components on the board and conduct it to the heat sink.

A suitable pad material is the Laird A15340-01 which may be found at Digi-Key (link here ).  This material has roughly half  the thermal resistance (e.g. better thermal conductivity) of other common pad materials and it is suitably "squishy" in that it will form around components and help fill small voids as it does so.

Unfortunately, this material is somewhat expensive in that it's available only as a rather large piece - about $32 (at the time of posting - not including shipping) for one that is 22.8x22.8cm square - but this will modify several RX-888s - but even at the price of $32, it's still a reasonable price to pay for improved reliability of a $150-$200 device!  If you do this, it's recommended that you work with other RX-888 owners to split the cost of the pad - but be sure to keep the pad - or any pieces that you cut from it - in a zip-bag or clean plastic cling film to prevent its surface from being contaminated with dirt and dust.  If you post/mail this pad material to someone else, be sure to protect it between two pieces of cardboard to prevent it from being mangled.

Note:  Others have obtained 5mm thick thermal pad material from other sources (e.g. Amazon) and while it likely does not have as low thermal resistance as the Laird product mentioned, reports indicate that it works adequately - most likely a result of the larger size of this pad compared to the original, drawing heat away from the entire bottom surface of the board.

Figure 7:
The new pad, installed, as viewed from the
end with the USB connector, near the ADC
and FX3 USB interface chip.
Click on the image for a larger version.

A rectangular piece of thermal pad 45mm x 65mm will cover the bottom of the board where there are heat-generating components and ensure superior heat transfer to the case.  Since this material is a bit "sticky", it may be a bit difficult to get it installed as it will be resistant to sliding, but a very light coating of white heat-sink grease on the side of the pad facing the heat sink material will provide sufficient lubrication to allow it to slide as the board is inserted along its mounting rails.

Comment:  This process is fairly messy, so if you plan to add a connector for an external clock input, I would suggest that you do so at the time that you install the new pad as you will probably not to repeat the process unnecessarily.

Remount the heat sinks.

As noted earlier, the four heat sinks (two on the "bottom" side opposite the label and one on each side) are held on by double-sided paper tape.  It is recommended that these be removed - along with any tape residue (best done with paint thinner and/or alcohol) - and be reattached with thermal epoxy.

Figure 8:
An RX888 (Mk2) in the process of gluing on the side heat
sinks, using a vise for clamping.  Alternatively, weight may
be placed on the heat sink(s) while the epoxy cures to
compress it and squeeze out excess - but note that until it
cures that the heat sinks may slide slowly out of position
if one isn't careful.
Click on the image for a larger version.

As noted previously, the heat sinks do not fit flat with each other so  it would be a good idea to assure that the surfaces are reasonably to maximize thermal conductivity by drawing the case and the mating surfaces of the heat sinks across 800-grid sandpaper (using a flat piece of metal or glass as a substrate) - taking care to prevent metal particles from getting onto the board or inside the case:  It would be best to remove the board and do this prior to the installation of the new thermal pad and wash any such particles from the case before reassembly.

Once the mating surfaces have been flattened and cleaned, using thermal epoxy (or the gray "JB-Weld") reattach the heat sinks one-at-a-time - preferably by compressing them in a vice or with a clamp to squeeze out as much adhesive as possible.

It's worth noting that even if you don't go through the trouble of flattening the heat sink and the surface of the case, the use of a void-filling adhesive will certainly offer far more efficient thermal transfer than  the original double-sided paper sticky tape along with it s rather large air gap between the two surfaces.

Out of curiosity I measured the difference in temperature between the heat sinks stuck on with double-sided tape and the exposed portion of the case right next to the heat sink and it was found to be about 3-5F (1.7-2.8C) - surprisingly good, actually.

Before and after thermal measurements

Figure 9:
Two RX888 Mk2's with reattached heat sinks, ready for a 
bit of clean-up and final assembly.
Click on the image for a larger version.

Using a thermal infrared camera and verifying with a thermocouple, temperature measurements were made of various components with an RX-888 operating at 130 Msps at an ambient temperature of 74F (23C) after 10 minutes of operation.  The readings were as follows:

With the original thermal pad, end plates removed - heat sink cooling by convection only:

ADC:  175F (79C)

FX3 (USB interface): 155F (68C)

Capacitor near 3.3 volt regulator:  145F (63C)

3.3V Regulator:  170F (77C)

1.8V Regulator:  178F (81C)

 

With Laird 45mm X 65mm pad - heat sink cooling by convection only:

ADC: 145F (63C)

FX3: 130F (54C)

Capacitor near 3.3 volt regulator:  125F (52C)

3.3V Regulator:  145F (63C)

1.8V Regulator:  150F (66C)

Note:  There is another capacitor near the 1.8 volt regulator, but it is temperature cannot be readily measured while the board was installed in the case, but other measurements made outside the case indicates that its temperature was at least as high as that of the capacitor near the 3.3 volt regulator.

Results and comments:

The replacement of the original thermal pad with one that is 45mm X 65mm in size to cover the bottom of the board where there are active components has resulted in a very significant heat reduction:  As with all electronics, reducing the temperature of the components will increase the operational lifetime.

Considering that one can use - as a guideline - the temperature rise above ambient, we can make some estimations as to what will happen if the modified RX-888 (Mk2) is operated at a higher temperature.  

For example, if we consider 212F (100C) to be the maximum allowed case temperature of any of the components, we can see that with the original thermal pad, this limit would occur with the ADC converter at an ambient temperature of around 111F (44C) - a temperature that one could reasonably expect during the summer in a room without air conditioning.  In contrast, with the larger pad the ADC's temperature would likely be closer to 185F (85) in the same environment.

With a small amount of air moving across the heat sinks, their temperature rise would also be lower, further reducing internal temperature - and even though it isn't strictly necessary, it wouldn't hurt to use a small fan - even on a modified RX-888 (Mk2) to cool it even more, and feel confident that it will still survive should that fan fail.

Finally, I would again remind the reader that I consider the RX-888 (Mk2) to be an excellent-performing and extraordinarily flexible device and well worth extra trouble to make it better! 

* * *

Since the original writing of this article, TAPR has started selling a kit that includes both a large thermal pad and an interface board that allows external clocking to be done safely - more information about that product - details here: https://tapr.org/product/rx888-clock-kit-and-thermal-pad/

* * *

Update - 10 September, 2025

After having been in the field for several years, I've had a number of RX-888s cross my workbench for repair - the vast majority of these having been damaged by excess voltage/current on an unprotected external clock input:    To learn a bit more about how to reliably and safely clock an RX-888 from an external source, read THIS article.

Another failure mode - not as common - is the failure of the AMS1117 3.3 volt regulator that feeds both the VGA and the A/D converter:  The voltage output drops below the voltage at which these can function properly - if not to zero.

This failure mode can be exacerbated by the lack of proper cooling and this is yet another reason why the installation of a "thermo pad" the covers the entire bottom of the circuit board is strongly recommended.

 

This page stolen from ka7oei.blogspot.com

 

[End]

A solid state replacement for an old radio's "vibrator" (Wards Airline 62-345)

Figure 1:
The front of the Wards Airline 62-345 with its rather
distinctive "telephone dial" tuning dial.
It's powered up and running from 12 volts!
Click on the image for a larger version.

Quite some time ago - a bit more than a decade - a friend of mine came to me with an old "Farm" radio - a Wards Airline 62-345.  This radio - from the 1930s - was designed to run from a 6 volt positive ground battery system  such as that which one might find in tractors and cars of that vintage.

