Multi-band transmitter and monitoring system for Eclipse monitoring (Part 1)

It should not have escaped your attention - at least if you live in North America - there there have been/will be two significant solar eclipses occurring in recent/near times:  One that occurred on October 14, 2023 and another eclipse that will happen during April, 2024.  The path of "totality" of the October eclipse happened to pass through Utah (where I live) so it is no surprise that I went out of my way to see it - just as I did back in 2012:  You can read my blog entry about that here.

 Figure 1:
The eclipse in progress - a few minutes
before "annularity".
(Photo by C. L. Turner)

I will shortly produce a blog entry related to my activities around the October 14, 2023 eclipse as well.

The October eclipse was of the "annular" type meaning that the moon is near-ish apogee meaning that the subtended angle of its disk is insufficient to completely block the sun owing to the moon's greater-than-average distance from Earth:  Unlike a solar eclipse, there is no time during the eclipse where it is safe to look at the sun/moon directly, without eye protection.

The sun will be mostly blocked, however, meaning that those in the path of "totality" experienced a rather eerie local twilight with shadows casting images of the solar disk:  Around the periphery of the moon it was be possible to make out the outline of lunar mountains - and those unfortunate to stare at the sun during this time will receive a ring-shaped burn to their retina.

From the aspect of a radio amateur, however, the effects of a total and annular solar eclipse are largely identical:  The diminution of the "D" layer and partial recombination of the "F" layers of the ionosphere causing what are essentially nighttime propagation conditions during the daytime - geographically limited to those areas under the lunar shadow.

In an effort to help study these sort of effects - and to (hopefully) better-understand the propagation effects, a number of amateurs went (and are) going out into the field - in or near the path of "totality" - and setting up simultaneous, multi-band transmitters.

Producing usable data

Having "Eclipse QSO Parties" where amateur radio operators make contacts during the eclipse likely goes back nearly a century - the rarity of a solar eclipse making the event even more enigmatic.  In more recent years amateurs have been involved in "citizen science" where they make observations by monitoring signals - or facilitate the making of observations by transmitting them - and this happened during the October eclipse and should also happen during the April event as well.

While doing this sort of thing is just plain "fun", a subset of this group is of the metrological sort (that's "metrology", no "meteorology"!) and endeavor to impart on their transmissions - and observations of received signals - additional constraints that are intended to make this data useful in a scientific sense - specifically:

• Stable transmit frequencies.  During the event, the perturbations of the ionosphere will impart on propagated signals Doppler shift and spread:  Being able to measure this with accuracy and precision (which are NOT the same thing!) adds another layer of extractable information to the observations.
  

• Stable receivers.  As with the transmitters, having a stable receiver is imperative to allow accurate measurement of the Doppler shift and spread.  Additionally, being able to monitor the amplitude of a received signal can provide clues as to the nature of the changing conditions.
  

• Monitoring/transmitting at multiple frequencies.  As the ionospheric conditions change, its effects at different frequencies also changes.  In general, the loss of ionization (caused by darkness) reduces propagation at higher frequencies (e.g. >10 MHz) and with lessened "D" layer absorption lower frequencies (<10 MHz) the propagation at those frequencies is enhanced.  With the different effects at different frequencies, being able to simultaneously monitor multiple signals across the HF spectrum can provide additional insight as to the effects.
  

To this end, the transmission and monitoring of signals by this informal group have established the following:

• GPS-referenced transmitters.  The transmitters will be "locked" to GPS-referenced oscillators or atomic standards to keep the transmitted frequencies both stable, accurate - and known to within milliHertz.
  

• GPS referenced receivers.  As with the transmitters, the receivers will also be GPS-referenced or atomic-referenced to provide milliHertz accuracy and stability.

With this level of accuracy and precision the frequency uncertainties related to the receiver and transmitter can be removed from the Doppler data.  For generation of stable frequencies, a "GPS Disciplined Oscillator" is often used - but very good Rubidium-based references are also available, although unlike a GPS-based reference, the time-of-day cannot be obtained from them.

Why this is important:

Not to demean previous efforts in monitoring propagation - including that which occurs during an eclipse - but unless appropriate measures are taken, their contribution to "real" scientific analysis can be unwittingly diminished.  Here are a few points to consider:

• Receiver frequency stability.  One aspect of propagation on HF is that the signal paths between the receiver and transmitter change as the ionosphere itself changes.  These changes can be on the order of Hertz in some cases, but these changes are often measured in 10s of milliHertz.  Very few receivers have that sort of stability and the drift of such a receiver can make detection of these Doppler shifts impossible.
  

• Signal amplitude measurement.  HF signals change in amplitude constantly - and this can tell us something about the path.  Pretty much all modern receivers have some form of AGC (Automatic Gain Control) whose job it is to make sure that the speaker output is constant.  If you are trying to infer signal strength, however, making a recording with AGC active renders meaningful measurements of signal strength pretty much impossible.  Not often considered is the fact that such changes in propagation also affect the background noise - which is also important to be able to measure - and this, too, is impossible with AGC active.
  

• Time-stamping recordings.  Knowing when a recording starts and stops with precision allows correlation with other's efforts.  Fortunately this is likely the easiest aspect to manage as a computer with an accurate clock can automatically do so (provided that one takes care to preserve the time stamps of the file, or has file names that contain such information) - and it is particularly easy if one happens to be recording a time station like WWV, WWVH, WWVB or CHU.

In other words, the act of "holding a microphone up to a speaker" or simply recording the output of a receiver to a .wav file with little/no additional context makes for a curious keepsake, but it makes the challenge of gleaning useful data from it more difficult.

One of our challenges as "citizen scientists" is to make the data as useful as possible to us and others - and this task has been made far easier with inexpensive and very good hardware than it ever has been - provided we take care to do so.  What follows in this article - and subsequent parts - are my reflections on some possible ways to do this:  These are certainly not the only ways - or even the best ways - and even those considerations will change over time as more/different resources and gear become available to the average citizen scientist. 

* * *

How this is done - Receiver:

The frequency stability and accuracy of MOST amateur transceivers is nowhere near good enough to provide usable observations of Doppler shift on such signals - even if the transceiver is equipped with a TCXO or other high-stability oscillator:  Of the few radios that can do this "out of the box" are some of the Flex transceivers equipped with a GPS disciplined oscillator.

To a certain degree, an out-of-the-box KiwiSDR can do this if properly set-up:  With a good, reliable GPS signals and when placed within a temperature-stable environment (e.g. temperature change of 1 degree C or so during the time of the observation) they can be stable enough to provide useful data - but there is no guarantee of such.

To remove such uncertainty a GPS-based frequency reference is often applied to the KiwiSDR - often in the form of the Leo Bodnar GPS reference, producing a frequency of precisely 66.660 MHz.  This combination produces both stable and accurate results.  Unfortunately, if you don't already have a KiwiSDR, you probably aren't going to get one as the original version was discontinued in 2022:  A "KiwiSDR 2" is in the works, but there' no guarantee that it will make it into production, let alone be available in time for the April, 2024 eclipse. 

Figure 2:
The RX-888 (Mk2) - a simple and relatively inexpensive
box that is capable of "inhaling" all of HF at once.
Click on the image for a larger version.

The RX-888 (Mk2)

A suitable work-around has been found to be the RX-888 (Mk2) - a simple direct-sampling SDR - available for about $160 shipped (if you look around).  This device has the capability of accepting an external 27 MHz clock (if you add an external cable/connector to the internal U.FL connector provided for this purpose) in which it can become as stable and accurate as the external reference.

This SDR - unlike the KiwiSDR, the Red Pitaya and others - has no onboard processing capability as it is simply an analog-to-digital coupled with a USB3 interface so it takes a fairly powerful computer and special processing software to be able to handle a full-spectrum acquisition of HF frequencies.

Software that is particularly well-suited to this task is KA9Q-Radio (link).  Using the "overlap and save" technique, it is extraordinarily efficient in processing the 65 Megasamples-per-second of data needed to "inhale" the entire HF spectrum.  This software is efficient enough that a modest quad-core Intel i5 or i7 is more than up to the task - and such PCs can be had for well under $200 on the used market.

KA9Q-Radio can produce hundreds of simultaneous virtual receivers of arbitrary modes and bandwidths which means that one such virtual receiver can be produced for each WSPR frequency band:  Similar virtual receivers could be established for FT-8, FT-4, WWV/H and CHU frequencies.  The outputs of these receivers - which could be a simple, single-channel stream or a pair of audio in I/Q configuration - can be recorded for later analysis and/or sent to another program (such as the WSJT-X suite) for analysis.

Additionally, using the WSPRDaemon software, the multi-frequency capability of KA9Q-Radio can be further-leveraged to produce not only decodes of WSPR and FST4W data, but also make rotating, archival I/Q recordings around the WSPR frequency segments - or any other frequency segments (such as WWV, CHU, Mediumwave or Shortwave broadcast, etc.) that you wish.

Comment:  I have written about the RX-888 in previous blog posts:

• Improving the thermal management of the RX-888 (Mk 2) - link 
• Measuring signal dynamics of the RX-888 (Mk 2) - link
  

Full-Spectrum recording

Yet another capability possible with the RX-888 (Mk2) is the ability to make a "full spectrum" recording - that is, write the full sample rate (typically 64.8 Msps) to a storage device.  The result are files of about 7.7 gigabytes per minute of recording that contain everything that was received by the RX-888, with the same frequency accuracy and precision as the GPS reference used to clock the sample rate of the '888.  

What this means is that there is the potential that these recordings can be analyzed later to further divine aspects of the propagation changes that occurred during, before and after the eclipse - especially by observing signals or aspects of the RF environment itself that one may not have initially thought to consider:  This also can allow the monitoring of the overall background noise across the HF spectrum to see what changes during the eclipse, potentially filling in details that might have been missed on the narrowband recordings.

Because such a recording contains the recordings of time stations (WWV, WWVH, CHU and even WWVB) it may be possible to divine changes in propagation delay between those transmit sites and the receive sites.  If a similar GPS-based signal is injected locally, this, too, can form another data point - not only for the purposes of comparison of off-air signals, but also to help synchronize and validate the recording itself.

By observing such a local signal it would be possible to time the recording to within a few 10s of nanoseconds of GPS time - and it would also be practical to determine if the recording itself was "damaged" in some way (e.g. missed samples from the receiver):  Even if a recording is "flawed" in some way, knowing the precise location an duration of the missing data allows this to be taken into account and to a large extent, permit the data "around" it to still be useful.

Actually doing it:

Up to this point there has been a lot of "it's possible to" and "we have the capability of" mentioned - but pretty much everything mentioned so far was used during the October, 2023 eclipse.  To a degree, this eclipse is considered to be a rehearsal for the April 2024 event in that we would be using the same techniques - refined, of course, based on our experiences.