How high voltage was made from low voltage DC in the 30's

As the technology of the time dictated, this radio has what's called a "vibrator" inside - essentially a glorified buzzer - that is used as a voltage chopper along with a transformer to convert the 6 volts from the battery to the 130-150 volts needed for the plates of the tubes within.  Not only did this vibrator do the chopping for the high voltage, but it also performed the duty of synchronously rectifying the AC waveform from the transformer as the pulses from it would naturally be in sync with the motion of the moving reed, briefly connecting the output of the transformer to the input of the high voltage DC supply when the voltage waveform from it was at the correct polarity.

These devices, as you would expect, don't have a particularly long lifetime as they are constantly buzzing, making and breaking electrical contact and causing a small bit of arcing - something that will inevitably wear them out.  Even if the contacts were in good shape, the many decades of time that have passed will surely cause these contacts to become oxidized - particularly since these devices are in rubber-sealed cans (to minimize noise and vibration) and the out-gassing of these materials is likely of no help in their preservation.

Figure 2:
The chassis of the radio.  The vibrator is in its original
can in the far right corner.
Click on the image for a larger version.

Such was the case with this radio.  Often, the judicious application of percussive repair (e.g. whacking with a screwdriver) can get them going and if the contacts are just oxidized, they will often clean themselves and work again - at least for a while.  In this case, no amount of whacking seemed to result in reliable operation, so a modern, solid-state approach was needed.

The solid-state replacement

As mentioned earlier, the job of the vibrator was to produce a chopped DC waveform, apply it to a transformer for "upping" the voltage and then use a separate set of contacts to perform synchronous rectification - and our solid-state replacement would need to do just that.  That last part - rectification - was easy:  Just two, modern diodes would do the job - but chopping the DC would require a bit more circuitry.

The owner of this radio also had a few other things in mind:  He changed it from 6 volts, positive ground to 12 volts, negative ground so that it could be readily operated from this more-common power scheme.  The change to 12 volt filaments required a bit of work, but since all of the tubes were indirectly heated, the filament supply could be rearranged - but some tubes had to be changed to accommodate different filament voltages and currents as follows:

• Oscillator and detector:  This was originally a 6D8 (6.3v @ 150mA) and it was replaced with a 6A8 (6.3V @ 300mA).  Other than filament current, these tubes are more or less the same.
  

• IF Amplifier:   The original 6S7 (6.3v @ 150mA) was retained.
  

• 2nd Detector/AVC/1st Audio:  The original 6T7 (6.3V @ 150mA) was retained.
  

• AF Output:  The original 1F5 (2.0v @ 150mA) was replaced with a 6K6 (6.3v @ 400mA).  The latter is a pentode, requiring a bit of rewiring and rebiasing to replace the original triode.

• Magic Eye tube:   The original 6N5 (6.3v @ 150mA) was replaced with a 6E5 (6.3v @ 300ma) - which is also more sensitive than the 6N5, giving a bit more deflection.

The 6T7 (150mA), 6A8 (300mA) and the #47 dial lamp (6.3v @ 150mA) are wired in parallel on the low side with one end of the filament grounded while the 6K6 (400mA), 6S7 (150mA) and 6E5 (300mA) are wired in parallel on the high side with one end of the filament connected to +12 volts.  You might notice a current imbalance here (600mA on the low side with 850mA on the high side) but this is taken care of with the addition of 30 ohms of resistance between the midpoint of the filament string and ground to sink about 200mA, getting us "close enough".

He also did some additional rebiasing and other minor modifications - particularly for the rewiring of the AF Output from the original 1F5 to a 6K6 as he swapped a triode for a pentode - which was then  wired as a triode.  The total current consumption of the radio at 13 volts is 1.6 amps - a bit more than half of that being the filament and pilot lamp circuits meaning that about 10 watts of power is being used/converted by the vibrator supply and consumed by the idle current of the audio output and other tubes.

The "chopper" circuit

The other issue with the 6 to 12 volt conversion is that of the primary of the high voltage transformer:  This transformer is center-tapped with that connection going to the "hot" side of the battery (which was originally at -6 volts) - but what this really means is that there's about 12 volts from end-to-end on the transformer at any instant.  We can deal with this difference simply by driving the transformer differently:  Rather than having the center tap "hot" with the DC voltage and alternatively grounding one end or the other as the vibrator did we can simply disconnect the transformer's center tap altogether and alternately apply 12 volts to either end, reversing the connection electronically to preserve the original voltage ratio between primary and secondary.

This feat is done using an "H" bridge - an array of four transistors that will do just what we need when driven properly:  Apply 12 volts to one side and ground the other - or flip that around, reversing the polarity.

Consider the schematic below:

Figure 3:
Solid state equivalent of a vibrator supply.  This version uses an "H" bridge, suitable for
the conversion of a 6 volt radio to 12 volt operation as detailed in the text.
Click on the diagram for a larger version.

This diagram shows a fairly simple circuit.  For the oscillator we are using the venerable CD4011 quad CMOS NAND gate with the first two sections wired to produce a square wave with a frequency somewhere in the 90-150 Hz region - the precise value not being at all critical.  The other two sections (U1c and U1d) take the square wave and produce two versions, inverted from each other.

Figure 4:
The top (component side) of the circuit.  This is built on a
piece of phenolic prototype board.
Click on the image for a larger version.

The section of interest is the "H" bridge consisting of transistors Q1 through Q4 wired as two sets of complimentary-pair Darlington transistors.   Here's how it works:

• Let us say that the output of U1c is high.  This causes the output of U1d to be low as it's wired as a logic inverter.
• The output of U1c being high will cause the top transistor (Q1 - a PNP Darlington) to be turned OFF, but at the same time the bottom transistor of this pair, Q2, will be turned ON, causing the connection marked "PIN 1" to be grounded.
• At the output of U1d - being low - we see that the bottom of this pair of transistors, Q4, is turned OFF, but the top transistor Q3 is turned ON causing V+ (12 volts) to appear at the connection marked "PIN 5".
• In this way, the low-voltage primary of the transformer has 12 volts across it.
• A moment later - because of the oscillator - the output of U1c goes low:  This turns off Q2 and turns on Q1 - and since this also causes the output of U1d to go high this, in turn, turns off Q4 and turns on Q3.  All of this causes "PIN 5" to now be grounded and "PIN 1" to be connected to V+ - thus applying the full 12 volts to the transformer in reverse polarity.

Also shown are D1 and D2, the solid-state replacements for the synchronous rectifier of the original vibrator.  While this could be a pair of high-voltage diodes (>=400 volts) we simply used half of a full-wave bridge rectifier from a junked AC-powered switching supply.  Finally, resistor R3 and capacitor C2 form a filter to keep switching noise and high-voltage spikes out of the power supply of U1 to prevent its destruction - a sensible precaution!

Now some of you might be concerned about "shoot through" - the phenomenon when both the "upper" transistors (Q1, Q3) might be on - if only for an instant - at the same time as the "lower" transistors (Q2, Q4) as the switching is done.  While this may happen to a small extent, it has negligible effect - particularly at the low switching frequency where this effect would constitute a very minuscule percentage of the switching period:  This circuit is efficient enough that no heat sinking is required on transistors Q1-Q4 and they get only barely warm at all.  Were I to build it again I might consider ways to minimize shoot-through, but this would come at the expense of simplicity which, itself, is a virtue - and since this circuit works just fine, would probably be not worth the effort.

Figure 5:
The bottom (wired side) of the circuit with flying leads
connecting to the original base socket.
Click on the image for a larger version.

These days one might consider building this same type of circuit using MOSFETs instead of Darlington transistors (e.g. P-channel for Q1 and Q3, N-channel for Q2 and Q4) and this should work fine - but the Darlington transistors were on hand at the time that this circuit was built and very easily driven by U1 - and the bipolar transistors are - at least in this case - arguably more rugged than the MOSFETs would be - particularly since there was no need to include a "snubber" network to suppress switching transients that might occur.  It's also worth noting that while standard MOSFET transistors would work fine for a 12 volt supply, you'd have to be sure to select "low gate threshold" devices to work efficiently at 6 volts or lower - something that would not really be an issue with the bipolar Darling transistors shown here.

This circuit is simple enough that it was wired onto a piece of phenolic prototyping board, snapped down to a size that will nicely fit into the original can that housed the vibrator.  To complete the construction, the top of the can - which was originally removed by careful filing and prying - was glued into its base using "shoe goo" - a rubber adhesive - keeping the board protected, but also allowing it to be easily disassembled in the future should modification/repair be necessary.

To be sure, the Internet is lousy with this same sort of circuit, but this version has worked very well.

What about the center tap version of the solid state vibrator?

You might ask yourself "what if we don't want to rewire a 6 volt radio to 12 volts?"  As noted previously, the boost transformer in the radio had its center tap connected to the "hot" side - which, in this case, would have been the negative terminal (because many vehicles had 6 volt, positive grounds at the time).  This circuit could be easily modified for that as you'd need only "half" an "H" bridge and the resistors driving the transistors would be changed to a lower value - perhaps 2.2k.  Depending on whether the it was positive-ground or negative ground, or whether the center-tap was grounded or "hot" - this would dictate whether you needed the PNP or NPN halves of the H-bridge.

(If you have a specific need, feel free to contact me by leaving a comment.)

* * * 

This page stolen from ka7oei.blogspot.com

 

 [END]

 

Modifying an "O2-Cool" battery fan to (also) run from 12 volts

A blog posting about a fan?  Really?

Why not!

Figure 1:
The modified fan on my cluttered workbench, running
from 13 volts.
The external DC input plug is visible on the lower left.
Click on the image for a larger version.

This blog post is less about a fan, but is more of an example of the use of a low-cost buck-type voltage converter to efficiently power a device intended for a lower voltage than might be available - in this case, a device (the fan) that expects 3 volts.  In many cases, "12" volts (which may be anything from 10 to 15 volts) will be available from an existing power source (battery, vehicle, power supply) and it would be nice to be able to run everything from that one power bus.

Background

Several years ago I picked up a 5" battery-operated DC fan branded "O2 Cool" that has come in handy occasionally when I needed a bit of airflow on a hot day.  While self-contained, using two "D" cells - it can't run from a common external power source such as 12 volts.

Getting 3 volts

Since this fan uses 3 volts, an obvious means of powering it from 12 volts would be to simply add a dropping resistor - but I wasn't really a fan of this idea (pun intended!) as it would be very wasteful in power and since doing this would effectively defeat the speed switch - which, itself is just a 2.2 ohm resistor placed in series with the battery when set to "low".

The problem is that the fan itself pulls 300-400 mA on high speed.  If I were to drop the voltage resistively from 12 volts (e.g. a 9 volt drop) - and if we assume a 300mA current - we would need to add (9/0.3 = ) 30 ohms of series resistance to attain the same speed on "high" as with the battery.  The "low speed" switch inserts a 2.2 ohm resistor, and while this works with its original 3 volt supply, adding this amount to 30 ohms would result in a barely noticeable difference in speed, effectively turning it into a single-speed fan.  By directly supplying the fan with something close to the original voltage, we preserve the efficacy of the high/low speed switch.

Fortunately, there's an answer:  An inexpensive buck converter board.  The board that I picked - based on the MP1584 chip - is plentiful on both EvilBay and Amazon, typically for less than US$2 each.  These operate at a switching frequency of about 1 MHz and aren't terribly prone to cause radio interference, having also been used to power 5 volt radios and even single-board computers (such as the Raspberry Pi) from 12 volts without issues.

These buck converters can handle as much as 24 volts on the input and provide up to 3 amps output - more than enough for our purpose - and can also be adjusted to output about any voltage that is at least 4 volts lower than the input voltage - including the nominal 3 volts that we need for the fan.

An additional advantage is the efficiency of this voltage conversion.  These devices are typically 80% efficient or better meaning that our 300 mA at 3 volts (about 0.9 watts of power) would translate to less than 100mA at 12 volts (a bit more than a watt).  Contrast this to the hypothetical resistive dropper (an efficiency of 25-30%) discussed earlier where we would be burning up nearly 3 watts in the 30 ohm resistor by itself !

Implementation

One of my goals was to retain the ability of this fan to run at 3 volts as it would still be convenient to have this thing run stand-alone from internal power.  Perhaps overkill, but to do this I implemented a simple circuit using a small relay to switch to the buck converter when external power was present and internal power when it was not, rather than parallel the buck converter across the battery.

If I never intended to use the internal "D" cells ever again I would have dispensed with the relay entirely and not needed to make the slight modifications to the switch board mentioned below.  In this case I would have had plenty of room in the case and freedom to place the components wherever I wished.  In lieu of the ballast of the battery to hold the fan down and stable, I would have placed some weight in the case (some bolts, nuts, random hardware, rocks) to prevent it from tipping over.

The diagram of this circuitry is shown below:

Figure 2:
Diagram of the finished/modified fan.
On the left, J1 is the center-positive coaxial power connector with diode D1 and self-resetting
resetting thermal fuse F1 to protect against reverse polarity.  The relay selects the source of power.
Click on the image for a larger version.

The original parts of are the High/Low switch, the battery and the fan itself on the right side of the schematic with the added circuits being the jack (J1), the self-resetting fuse (F1), D1, R1, the buck converter and the relay (RLY).

How it works:

When no external power is applied, the relay (RLY) is de-energized and via the "NC" (Normally-Closed) contacts, the battery is connected to the High/Low switch and everything operates as it originally did.

External power is applied via "J1" which is a coaxial power jack, wiring the center pin as positive:  The connector that I used happens to have a 2.5mm diameter center pin and expects an outer shell diameter of 5.5mm.  There's nothing special about this jack except that I happen to have it on-hand.

When power is applied, the relay is energized and the high/low switch is disconnected from the battery but is now connected, via the "NO" (Normally Open) contacts, to the OUT+ terminal of the buck converter.  

Ideally, a small 12 volt relay would be used, but the smallest relay that I found in my junk box was a 5 volt unit, requiring that the coil voltage be dropped.  Measuring the relay coil's resistance as 160 ohms, I knew via Ohm's law that it required about 30 mA (5/160 = 0.03) and if we were to use 12 volts, we'd need to drop (12 - 5 =) 7 volts.  The resistance needed to drop 7 volts is therefore (7/0.03 = ) 233 ohms - but since I was more likely to operate it from closer to 13 volts much of the time I chose the next higher standard value of resistance, 270 ohms to put in series for R1.

Figure 3:
Modification of the switch board.  The button is
the positive battery terminal and traces are cut to
isolate it to allow relay switching.
Click on the image for a larger version.