While this blog will mostly refer to my efforts (because I was there!) there were a number of similarly-equipped parties out in the fields and at home/fixed stations transmitting and receiving and it is the cumulative effort - and especially the discussions of what worked and what did not - that will be valuable in preparation for the April event.  Not to be overlooked, this also gives us valuable experience with propagation monitoring overall - an ongoing effort using WSPRDaemon - where we have been looking for/using other hardware/software to augment/improve our capabilities.

In Part 2 I'll talk about the receive hardware and techniques in more detail.

Stolen from ka7oei.blogspot.com

[END]

Remote (POTA) operation from Canyonlands National Park (K-0010)

As I am wont to do, I recently spent a week camping in the "Needles" district of Canyonlands National Park.  To be sure, this was a bit closer to "glamping" in the sense that we had a tent, a flush-toilet a few hundred feet away, plenty of food, solar panels for power and didn't need to haul our gear in on our backs - at least not any farther than between the vehicle(s) and the campsite.

While I did hike 10s of miles during the week, I didn't hike every day - and that left a bit of "down time" to relax and enjoy the local scenery.

As a first for me - even though I have camped there many times and have even made dozens of contacts over the years on HF - I decided to do a real POTA (Parks On The Air) activation.  In the days before departure I finally got around to signing up on the pota.app web site and just before I left the area of cell phone coverage (there is none at all anywhere near where we were camping) I scheduled an activation to encompass the coming week as I had no idea exactly when I would be operating - or on what bands.

Figure 1:
The JPC-7 loaded dipole at 10', backgrounded by red rock.
Click on the image for a larger version.

* * *

It wasn't until the day after I arrived that I finally had time to operate.  As it was easiest and most convenient to do so, I deployed my "modified" JPC-7 loaded dipole antenna (an antenna I'll describe in greater detail in a future post) affixing it atop a tripod light stand that could be telescoped to about 10 feet (3 meters) in height - attaching one of its legs to the swing-out grill of the fire pit to prevent it from falling over.  Being only about 10 feet from the picnic table, it offered a relatively short cable run and when it came time to tune the antenna, I simply disconnected it from the input of the tuner, connected it to my NanoVNA and adjusted the coils:  In so-doing, I could change bands in about two minutes.

The radio that I usually used was my old FT-100 - typically running at 50 watts on CW, 100 watts on SSB, but I would occasionally fire up my FT-817  and run a few contacts on that as well.  As you would expect, the gear was entirely battery-powered as there is not a commercial power line within 10s of miles of this place:  Often, one of my batteries would be off being charged from a solar panel, requiring that I constantly rotate through them.

* * *

For reasons of practicality - namely the fact that I would be operating in (mostly) daylight - and for reasons related to antenna efficiency, I mostly operated on 30 meters and higher.  Because we were outside, this made a computer screen very difficult to see so I logged on a piece of paper - also convenient because this method required no computer or batteries!  The very first contact - a Park-to-Park - occurred on 15 meter SSB, but I quickly QSY'ed down to 17 meters and worked a few dozen stations on CW - breaking in my "CW Morse" paddle for the first time on the air:  It would seem that my scheduling the activation and my Morse CW being spotted by the Reverse Beacon Network caused the notice to go out automatically where I was quickly pounced on.

In using this paddle - made by CW Morse - for the first time I quickly discovered several things:

• I've seen others using this paddle by holding it in their hand - but I was completely unable to do that:  I would get into the "zone" while sending and inevitably put my fingers on the "dit" and "dah" paddle's tension adjustment screws, causing me to send random elements:  At first I thought that something was amiss - perhaps RF getting into the radio - but one of the other folks I was with (who are also hams) pointed out what I was doing.
  

• Since my CW Morse paddle has magnets in the base - and since the picnic table's top was aluminum - I stuck it to the bottom of a cast-iron skillet which solved the first problem, but I quickly discovered that the bottom of a well-used skillet is really quite smooth and lubricated with a fine layer of carbon.  What this meant was that not only did I have to use my other hand to keep the key from sliding around, I started looking like the carbon-covered operators of high-power Poulsen Arc transmitters of a century ago:  My arm and hand quickly got covered with a slight residue of soot!  I then made it a practice to at least wipe down the bottom of the pan before operating.
  

• During contacts, I would randomly lose the "Dah" contact.  I was presuming that this was from dust getting into the contacts (I'm sitting outside!) as it usually seemed to "fix" itself when I would lean over and blow into the paddle, but in once instance when this didn't work at all I wiggled/rotated the 3.5mm TRS jack on the back and it started working again.  I'm thinking that the issue was just a flaky contact on the jack.

At some point I'll need to figure out a better means of holding this paddle down to keep it from sliding about - perhaps a small sheet of steel with bumpers and rubber feet - or simply learn to use the paddle with a much lighter touch!

Figure 2:
Operating CW from the picnic table, the paddle on a skillet!
Click on the image for a larger version.

With a few dozen CW contact under my belt I readjusted the antenna and QSYed down to 20 meter SSB where I worked several pages of stations, my voice getting a bit hoarse before handing the microphone over to Tim, KK7EF who continued working the pileup under my callsign.

* * *

After a while, we had to shut down as we needed the picnic table to prepare dinner - but this wasn't the last bit of activation:  Over the next few days - when time was available - I would often venture out on 40, 30, 20 and 17 meter CW - occasionally braving 17 meter SSB:  I generally avoided 20 meter SSB as the band generally seemed to be a bit busy - particularly during the weekend when some sort of activity caused the non-WARC bands to be particularly full.

* * *

By the end of the trip I had logged about 387 total contacts - roughly 2/3 of them being CW.  When I got home I had to transcribe the paper logs onto the computer and learned something doing this:  If you do such a transcription, try to avoid doing so late at night when you are tired - and always wait until the next day - whether you were tired or not - and go back and re-check your entries BEFORE uploading the logs to LOTW, eQSL and/or the POTA web site!  Being tired, I hadn't thought the above through very well and later had to go back and make corrections and re-upload.

This page stolen from ka7oei.blogspot.com

[END]

Resurrecting my FE-5680A Rubidium frequency reference

Fig 1:
The Hammond 1590 aluminum case
housing the FE-5860A rubidium osc-
oscillator and other circuitry - the
markings faded by time and heat.
Click on the image for a larger version.

Recently I was getting ready for the October 14, 2023 eclipse, so I pulled out my two 10 MHz rubidium frequency references (doesn't everyone have at least one?) as I would need an accurate and (especially) stable frequency reference for transmitting:  The details of what, why and how will be discussed in a post to be added in the near future.

The first of these - my Efratom LP-101 - fired up just fine, despite having seen several years of inactivity.  After letting it warm up for a few hours I dialed it in against my HP Z3801 GPSDO and was able to get it to hold to better than 5E-11 without difficulty.

My other rubidium frequency reference - the FEI FE-5680A - was another matter:  At first, it seemed to power up just fine:  I was using my dual-trace oscilloscope, feeding the 'Z3801 into channel 1 and the '5680A into channel 2 and watching the waveforms "slide" past each other - and when they stop moving (or move very, very slow) then you know things are working properly:  See Figure 2, below, for an example of this.

That did happen for the '5680A - but only for a moment:  After a few 10s of seconds of the two waveforms being stationary with respect to each other, the waveform of the '5680A suddenly took off and the frequency started "searching" back and forth, reaching only as high as a few Hz below exactly 10 MHz and swinging well over 100 Hz below that.

My first thought was something along the lines of "Drat, the oven oscillator has drifted off frequency..."

Fig 2:
Oscillogram showing the GPS reference (red)
and the FE-5680A (yellow) 10 MHz signals
atop each other.  Timing how long it takes for the
two waveforms "slide" past each other (e.g. drift
one whole cycle) allows long-term frequency
measurement and comparison.
Click on the image for a larger version.

As it turns out, that was exactly what had happened.

Note: 

 I've written a bit more about the aforementioned rubidium frequency references, and you can read about them in the links below:

• A portable 10 MHz Rubidium Frequency Refrence using the Efratom LPRO-101  - LINK 

• A portable 10 MHz Rubidium Frequency Reference using the FE-5680A - LINK.
  

Oscillator out of range

While it is the "physics package" (the tube with the rubidium magic inside) that determines the ultimate frequency (6834683612 Hz, to be precise) it is not the physics package that generates this frequency, but rather another oscillator (or oscillators) that produce energy at that 6.834682612 GHz frequency, inject it into the cavity with the rubidium lamp and detect a slight change in intensity when it crosses the atomic resonance.

In this unit, there is a crystal oscillator that does this, using digital voodoo to produce that magic 6.834682612 GHz signal to divine the hyperfine transition.  This oscillator is "ovenized" - which is to say, the crystal and some of the critical components are under a piece of insulating foam, and attached to the crystal itself is a piece of ceramic semiconductor material - a PTC (positive temperature coefficient) thermistor - that acts as a heater:  When power is applied, it produces heat - but when it gets to a certain temperature the resistance increases, reducing the current consumption and the thermal input and the temperature eventually stabilizes.

Because we have the rubidium cell itself to determine our "exact" frequency, this oven and crystal oscillator need only be "somewhat" stable intrinsically:  It's enough simply to have it "not drift very much" with temperature as small amounts of frequency change can be compensated, so neither the crystal oven - or the crystal contained within - need to be "exact".

Fig 3:
The FE-5680A itself, in the lid of the
case of the 1590 box to provide heat-
sinking.  As you can see, I've had this
unit open before!
Click on the image for a larger version.

What is required is that this oscillator - which is "pullable" (that is, its precise frequency is tuned electronically) - must be capable of covering the exact frequency required in its tuning range:  If this can't happen, it cannot be "locked" to the comparison circuitry of the rubidium cell.

The give-away was that as the unit warmed up, it did lock, but only briefly:  After a brief moment, it suddenly unlocked as the crystal warmed up and drifted low in frequency, beyond the range of the electronic tuning.

Taking the unit apart I quickly spotted the crystal oscillator under the foam and powering it up again, I kept the foam in place and watched it lock - and then unlock again:  Lifting the foam, I touched the hot crystal with my finger to draw heat away and the unit briefly re-locked.  Monitoring with a test set, I adjusted the variable capacitor next to the crystal and quickly found the point of minimum capacitance (highest frequency) and after replacing the foam, the unit re-locked - and stayed in lock.

Bringing it up to frequency

This particular '5680A is probably about 25 years old - having been a pull from service (likely at a cell phone site) and eventually finding its way onto EvilBay as surplus electronics.  Since I've owned it, it's also seen other service - having been used twice in in ground stations used for geostationary satellite service as a stable frequency reference, adding another 3-4 years to its "on" time.