The diode D1 is a standard 1 amp diode - I used a 1N4003 as it was the first thing that I found in my parts bin, but about any diode rated for 1 amp or greater could be used, instead.  Placing it in reverse-bias across the input of the buck converter means that if the voltage was reversed accidentally, it would conduct, causing the self-resetting thermal fuse F1 to "blow" and protect the converter.  I chose a thermal fuse that has several times the expected operating current so I selected a device that would handle 500-800 mA before it would open.

Modification to the switch board

The High/Low switch board also houses the positive battery contact, but since it is required that we disconnect the battery when running from external power, a slight modification is required, so a few traces were cut and a jumper wire added to isolate the tab that connects to the positive end of the battery as seen in Figure 3.

Figure 4:
The top of the board battery board. The
connection to the Batt+ is made by soldering to
the tab.
Click on the image for a larger version.

Near the top of the photo in Figure 3 we see that the trace connecting end of the 2.2 ohm resistor has been separated from the battery "+" connector (the round portion) and also along the bottom edge where it connects to the switch.  Our added jumper wire then connects the resistor to the far end of the switch where the trace used to go and we see the yellow wire go off to the "common" contact of the relay.

In Figure 4 we can see the top of the board with the 2.2 ohm resistor - but we also see the wire (white and green) that connects to one of the tabs for the Battery + button on the bottom of the board:  The wire was connected on this side of the circuit board to keep it out of the way round battery tab and the "battery +" connection.

The mechanical parts

For a modification like this, there's no need to make a circuit board - or even use prototyping boards.  Because we are cramming extra components in an existing box, we have to be a bit clever as to where we put things in that we have only limited choices.

Figure 5:
Getting ready to install the connector after
a session of drilling and filing.
Click on the image for a larger version.

In the case of the coaxial power connector, there was only one real choice for its location:  On the side opposite the power switch, near the front, because if it were placed anywhere else it would interfere with the battery or with the fan itself as the case was opened.

Figure 5 shows the location of this connector.  Inside the box. this is located between two bosses and there is just enough room to mount it.  To do this, small holes were drilled into the case at the corners of the connector and a sharp pair of flush-cut diagonal nippers were used to open a hole.  From here it was a matter of filing and checking until the dimensions of the hole afforded a snug fit of the connector.

Figure 6:
A close-up of the buck converter board with the
attached wires and BATT- spring terminal.
The tiny voltage adjustment potentiometer is
visible near the upper-left corner of the board.
Click on the image for a larger version.

Wires were soldered to the connector before it was pressed into the hole and to hold it in place I used "Shoe Goo" - a rubber adhesive - as I have had good luck with this in terms of adhesion:  I could have used cyanoacrylate ("Super" glue) or epoxy, but I have found that the adhesive bonds of these tend to be a bit more brittle with rapid changes of temperature, mechanical shock or - most applicable here - flexing - something that the Shoe Goo is meant to do.

Because this jack is next to the battery minus (-) connector, a short wire was connected directly to it, and another wire was run to the location - in the adjacent portion of the case - where the buck converter board would be placed.

Figure 6 shows the buck converter board itself in front of the cavity in which it will be placed, next to the negative battery "spring" connector.  Diode D1 is soldered on the back side of this board and along the right edge, the yellow self-resetting fuse is visible.  Like everything else the relay was wired with flying leads as well, with resistor R1 being placed at the relay for convenience.

Figure 7:
The relay, wired up with the flying leads.
Click on the image for a larger version.

Figure 7 shows the wiring of the relay.  Again, this was chosen for its size - but any SPDT relay that will fit in the gap and not interfere mechanically with the battery should do the job.

The red wire - connected to the resistor - comes from the positive connector on the jack and the "IN+" of the buck converter board - the orange wire is the common connection of the High/Low switch, the white/violet comes from the "OUT+" of the buck converter and goes to the N.O. (Normally Open) contact on the relay, the white/green goes to the N.C. (Normally Closed) relay contact and the black is the negative lead attached to the coil.

Everything in its place

Figure 8 shows the internals of the fan with the added circuitry.  Shoe Goo was again employed to hold the buck converter board and the relay in place while the wires were carefully tucked into rails that look as though they were intended for this!

Now it was time to test it out:  I connected a bench power supply to the coaxial connector and set the voltage of my external test power supply at 10 volts - enough to reliably pull in the relay - and set the fan to low speed.  At this point I adjusted the (tiny!) potentiometer on the buck converter board for an output of 3.2 volts - about that which could be expected from a very fresh pair of "D" cells.

Figure 8:
Everything wired and in its final locations.  On the far left is
the switch board.  To the left of the hinge is the relay with the
buck converter on the right side of the hinge.  The jack and
negative battery terminal is on the far right of the case.
Click on the image for a larger version.

The result was a constant fan speed as I varied the bench supply from 9 to 18 volts indicating that the buck converter was doing its job.

The only thing left to do was to make a power cord to keep with the fan.  As is my wont, I tend to use Anderson Power Pole connectors for my 12 volt connections and I did so here.

As I also tend to do, I always attach two sets of Anderson connectors to the end of my DC power cords - the idea being that I would not "hog" DC power connections and leave somewhere to plug something else in.  While the power cord for the fan was just 22 gauge wire, I used heavier wire (#14 AWG) between the two Anderson connectors so that I could still run high-current devices.

* * *

Does it work?

Of course it does - it's a fan!

The relay switches over at about 8.5 volts making the useful voltage range via the external connector between 9 and 16 volts - perfect for use with an ostensibly "12 volt" system where the actual voltage can vary between 10 and 14 volts, depending on the battery chemistry and type.

Figure 9:
The fan, folded up with power cord.  The two
power pole connectors and short section of heavy
conductor can be just seen.
Click on the image for a larger version.

Without the weight of the two "D" batteries, the balance of the fan is slightly precarious and prone to tip forward slightly, but this could be fixed by leaving batteries in the unit - but this is not desirable for long-term storage as leakage is the likely result.

Alternatively, one may place some ballast in the battery compartment (large bolt wrapped in insulation, a rag, paper towel, etc.) or simply by placing something (perhaps a rock or two) on the top.  Alternatively, since the fan is typically placed on a desktop, it is often tilted slightly upwards and that offsets the center of gravity in our favor and this - plus the thrust from the airflow - prevents tipping.

This page stolen from ka7oei.blogspot.com

[End]

It *is* possible to have an RF-quiet home PV (solar) electric system!

Figure 1:  Half of the array on my garage - the other half is
on the west-facing aspect.
There's a bit of shade in the morning around the end of June,
but it detracts little during the peak solar production
of the day - the hours on either side of "local" noon.
Click on the image for a larger version

For the past several years an incremental nemesis of amateur radio operation on the HF bands is solar power and the cover article of the April 2016 issue of QST magazine, "Can Home Solar Power and Ham Radio Coexist?" (available online HERE) brings this point home.

Personally, I thought that the article was a bit narrow in its scope, with an unsatisfying conclusion (e.g. "The QRM is still there after a lot of effort and expense, but I guess that it's OK") - but this impression is understandable owing to the constraints of the medium (magazine article) and the specific situation faced by the author.

Solar power need not cause QRM:

I can't help but wonder if others that read the article presumed that amateur radio and home solar were incompatible - but I know from personal experience that this is NOT necessarily the case:  There are configurations that will not produce detectable QRM on amateur bands from 160 meters and higher.