As quartz crystals age, they inevitably change frequency:  In general, they tend to drift upwards if they are overdriven and slowly shed material - but this practice is pretty rare these days, so they seem to tend to drift downwards in frequency with normal aging of the crystal and nano-scale changes in the lattice that continue after the quartz is grown and cut:  Operating at elevated temperature - as in an oven - tends to accelerate this effect.

By adjusting the trimmer capacitor and noting the instantaneous frequency (e.g. adjusting it mechanically before the slower electronic tuning could take effect) I could see that I was right at the ragged edge of being able to net the crystal oscillator's tuning range with the variable capacitor at its extreme low end, so I needed to raise the natural frequency a bit more.

If you need to lower a crystal's frequency, you have several options:

• Place an inductor in series with the crystal.  This will lower the crystal's in-circuit frequency of operation, but since doing so generally involves physically breaking an electrical connection to insert a component, this is can be rather awkward to do.

Fig 4:
The tip of the screwdriver pointing at the added 2.2uH
surface-mount inductor:  It's the black-ish component
at sort of a diagonal angle, wired across the two
crystal leads.
Click on the image for a larger version.

• Place a capacitor across the crystal.  Adding a few 10s of pF of extra capacitance can lower a crystal's frequency by several 10s or hundreds of ppm (parts-per million), depending on the nature of the crystal and the circuit.

Since the electrical "opposite" of a capacitor is an inductor, the above can be reversed if you need to raise the frequency of a crystal:

• Insert a capacitor in series with the crystal.  This is a very common way to adjust a crystal's frequency - and it may be how this oscillator was constructed.  As with the inductor, adding this component - where none existed - would involve breaking a connection to insert the device - not particularly convenient to do.
  

• Place an inductor across the crystal.  Typically the inductance required to have an effect will have an impedance of hundreds of ohms at the operating frequency, but this - like the addition of a capacitor across a crystal to lower the frequency - is easier to do since we don't have to cut any circuit board traces.
  

With either method of tweaking the resonance of the oscillator circuit, you can only go so far:  Adding reactance in series or parallel will eventually swamp the crystal itself, potentially making it unreliable in its oscillation - and if that doesn't happen, the "Q" is diminished, potentially reducing the quality of the signal produce and furthermore, taking this to an extreme can reduce the stability overall as it starts to become more temperature sensitive with the added capacitor/inductor than just the crystal, alone.

In theory, I could have placed a smaller fixed capacitor in series with the trimmer capacitor  - or used a lower-value capacitor - but I chose, instead, to install a fixed-value surface-mount inductor in parallel with the crystal as it would not require cutting any traces.  Prior to doing this I checked to see if there was any circuit voltage across the crystal, but there was none:  Had I seen voltage, adding an inductor would have shorted it out and likely caused the oscillator to stop working and I would have either reconsidered adding a series capacitor somewhere or, more likely I would have placed a large-value (1000pF or larger) capacitor in series with the inductor to block the DC.

"Swagging" it, I put a 2.2uH 0805 surface-mount inductor across the crystal and powered up the '5680A and after a 2-3 minute warm-up time, it locked.   After it had warmed up for about 8 minutes I briefly interrupted the power and while it worked to re-establish lock I saw the frequency swing nearly 100 Hz below and above the target indicating that it was now more less in the center if its electronic tuning range indicating success!  As can be seen from Figure 4, there is likely enough room to have used a small, molded through-hole inductor instead of a surface-mount device.

Fig 5:
The crystal is under the round disk (the PTC
heater) near the top of the picture and the
adjustment capacitor is to the right of the
crystal.
Click on the image for a larger version.

With a bit of power-cycling and observing the frequency swing while the oscillator was hot, I was able to observing the electronic tuning range and in so-doing, increase the capacitance of the trimmer capacitor very slightly from minimum indicating that I now had at least a little bit of extra adjustment room - but not a lot.  Since this worked the first time I didn't try a lower value of inductance (say, 1uH) to further-raise the oscillator frequency, leaving well-enough alone.

Buttoning everything back up and putting it back in its case, everything still worked (always gratifying!) and I let the unit "burn in" for a few hours.

Comparing it to my HP Z8530 GPS Disciplined oscillator via the oscilloscope (see Figure 2) it took about 20 minutes for the phase to "slide" one entire cycle (360 degrees) indicating that the two 10 MHz signal sources are within better than 10E-10 of each other - not too bad for a device that was last adjusted over a decade ago and as seen about 15000 operational hours since!

 

* * *

 

Follow-up:  A few weeks after this was originally posted I had this rubidium reference with me at the October, 2024 Eclipse event as a "hot standby", its frequency being compared to the LPRO-101 - which was the active, on-the-air unit - using an oscilloscope.

 

This (repaired) unit fired up and locked within 5 minutes at the cool (45F/7C) ambient temperature and remained stable for the several hours that it was powered up.

 

I used this unit again form the April, 2024 Eclipse - this time, from a location in central Texas and, again, it worked perfectly.

* * * * *

 

This page stolen from ka7oei.blogspot.com

 

[END]

 

Measuring signal dynamics of the RX-888 (Mk2)

As a sort of follow-up to the previous posting about the RX-888 (Mk2) I decided to make some measurements to help characterize the gain and attenuation settings.

The RX-888 (Mk2) has two mechanisms for adjusting gain and attenuation:

• The PE4312 attenuator.  This is (more or less) right at the HF antenna input and it can be adjusted to provide up to 31.5dB of attenuation in 0.5dB steps.
  

• The AD8370 PGA.  This PGA (Programmable Gain Amplifier) can be adjusted to provide a "gain" from -11dB to about 34dB.