Before I continue, let me state a few things important to the context of this article:

Expertise in HF radio interference and home solar installations seems to mutually exclusive - which is to say that you will be hard-pressed to find anyone who is familiar with aspects of both.  This means that in the solar industry itself, you will not likely find anyone who can offer useful advice in putting together a system that will not contribute to the crescendo of electrical noise.

I have heard that many installers (at least in my area) will strongly pressure their potential customers to use microinverter-based systems - and this my experience as well:  From the very start of the process, I was adamant that the design of my system would be series string using SunnyBoy inverters which were known to me to be RF-quiet.  If your installer will not work with you toward your goals, consider a different company.

Designing an "RF-quiet" system as described here may incur a trade-off in available solar production as the use of microinverters can eke out additional efficiencies when faced with issues such as shading and complicated roofs that present a large number of aspects with respect to insolation (e.g. amount of light energy that can be converted to electricity).  Only in the analysis of proposed systems appropriate for your case can you reasonably predict the magnitude of this and whether or not you find it to be acceptable.

What is presented here is my own experience and that of other amateur radio operators with similar PV (PhotoVoltaic) system.  The scope of this experience is necessarily limited owing to the fact that when spending tens of thousands of dollars, one will understandably "play it safe" and pick a known-good configuration.

I will be the first to admit that there are likely other "safe" (low RF noise) combinations of PV equipment that can be demonstrated to be "clean" in terms of radio frequency interference.  I have anecdotally heard of other configurations and systems, but since I have not looked at them first-hand, I am not willing to make any recommendations that could result in the outlay of a large amount of money.  For this reason, please don't ask me a question like "What about inverter model 'X' - does it cause RFI?" as I simply cannot answer from direct experience.

An example system:

The system at my house consists of two series-string Sunny Boy grid-tie inverters:  I can unequivocally state that this system, which has both a SB 5000TL-US-22 (5 kW) and an SB3.8-1SP-US-40 (3.8kW) does not cause any detectable RF interference on any HF frequency or 160 meters - and I have yet to detect any interference on 6 meters, 2 meters or 70cm.  Near the LF and lower MF band (2200 and 630 meters, respectively) some emissions from these inverters can be detected - but none of the switching harmonics (about 16 kHz) land within either of these bands.  

Figure 2: 
One of two inverters in the garage. 
The Ethernet switch (upper right) produces
more RF noise than the inverter!
Click on the image for a larger version

This PV system is very simple:  I have a detached garage with a north-south ridge line meaning that the roof faces east and west.  While this orientation may seem to be less than ideal compared to a south-facing roof, it actually produces equal or greater power during the summer than a south-facing roof - and there are two usable surfaces onto which one can place panels (east and west) whereas one would typically not place any panels on a north-facing roof.  This means that one may be able to put twice as many panels on a symmetrical east-west facing roof than a south-facing roof.

Simple roof configuration can equal low noise:

The "simple" roof also has another advantage:  All panels on the faces are oriented the same and a larger number of panels may simply be wired in series.

This simple fact means that known-quiet series-string inverters may be used and known noise-generating components may be omitted from the system - namely, many models of "microinverters" and optimizers.  Both of these devices - despite being very different in their operation - are installed on a "per panel" basis and able to adjust the overall contribution of each panel to maximize the energy input of the entire solar power system.

Having each panel individually optimized for output power sounds like a good idea - and in most cases it is - but this nicety should be taken in context with the goals in mind - but considering that the panels themselves represent a rather small portion of the overall system cost, efficiency losses from not having optimizers can often be offset with the addition of more panels.  To be fair, it is not always possible to simply "add more panels" to make up for loss of production - but this must be carefully weighed against a major goal, which is to produce a "noise free" PV system.

The options have changed:

Since the 2016 article was written, the number of options for series-string inverters has significantly increased and the prices have gone down, allowing options to be considered now that may have been dismissed at that time.  Take the article as an example.

From the photographs accompanying the article, there appear to be two different aspects of panels:  A large array consisting of 30 panels, all seeming to face the same direction;  a smaller array of 8(?) panels:  There appears to be an array of 4 panels, but let us presume that this is an independent energy system.

Assuming that each panel is rated for 300 watts (likely higher than a circa-2016 panel) and that one would wish to limit the maximum open-circuit potential to about 450 volts, this implies the use of at least four MPPT circuits:  The 8 panel array and three arrays consisting of 10 series panels, each.  The maximum output of this system would theoretically be about 11.4 kW - but since one can optimistically expect to attain only about 80% of this value in a typical installation the use of an inverter system capable of 10 kW, as stated in the article, is quite reasonable.

Back in 2016, it would be reasonable to have a 10kW series string inverter with two MPPT inputs representing two separate inputs that could be independently optimized.  If such an inverter were used, this would mean that one input would have just 8 panels and the other would have all 30 panels on the main array - not particularly desirable in terms of balancing.  While all 30 of the panels in the larger array would ostensibly be producing the same output, snow, leaves and shading might cause the loss of efficiency should certain parts be thus impaired.

Having already ruled out the optimizing of each panel independently in the interest of having a "known-quiet" system, we might want to split things up a bit.  As an example, a single 10kW inverter with two MPPT inputs could be replaced with a pair of 5 kW inverters, each with 3 MPPT inputs and having a total of six independent DC inputs allowing the 8 panels of the isolated roof to be optimized together and the remaining 30 panels being divided into 5 arrays of about 6 panels, each.

The 2016 article did not mention the price the system, but a reasonable estimate for that time would be around US$35000 - and it was mentioned, in passing, that the cost of RFI mitigation might have been about 10% of the total system cost, implying about $3500 - about the cost of two Sunny Boy  SB5.0 5 kW series-string inverters, each with three MPPT inputs.

Replicating success:

At least two other local amateur radio operators used the same recipe for low-noise PV systems:  Series-string SunnyBoy grid-tie inverters - specifically the SB 3800TL, SB 5000TL and SB3.8s.  In none of these cases could RFI be detected that could be attributed to the inverter - and the only noise to be detected was with a portable shortwave receiver held within a few inches of the display.

What is known not to be quiet:

From personal experience I know for certain that microinverters such as the older Enphase M190 can be disastrous for HF, VHF and UHF reception.  As noted in the QST article, the Enphase power optimizers (model number not mentioned) also caused QRM.

Figure 3:
The two Tesla Powerwalls, gateway and electrical sub-
panels for the system located remotely on the east wall
of the house.
Click on the image for a larger version

Additionally, it has been observed that the Solaredge inverters - particularly coupled with optimizers - have caused tremendous radio frequency interference:  The aforementioned April, 2016 QST article about solar RFI deals with this very combination.

It probably won't work in all cases.

Compared to some installations that I have seen, my system - or the one in the 2016 article - are very simple cases - and there are a number of practical limitations, which include:

• A "minimum" array size limitation.  Taking the Sunny boy SB5.0 as an example, there is a 90 volt minimum input which means that one would (very conservatively) want at least four 60-cell panels on each circuit.  This limitation may affect what areas on a roof may be candidates for placement of solar panels, reducing the total system capacity as compared to what might be possible with individually-optimized panels.

• Systems with complicated shading.  If there are a number of trees - or even antennas and structures - portions of sub-strings may be shaded, causing reduction in output and compared to individually-optimized panels, series-strings are at a disadvantage, but careful selection of sub-string geometry can help.  For example, if a tower shades a series of panels during the period of highest production, placing all of those panels on one particular string can help isolate the degradation - but this sort of design consideration will require careful analysis of each situation.

Final words:

The design, configuration and layout of a home (or any) PV system is more complicated than depicted here and any system to be considered would have to take into account.  While I am certain that there are other ways to make an "RF Quiet" PV system, this article was intended to be limited to configurations and equipment with which I have direct experience.

Again, the likelihood of finding a "solar professional" who thoroughly understands RFI issues and knows which type of equipment is RF-quiet is unlikely, so it is up to you as the potential recipient of QRM to do the research.

Other articles at this blog on related topics:

• The Solar Saga Part 1: Avoiding Interference - (Why I did not choose Microinverters) - link
  
• The Solar Saga Part 2:  Getting the system online - link
• Does the Tesla Powerwall 2 produce Radio Frequency Interference? - link
• Reducing RFI from the Tesla Powerwall 2 - link 
• Tesla Powerwall susceptibility to RF Transmissions - and how to deal with it - link 
  

This page stolen from ka7oei.blogspot.com

[End]

Characterizing spurious (Harmonic) responses of the SDRPlay RSP1a (and other models)

The SDRPlay RSP2pro (left) and RSP1a receivers (right)

The SDRPlay RSP1a is a popular Software Defined Radio (SDR).  This device, connected to and powered by the computer via a USB cable covers from VLF through UHF and low microwave frequencies.

This receiver shares a similar internal architecture of similar devices such as the RTL-SDR dongle and the AirSpy in that an analog frequency converter (mixer) precedes the analog-to-digital converter:  In the case of the SDRPlay, the frequency to which the receiver is tuned is (usually) converted to baseband I/Q signals, with the "center" frequency being at zero Hz (DC). ¹

Note:

For the purposes of this discussion, there is no difference between the RSP1a and some of the other receivers in the product lineup (e.g. RSPDuo, RSPdx and the discontinued RSP1, RSP2 and RSP2pro) in terms of harmonic response across the 2-30 MHz range as they all have about the same 12 MHz and 30 MHz cut-off frequencies on their input filtering - properties that would affect HF reception across the 2-30 MHz range in terms of harmonic response.

This issue was noted at least as far back as 2017 in the SDRPlay forum - See this thread:  https://www.sdrplay.com/community/viewtopic.php?t=2280

Imperfect mixers

By its nature, a frequency mixer is a non-linear device.  Ideally, the two frequencies applied to a mixer would yield just two more - the sum and difference.  For example, if we applied a 5 MHz signal and a 1 MHz signal to a mixer, it would output both the sum of 6 MHz and the difference of 4 MHz - and this is true, but there's more to the story.

In our example - with a real-world mixer, we will also get additional products - including those related to the harmonics of the local oscillator and the applied signal.  Because of this, we will see weaker signals at:

• 11 MHz (2 * 5 MHz + 1 MHz) 
• 9 MHz (2 * 5 MHz - 1 MHz) 
• 7 MHz (5 MHz + 2 * 1 MHz) 
• 3 MHz (5 MHz - 2 * 1 MHz) 
• And so on.

Typically, these "other" signals will be quite a bit weaker than the original - but they will still be present, possibly at a high enough level to cause issues such as spurious signals - a problem with both receivers and transmitters.  Typically, these are tamed by proper design of the mixer, proper selection of frequencies and careful filtering around the mixer to limit the energy of these "extra" signals.

Note:  There will be a response at 5x the center frequency as well, but it is suppressed better than the 3x response by the mixer and - for the 80 meter amateur band and higher - these responses are suppressed reasonably well by the filtering.

SDRPlay's poor harmonic response suppression on 80 meters and below.

ANY receiver will experience spurious responses related to mixing products.  Typically, filtering is employed to remove/minimize such responses, but for a wide-bandwidth receiver such an SDR, doing this is complicated by the fact that being able to cover wide swaths of bandwidth would ideally require a large number of overlapping filters.

An example of a radio where this is done - albeit of different architecture - is the Icom IC-7300 which has nine overlapping band-pass filters that cover 160 through 10 meters.  While the reasons for the '7300 having many filters has as much to do with its being a "direct sampling" ² type of SDR, good filtering on the signal path of any type of receiver - SDR or "HDR" (Hardware Defined Radio - or an "old school" analog type) is always a good idea

If this many filters had been implemented on the SDRPlay, there would be enough filtering to prevent a significant harmonic response.  In the case of the RSP1a, this was not done - partly to allow 5-8 MHz of continuous coverage without being significantly impacted by the filters in many cases, but more likely it was done due to practical reasons of economics ³ :  There are just three filters used for covering all of the "HF" amateur bands 160 through 10 meters:  One that covers up to 2 MHz, another that covers 2-12 MHz and third that covers 12-30 MHz:  This information is covered in the RSP1a technical information document ( https://www.sdrplay.com/wp-content/uploads/2018/01/RSP1A-Technical-Information-R1P1.pdf )

The sensitivity to harmonics was tested with the RSP1a's local oscillator (but not necessarily the virtual receiver) tuned to 3.7 MHz ⁴ .  For reasons likely related to circuit symmetry, it is odd harmonics that will elicit the strongest response which means that it will respond to signals around (3.7 MHz * 3) = 11.1 MHz.  "Because math", this spurious response will be inverted spectrally - which is to say that a signal that is 100 kHz above 11.1 MHz - at 11.2 MHz - will appear 100 kHz below 3.7 MHz at 3.6 MHz.  (It's likely that there are also weaker responses at frequencies around 5 times the local oscillator, but these are - for the most part - adequately suppressed by the filtering.)

In other words, the response to spurious signals follow this formula:

Apparent signal = Center frequency + ((Center frequency * 3) - spurious signal) )

Where:

• Center frequency = The frequency to which the local oscillator on the RSP is tuned.  In the example above, this is 3.7 MHz.
  
• Spurious signal = The frequency of spurious signal which is approximately 3x the center frequency.  In the example above, this is 11.2 MHz.
  
• Apparent signal = Lower frequency where signal shows up.   In the example above, this is 3.6 MHz.
  

In our example - a tuned frequency of 3.7 MHz - the 3rd harmonic would be within the passband of the 2-12 MHz filter built into RSP1a meaning that the measured response at 11.2 MHz will reflect the response of the mixer itself, with little effect from the filter as the 2-12 MHz filter won't really affect the 11 MHz signal - and according to the RSP1a documentation (link), this filter really doesn't "kick in" until north of 13 MHz.

In other words, in the area around 80 meters, you will also be able to see the strong SWBC (Shortwave Broadcasting) signals on the 25 meter band around 11 MHz.

How bad is it?