Note:

While this blog posting has specific numbers related to the RX-888 (Mk2), its general principles apply to ALL receivers - particularly those operating as "Direct Sampling" HF receivers.  A few examples of other receivers in this category include the KiwiSDR and Red Pitaya - to name but two.

Other article RX-888 articles:

RX-888 Thermal issues:  I recently posted another article about the RX-888 (Mk2) discussing the thermal properties of its mechanical construction - and ways to improve it to maximize reliability and durability.  You can find that article here:  Improving the thermal management 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 clock an RX-888 from an external source, read THIS article.

* * * * *

Taking measurements

To ascertain the signal path properties of an RX-888 (Mk2) I set its sample rate to 64 Msps and using both the "HDSDR" and "SDR Radio" programs (under Windows - because it was convenient) and a a known-accurate signal generator (Schlumberger Si4031) I made measurements at 17 MHz which follow:

Gain setting (dB)	Noise floor (dBm/Hz)	Noise floor (dBm in 500Hz)	Apparent Clipping level (dBm)
-25	-106	-79	>+13dBm
+0	-140	-113	+3
+10	-151	-124	-8
+20	-155	-128	-18
+25	-157	-130	-23
+33	-158	-131	-31

Figure 1:  Measured performance of an RX-888 Mk2.  Gain mode is "high" with 0dB attenuation selected.

For convenience, the noise floor is shown both in "dBm/Hz" and in dBm in a 500 Hz bandwidth - which matches the scaling used in the chart below.  As the programs that I used have no direct indication of A/D converter clipping, I determined the "apparent" clipping level by noting the amplitude at which one additional dB of input power caused the sudden appearance of spurious signals.  Spot-checking indicated that the measured values at 17 and 30 MHz were within 1 dB of each other on the unit being tested.

Determining the right amount of "gain"

It should be stated at the outset that most of the available range of gain and attenuation provided by the RX-888's PE4312 step attenuator and AD8370 variable gain amplifier are completely useless to us.  To illustrate this point, let's consider a few examples.

Consider the chart below:

Figure 2:  ITU chart showing various noise environments versus frequency.

This chart - from the ITU - shows predicted noise floor levels - in a 500 Hz bandwidth - that may be expected at different frequencies in different locations.  Anecdotally, it is likely that in these days of proliferating switch-mode power supplies that we really need another line drawn above the top "Residential" curve, but let's be a bit optimistic and presume that it still holds true these days.

Let us consider the first entry in Figure 1 showing the gain setting of 0dB.  If we look at the "Residential" chart, above, we see that the curve at 30 MHz indicates a value very close to the -113dBm value in the "dBm in 500 Hz" column.  This tells us several things:

• Marginal sensitivity.  Because the noise floor of the RX-888 (Mk2) and that of our hypothetical RF environment are very close to each other, we may not be able to "hear" our noise floor at 30 MHz (e.g. the 10 meter amateur band).  One would need to do an "antenna versus no antenna" check of the S-meter/receiver to determine if the former causes an increase in signal level:  If not, additional gain may be needed to be able to hear signals that are at the noise floor.
  

• More gain may not help.  If we do perform the "antenna versus no antenna" test and see that with the antenna connected we get, say, an extra S-unit (6dB) of noise, we can conclude that under those conditions that more gain will not help in absolute system sensitivity.
  

Thinking about the above two statements a bit more, we can infer several important points about operating this or any receiver in a given receive environment:

• If we can already "hear" the noise floor, more gain won't help.  In this situation, adding more gain would be akin to listening to a weak and noisy signal and expecting that increasing the volume would cause the signal to get louder - but not the noise.  
  

• More gain than necessary will reduce the ability of the receiver to handle strong signals.  The HF environment is prone to wild fluctuations and signals can go between well below the local noise floor and very strong, so having any more gain that you need to hear your local noise floor is simply wasteful of the receiver's signal handling capability.  This fact is arguably more important with wide-band, direct-sampling receivers where the entire HF spectrum impinges on the analog-to-digital converter rather than a narrow section of a specific amateur band as is the case in "conventional" analog receivers.

Let us now consider what might happen if we were to place the same receiver in an ideal, quiet location - in this case, let's look at the "quiet rural" (bottom line) on the chart in Figure 2.

Again looking at the value at 30 MHz, we see that our line is now at about -133dBm (in 500 Hz) - but if we have our RX-888 gain set at 0 dB, we are now ((-133) - (-113) = ) 20 dB below the noise floor.  What this means is that a weak signal - just at the noise floor - is more than 3 S-units below the receiver sensitivity.  This also means that a receiver that may have been considered to be "Okay" in a noisy, urban environment will be quite "deaf" if it is relocated to a quiet one.

In this case we might think that we would simply increase our gain from 0 dB to +33dB - but you'll notice that even at that setting, the sensitivity will be only -131dBm in 500 Hz - still a few dB short of being able to hear the noise in our "antenna versus no antenna" test.

Too much gain is worse than too little!

At this point I refer to the far-right column in Figure 1 that shows the clipping level:  With a gain setting of +33dBm, we see that the RX-888 (Mk2) will overload at a signal level of around -31dBm - which translates to a  signal with a strength a bit higher than "S9 + 40dB".  While this sound like a strong signal, remember that this signal level is the cumulative TOTAL of ALL signals that enter the antenna port.  Thinking of it another way, this is the same as ten "S9+30dB" signals or one hundred "S9+20dB" signals - and when the bands are "open," there will be many times when this "-31dBm" signal level is exceeded from strong shortwave broadcast signals and lightning static.