Measurements were taken at a number of frequencies and the amount of attenuation is indicated in the table below.  These values are from measurement of a recent-production RSP1a and spot-checking of a second unit using a calibrated signal generator and the "HDSDR" program:

LO Frequency	Measured Attenuation at 3X LO frequency	Attenuation in "S" Units
2.1 MHz	21 dB (@ 6.3 MHz)  - Using 2-12 MHz filter	3.5
2.5 MHz	21 dB (@ 7.5 MHz)	3.5
3.0 MHz	21 dB (@ 9.0 MHz)	3.5
3.7 MHz	21 dB (@ 11.1 MHz)	3.5
4.1 MHz	23 dB (@ 12.3 MHz)	3.8
4.5 MHz	30 dB (@ 13.5 MHz)	5
5.0 MHz	39 dB (@ 15.0 MHz)	6.5
5.5 MHz	54 dB (@ 16.5 MHz)	9
6.0 MHz	54 dB (@ 18.0 MHz)	9
6.5 MHz	66 dB (@ 19.5 MHz)	11
12.0 MHz	21 dB (@ 36.0 MHz) - Using 12-30 MHz filter	3.5
12.5 MHz	21 dB (@ 37.5 MHz)	3.5
13.5 MHz	22 dB (@ 40.5 MHz)	3.7
14.5 MHz	26 dB (@ 43.5 MHz)	4.3
15.5 MHz	31 dB (@ 46.5 MHz)	5.2
16.5 MHz	35 dB (@ 49.5 MHz)	5.8
17.5 MHz	39 dB (@ 52.5 MHz)	6.5
18.5 MHz	43 dB (@ 55.5 MHz)	7.2
19.5 MHz	46 dB (@ 58.5 MHz)	7.7
20.5 MHz	50 dB (@ 61.5 MHz)	8.3
21.5 MHz	53 dB (@ 64.5 MHz)	8.8

Table 1:  Measured 3rd harmonic response of the RSP1a

Interpretation:

• In the above chart we see the local oscillator frequency in the left column, the measured attenuation of the 3rd harmonic response (and its frequency) in the center column, and that amount of attenuation expressed in "S" units.  Here, an "S" unit is based on the IARU standard (Technical recommendation R.1) of 6 dB per S unit, which is reflected in programs like SDRUNO, HDSDR and many others.
  
• The attenuation of the 3rd harmonic response was measured by first noting the signal level required to obtain a given reading - typically "S-9" near the fundamental frequency - and then observing the level required to obtain that same reading - within +/-1dB - near the 3rd harmonic frequency, using the relationship formula, above.
  
• Below the cutoff frequency of the relevant filter (nominally 12 MHz for receive frequencies in the range of 2 to 12 MHz, nominally 30 MHz for receive frequencies in the range of 12 to 30 MHz) the harmonic response is limited to that of the mixer itself, which is about 21 dB.
  
• We can see that on the 2 to 12 MHz segment, the attenuation related to the 3rd harmonic doesn't exceed 40 dB (which is the low end of what I would call "OK, but not great) until one gets above about 5 MHz (which translates to 15 MHz) and it doesn't get to the "goodish" range (50dB or more) until north of about 5.5 MHz which is borne out by the filter response charts published by SDRPlay.
• On the 12 to 30 MHz band the 2-12 MHz filter has practically negligible effect until one gets above about 20 meters, at which point it gets into the "OK, but not great" range by about 18 MHz, and it doesn't really get "goodish" until north of 20.5 MHz.
• Similarly, the "12-30 MHz filter isn't particularly sharp, either.  What this means is that strong 6 meter signals may well appear in the 16.5 to 17.5 MHz range as frequency inverted representations.
• If there is a relatively strong signal source in the area of the 3rd harmonic response, it will likely appear at the lower receive frequency where the attenuation of the filter is less than 40 dB or so.  The severity of this response will, of course, depend on the strength of that signal, the amount of attenuation afforded by the filters at that frequency, and the amount of noise and other signals present in the range of the fundamental frequency response.
  

Based on the above data, we can deduce the following:

• When the RSP1a is tuned between 2 MHz and (below) 12 MHz, it is using its "2-12 MHz" filter. In this range - and below approx. 4 MHz - the 12 MHz cut-off of the filter has negligible effect in reducing 3rd harmonic response.
• What this means is that signals from 6-12 MHz will appear more or less unhindered (aside from the 21 dB reduction afforded by the mixer) when the local oscillator of the receiver is tuned between 2 and 4 MHz.
• The 3rd harmonic response across 2-4 MHz - which is the 6-12 MHz frequency range - can contain quite a few strong signals and noise sources such as those from shortwave broadcast stations.
  
• When the RSP1a is tuned between 12 MHz and (below) 30 MHz, it is using its "12-30 MHz" filter.  Below about 14 MHz, the 30 MHz cut-off of the filter has negligible effect in reducing 3rd harmonic response.
• Signals from 36-40 MHz will appear with just 21-26 dB attenuation when tuned in the range of 12-13.5 MHz.
• In most cases there are probably few signals in the 36-40 MHz range that are likely to be an issue when tuning in the 12-13.5 MHz range.

80 meter example:

Connecting the RSP1 to a known-accurate signal generator set to -40dBm, the signal level at 3.6 MHz was measured:  Maintaining the signal level, the generator was retuned to 11.2 MHz and the resulting signal level was measured to be 21 dB (a bit more than 3 "S" units) lower than that at 3.6 MHz.

What this means is is that a "20 over S-9" signal at 11.2 MHz will show up as an S-9 signal at 3.7 MHz, and an S-9 signal at 11.2 MHz will be around S-6 at 3.7 MHz.  In other words, even a "weak-ish" signal at the 3rd harmonic will show up at the lower frequency.

80/60 meter example:

If you run the RSP1a in a wider bandwidth mode, it is possible to simultaneously see and tune a greater frequency range.  For example, let us presume that you wish to cover both 80 and 60 meters using a single RSP1.  To do this, you could set the center (LO) frequency to 4.5 MHz and set the sample rate to 5.376 MHz and use the 5 MHz band-pass filter built into the RSP1's converter/mixer chip (the Msi001) to prevent in-band aliasing.

In this configuration 20 meter signals will appear at the top of 80 meters owing to the relationship in the formula that we described above.  Taking the 20 meter FT-8 subband at 14.074 MHz as an example, we see that:

4.5 MHz + ((4.5 MHz * 3) - 14.074) = 3.926 MHz

In other words, we will see spectrally-inverted representations of 20 meter FT-8 signals around 3.926 MHz, and the rest of the (upper) portion of 20 meters across the rest (lower) portion of 80 meters where USB signals on 20 meters will show up as LSB signals on 80.  We know from the chart above that those signals will be attenuated by between 30 and 39 dB (about 5-6 S-units).  This might sound like a lot of attenuation, but it means that a "20 over" signal on 20 meters will appear at around S-7 to S-8 on 80 meters - still quite respectable most certainly very audible!

More about filtering and harmonic response

While these spurious responses may not be too much of a problem for the casual user, it will be necessary to add additional filtering to allow the RSP1a to function on par with a modern, SDR receiver from one of the major manufacturers.

Unfortunately, the filtering in the RSP1a is not sufficient in the 80 meter case mentioned above as it doesn't have octave filters (or similar) - but what about 60 or 40 meters?

The table above answers this question.  In the case of 60 meters - with the receiver tuned to 5.3 MHz - our 3rd harmonic will land on 15.9 MHz.  Based on measurements of the receiver the response of signals around 15 MHz - which corresponds to the 19 meter Shortwave Broadcast Band - will be a bit more than 40 dB down from 40 meters with about 20 dB of this being due to the roll-off of the 2-12 MHz filter - but because this frequency range is inhabited by very strong shortwave broadcasters they are likely to still be quite audible around 60 meters.

The situation is a bit better for 40 meters where the 3rd harmonic is around the 15 meter band.  There, the 2-12 MHz filter knocks signals down by 50dB or more, putting them about 70dB below the 40 meter response - on par with about any respectable receiver.

What this means is that for amateur bands below 40 meters it is suggested that additional filtering be applied.