In the case of too-little gain, only the weakest signals, below the receiver's noise floor will be affected - but if the A/D converter in the receiver is overloaded, ALL signals - weak or strong - are potentially disrupted as the converter no longer provides a faithful representation of the applied signal.  When the overload source is one or more strong transmissions, a melange of all signals present is smeared throughout the receive spectrum consisting of many mixing products, but if the overload is a static crash, the entire receive spectrum can be blanked out in a burst of noise - even at frequencies well removed from the original source of static.

Most of the adjustment range is useless!

Looking carefully at Figure 1 at the "noise floor" columns, you may notice something else:  Going from a gain of 0 dB to 10 dB, the noise floor "improves" (is lower) by about the same amount - but if you go from 25 dB gain to 33 dB gain we see that our noise floor improves by only 1 dB - but our overload threshold changes by the same eight dB as our gain increase.

What we can determine from this is that for practical purposes, any gain setting above 20 dB will result in a very little receiver sensitivity improvement while causing a dramatic reducing in the ability of the receiver to handle strong signals.

Based on our earlier analysis in a noise "Urban" environment, we can also determine that a gain setting lower than 0 dB will also make our receiver too-insensitive to hear the weakest signals:  The gain setting of -25dB shown in Figure 1 with a receive noise floor of -79dBm (500 Hz) - which is about S8 - is an extreme example of this.

Up to this point we have not paid any attention to the PE4312 attenuator as all measurements were taken with this set to minimum.  The reason for this is quite simple:  The noise figure (which translates to the absolute sensitivity of a receiver system) is determined by the noise generation of all of the components.  As reason dictates, if you have some gain in the signal path, the noise contribution of the devices after the gain have lesser effects - but any loss or noise contribution prior to the gain will directly increase the noise figure.

Note:

For examples of typical HF noise figure values, see the following articles:

• Measurements on a Multiband R2P Pro Low-Noise Amplifier System - link to the Web Archive.  Refer to page 9 section 7.
• Antenna and Receiver Noise Figure - link.
  

Based on the articles referenced above, having a receiver system with a noise figure of around 15dB is the maximum that will likely permit reception at the noise floor of a quiet 10 meter location.  If you aren't familiar with the effects of noise figure - and loss - in a receive signal path, it's worth playing with a tool like the Pasternack Enterprises Cascaded Noise Figure Calculator (link) to get a "feel" of the effects.

I do not have the ability to measure the precise noise figure of the RX-888 (Mk2) - and if I did do so, I would have to make such a measurement using the same variety of configurations depicted in Figure 1 - but we can know some parameters about the worst-case:

• Bias-Tee:  Estimated insertion loss of 1dB
  
• PE4312:  Insertion loss of 1.5dB at minimum attenuation
• RF Switch (HF/VHF):  1dB loss
  
• 50-200 Ohm transformer:  1dB loss
  
• AD8370 Noise figure:  8dB (at gain of 20dB)

The above sets the minimum HF floor noise figure of the RX-888 (Mk2) at about 12.5dB with an AD8370 gain setting of 20dB - but this does not include the noise figure of the A/D converter itself - which would be difficult to measure using conventional means.

On important aspect about system noise figure is that once you have loss in a system, you cannot recover sensitivity - no matter how much gain or how quiet your amplifier may be!  For example, if you have a "perfect" 20 dB gain amplifier with zero noise, if you place a 10 dB attenuator in front of it, you have just turned it into an amplifier with 10 dB noise figure with 10dB gain and there is nothing that can be done to improve it - other than get rid of the loss in front of the amplifier.

Similarly, if we take the same "perfect" amplifier - with 20dB of gain - and then cascade it with a receiver with a 20dB noise figure, the calculator linked above tells us that we now have a system noise figure of 3 dB since even with 20dB preceeding it, our receiver still contributes noise!

If we presume that the LTC2208 A/D converter in the RX-888 has a noise figure of 40dB and no gain (a "ballpark" value assuming an LSB of 10 microvolts - a value that probably doesn't reflect reality) our receive system will therefore have a noise figure of about 22dB.

What this means is that in most of the ways that matter, the PE4312 attenuator is not really very useful when the RX-888 (Mk2) is being used for reception of signal across the HF spectrum, in a relatively quiet location on an antenna system with no additional gain.

Where is the attenuator useful?

From the above, you might be asking under what conditions would the built-in PE4312 attenuator actually be useful?  There are two instances where this may be the case - and this would be applied ONLY if you have been unable to resolve overload situations by setting the gain of the AD8370 lower.

• In a receive signal path with a LOT of amplification.  If your receive signal path has - say - 30dB of amplification (and if it does, you might ask yourself "why?") a moderate amount of attenuation might be helpful.
  

• In a situation where there are some extremely strong signals present.  If you are near a shortwave or mediumwave (AM broadcast) transmitter that induces extremely strong signals in the receiver that cause intractable overload, the temporary use of attenuation may prevent the receiver from becoming overloaded to the point of being useless - but such attenuation will likely cause the complete loss of weaker signals.  In such a situation, the use of directional antennas and/or frequency-specific filtering should be strongly considered!
  

Improving sensitivity

Returning to an earlier example - our "Quiet Rural" receive site - we observed that even with the gain setting of the RX-888 (Mk2) at maximum, we would still not be able to hear our local noise floor at 30 MHz - so what can be done about this?

Let us build on what we have already determined:

• While sensitivities is slightly improved with higher gain values, setting the gain above 20dB offers little benefit while increasing the likelihood of overload.
  