The best solution - and recommended for any software-defined radio (or even older "hardware-defined radios") is to have band-pass filter designed for the specific amateur band in question. This will not only significantly attenuate the harmonic response, but it will also reduce the total amount of RF energy entering the receiver, reducing the probability of overload.  The obvious down-side is that it will reduce the flexibility of the receiver in that unless you change/remove it, you won't be able to receive signals well outside the filter's design range.

Another possibility is to add a low-pass filter that is designed to cut off signals above the band of interest.  For example, if you have a filter that cuts off sharply above 8 MHz, you will be able to tune 80-40 meters and get reasonable attenuation of the 3rd harmonic response across this entire frequency range.

In the case of 160 meters the RSP1a will automatically select the 0-2 MHz low-pass filter and the 3rd harmonic response will be a respectable 50-ish dB down, depending on frequency.

On 20 meters - where the 3rd harmonic is around 42 MHz - the "12-30 MHz" filter will be selected, but the published response of this filter shows that at 42 MHz its attenuation will be quite limited.  Practically speaking, it is unlikely that there will be any signals in this frequency range so there being "only" 20-30dB of attenuation is unlikely to cause a problem in most cases, but one should be aware of this.

What can be done:

In short, none of the currently-made SDRPlay receivers - by themselves - will offer very good performance in terms of harmonic rejection between 2 and 5 MHz and it will be particularly bad on the 80 meter band where strong 25 meter SWBC signals can appear:  It is interesting that the ARRL review of the RSPdx (Link here) didn't catch this issue.

It is unfortunate that the designers of the SDRPlay receivers did not add at least one additional low-pass filter in the signal path to quash what is a rather strong response in the 2-6 MHz range - particularly on 80 meters, one of the most popular bands.  A low-pass filter with a cut-off frequency of 6 MHz (with attenuation becoming significant above 7 MHz) would ameliorate the harmonic response when tuning across this band.  This problem is made even worse by the fact that even antennas that aren't particularly resonant at their harmonic responses (e.g. the antenna for 80 meters) will likely do quite a decent job of receiving signals in the 11-12 MHz area.

The only real "fix" for this is to install additional filtering between the SDRPlay receiver and the antenna.  If single-band operation is all that is desired, the best choice will be a band-pass filter designed for the frequency range in question ⁵ - but unless you are dedicating the receiver just for that one band, this isn't really desirable unless you can easily switch/bypass the filter when tuning elsewhere.

A more flexible solution would be to use a low-pass filter.  As we noted above, the 12 MHz roll-off of the built-in (2-12 MHz) filter just doesn't do much to suppress signals from 20 meters, but if we had a filter that had a sharp cut off beginning, say, at 8 MHz, we could use it for 80, 60 and 40 meters - such a filter is depicted schematically, below:

8 MHz low-pass filter schematic, designed using ELSIE

This filter is pretty easy to build:

• Capacitors 1 and 5 each consist of a 100pF and 470pF in parallel
• Capacitor 3 consists of a 680pf and 220pF in parallel - although you could probably get away with two 470pF capacitors in parallel in a pinch.
  
• Inductors 2 and 4 consist of 16 turns on a T50-2 (or 18 turns on a T37-2) toroid using small wire - 24-30 AWG (0.6-0.25mm dia.) is fine

A small scrap of PC board material - about 2"x4" (5x10cm) is more than large enough to accommodate with the capacitors soldered directly to the foil and inductors held aloft by the capacitors.  The connectors should be attached to the PC board directly - or with short lengths of coax, keeping the ground (shield) lead length to an absolute minimum to minimize the probability of ground-loop induced noise currents.

If you have access to a NanoVNA it's quite easy to check the performance.  If anything, it may be necessary to spread the turns across the toroid or remove one turn - but this design is quite forgiving and it should work adequate with no tuning at all.

The obvious down-side for this is that if you are tuning all over the HF spectrum (above 7.5-8 MHz in the case of the filter above) you'd have to manually remove or bypass any such filtering when you tuned beyond the range that the added filter would pass.

 

Footnotes:

1. The receivers mentioned at the beginning of the article (SDRPlay, AirSpy HF, RTLSDR, etc.) have analog-to-digital converters that cover only a portion of the HF spectrum, using a frequency mixer to convert a range of frequencies from the range of interest to a lower frequency, which is then fed into the converter.  Limiting the amount of spectrum being ingested by the receiver - particularly when appropriate filtering is used - can improve performance, reduce cost, and especially reduce the total amount of data, allowing a modest computer (older PC, Raspberry Pi) to be used with it.
   
2. A "direct sampling" type of receiver - such as that found in the Icom IC-7300, IC-7610, the KiwiSDR, Red Pitaya and the RX-888 (when used at HF) and others like them simply "inhale" large swaths of spectrum all at once.  Because the analog-to-digital converter itself has a limited amount of total RF signal power that it can handle, radios like the Icoms have filtering that allow the passage of only the (relatively) small portion of the HF spectrum around that to which the receiver is tuned, reducing the probability of overload from strong signals on frequencies well away from those of interest.  Other direct-sampling receivers such as the KiwiSDR, Red Pitaya and RX-888 do not necessarily have band-specific filtering as they are intended to be able to receive multiple frequencies across the entire HF spectrum at once and as such, much more care is required in implementation to prevent overload/distortion for these devices.
   
3. In the case of the (currently-produced) RSP receivers, the filtering varies depending on model:  In the case of the RSP1a, it has a band-pass filter that covers 2-12 MHz while other models have used just a 12 MHz low-pass - the former being capable of rejecting AM broadcast band (e.g. mediumwave) signals from the input of the receiver when tuned to HF, and the latter not.  Some units additionally have a separate "notch" (reject) filter that is designed to remove just AM broadcast-band signals.  The situation described in this article - the reception of signals around 11 MHz when tuned to 80 meters - is related to the fact that the 2-12 MHz filter represents a 6:1 frequency range which means that over the lower portion of this spectrum, the 12 MHz cut-off of this filter cannot possibly remove responses to the third harmonic, hence the issue described here.
4. If you are using a program like SDRUno it may not be readily apparent to what frequency the receiver's local oscillator is tuned.  If set to "Zero IF" mode, the local oscillator will be tuned at the same place as the center of the waterfall display when it is fully zoomed out - typically indicated by a slight line at the "Zero Hz" frequency there there is a slight amount of noise energy.  By default, one cannot directly tune the local oscillator ("Zero IF" frequency) in SDRUno.  If you use the "HDSDR" program by I2PHD (et al) you can independently tune the local oscillator and the frequency of the virtual receiver.
   
5. SDRPlay receivers are currently in use at a number of well known and public WebSDRs around the world as the "acquisition device" (e.g. receiver).  In most cases these receivers - because they are used only for specific amateur bands - are preceded by a band-pass filter for the band that they are covering, completely eliminating issue noted in this article.  It was during testing at one of these WebSDRs - a receiver on 80 meters that does not (yet) have additional filtering - that signals were noted across the 80 meter band in the middle of the day that should not have been there at all - and these signals were quickly realized to be the result of a harmonic response in the front end.  These responses were then verified and quantified using two other RSP1a receivers (of different production runs) and test equipment during the preparation of this article.  When it was convenient to do so, a low-pass filter with a cut-off frequency of 7.5 MHz was installed on this receiver, solving the image problems.
   

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This page stolen from ka7oei.blogspot.com

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