• In a "Quiet Rural" situation, our 30 MHz noise floor is about -133dBm (500 Hz BW) which means that our receiver needs to attain a lower noise floor than this:  Let's presume that -136dBm (a value that is likely marginal) is a reasonable compromise.

With a "gain" setting of 20dB we know that our noise floor will be around -128dBm (500 Hz) and we need to improve this by about 8 dB.  For straw-man purposes, let's presume that the RX-888 (Mk2) at a gain setting of 20dB has a noise figure of 25dB, so let's see what it takes for an amplifier that precedes the RX-888 (Mk2) to lower than to 17dB or so using the Pasternak calculator above:

• 10dB LNA with 7 dB noise figure:  This would result in a system noise figure of about 16 dB - which should do the trick.

Again, the above presumes that there is NO  loss (cable, splitters, filtering) preceding the preamplifier.  Again, the presumed noise figure of 25dB for the RX-888 (Mk2) at a gain setting of 20 is a bit of a "SWAG"  - but it illustrates the issue.

Adding a low-noise external amplifier also has another side-effect:  By itself, with a gain setting of +33, the RX-888 (Mk2)'s overload point is -31dBm, but if we reduce the gain of the RX-888 to 20dB the overload drops to -18dBm - but adding the external 10dB gain amplifier will effectively reduce the overload to -28dBm, but this is still 5 dB better than if we had turned the RX-888's gain all of the way up!

Taking this a bit further, let's presume that we use, instead, an amplifier with 3dB noise figure and 8 dB gain:  Our system noise figure is now about 17dB, but our overload point is now -26dBm - even better!

The RX-888 is connected to a (noisy) computer!

Adding appropriate amounts of external gain has an additional effect:  The RX-888 (and all other SDRs) are computer/network connected devices with the potential of ingress of stray signals from connected devices (computers, network switches, power supplies, etc.).  The use of external amplifiers can help override (and submerge) such signals and if proper care is taken to choose the amount of gain of the external amplification and properly choose gain/attenuation settings within the receiver, superior performance in terms of sensitivity and signal-handling capability can be the result.

Additional filtering

Only mentioned in passing, running a wideband, direct-sampling receiver of ANY type (be it RX-888, KiwiSDR, Red Pitaya, etc.) connected to an antenna is asking a lot of even 16 bits of conversion!  If you happen to be in a rather noisy, urban location, the situation is a bit better in the sense that you can reduce receiver gain and still hear "everything there is to hear" - but if you have a very quiet location that requires extra gain, the same, strong signals that you were hearing in the noisy environment are just as strong in the quiet environment.

Here are a few suggestions for maximizing performance under the widest variety of situations:

• Add filtering for ranges that you do not plan to cover.  In most cases, AM band (mediumwave) coverage is not needed and may be filtered out.  Similarly, it is prudent to remove signals above that in which you are interested.  For the RX-888 (Mk2), if you run its sampling rate at just 65 MHz or so, you should install a 30 MHz low-pass filter to keep VHF and FM broadcast signals out.
  

• Add "window" filtering for bands of interest.  If you are interested only in amateur radio bands, there are a lot of very strong signals outside the bands of interest that will contribute to overload of the A/D converter.  It is possible to construct a set of filters that will pass only the bands of interest - but this does not (yet?) seem to be a commercial product.  (Such a product may be available in the near future - keep a lookout here for updates.)
  

• Add a "shelving" filter.  If you examine the graph in Figure 2 you will notice that as you go lower in frequency, the noise floor goes UP.  What this means is that at lower frequencies, you need less receiver sensitivity to hear the signals that are present - and it also means that if you increasingly attenuate those lower frequencies, you can remove a significant amount of RF energy from your receiver without actually reducing the absolute sensitivity.  A device that does just this is described in a previous blog article "Revisiting the limited-attenuation high-pass filter - again (link)".  While I do not offer such a filter personally, such a device - along with an integrated 30 MHz low-pass filter - may be found at Turn Island Systems - HERE.
  

Conclusions:

• The best HF weak-signal performance for the RX-888 (Mk2) will occur with the receiver configured for "High" gain mode, 0 dB attenuation and a gain setting of about 20dB.  Having said this, you should always to the "antenna versus no antenna" test:  If you see more than 6-10dB increase in the noise level at the quietest frequency, you probably have too much gain.  Conversely, if you don't see/hear a difference, you probably need more gain - taking care in doing so.
  

• For best HF performance of this - or any other wideband, direct-sampling HF SDR (RX-888, KiwiSDR, Red Pitaya, etc.) additional filtering is suggested - particularly the "shelving" filter described above.

• In situations where the noise floor is very low (e.g. a nice, receive quiet location) many direct-sampling SDRs (RX-888, KiwiSDR, Red Pitaya) will likely need additional gain to "hear" the weaker signals - particularly on the higher HF bands.  While some of these receivers offer onboard gain adjustment, the use of external high-performance (low-noise) amplification (along with filtering and careful adjustment of the devices' gain adjustments) will give improved absolute sensitivity while helping to preserve large-signal handling capability.
  

• Because the RX-888 is a computer-connected device, there will be ingress of undesired signals from the computer and the '888's built-in circuitry.  The use of external amplification - along with appropriate decoupling (e.g. common-mode chokes on the USB cable and connecting coaxial cables) can minimize the appearance of these signals.
  

 

This page was stolen from ka7oei.blogspot.com.

[End]

 

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

 

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