Frequency response of the RX-888 SDR at the high and low ends (above 30 MHz and below 1.5 MHz)

Figure 1:
The RX-888 Mk2.

The RX-888 Mk 2 (hereafter referred to as the  RX-888 or '888) is a versatile device, essentially providing a means by which "all of HF" (0-30 MHz - or even 0-60 MHz) may be sampled and presented to a computer for processing via a multi-gigabit USB3 interface.  As it has no onboard signal processing, this device is practically "future proof" in that as all computations are performed on the host computer and there are no frequency or bandwidth limitations regarding the sort of signals - or how many - may be processed, presuming adequate processing capacity.

Comment:

I've seen at least three different "sub-versions" of the RX-888 Mk2 - each one looking slightly different (different circuit board color and other minor differences) - since this device was released.  It's also very likely that components also vary a bit with different manufacturers so the actual frequency response of units of different "builds" may also change.  Unfortunately, I only "have what I have" and haven't been able to compare differences - if any.

The highs and the lows

Like any receiver, it has limits of its frequency response - both at the upper end where the high-pass filter dominates and at the bottom end where the component selection as well as the design itself will limit low-frequency response.

Let's look at the low end first.

The lows

The low end limit to the frequency response (somewhere below 1 MHz) of the '888 has not previously been well defined.  This low frequency response is set by component limitations within the HF signal path, including:

• Coupling capacitors.  DC blocking capacitors in series with the signal path will act as high-pass filters, rolling off the low frequencies.
• The Bias-Tee inductor.  The RX-888 has the ability to supply power via the antenna port to an amplifier.  This inductor has a finite inductance and it, too, will force a high-pass response as well.  This inductor's value was measured as being 10uH (nominal) which presents a reactance of 50 ohms at about 800 kHz.  This is the major contributor to low-frequency roll-off as discussed below.
  
• The coupling transformer.  The RX-888 has a transformer that couples the input of the variable gain amplifier (VGA) from the attenuator.  As with any transformer, this, too, has defined low-frequency response.  This transformer was measured and found to have an inductance of 125uH of its primary (a reactance of 50 ohms at 64 kHz) with the secondary (the side facing the VGA) being about 760 uH.
  

This low-frequency roll-of is not uncommon in broadband receivers:  Most amateur transceivers suffer severe performance degradation at LF and VLF frequencies for the simple reason that the designers presume (correctly!) that very few of the users of that gear would ever be interested in that range - and making this assumption simplifies the design somewhat and reduces cost. 

Using a signal generator with a constant output, the response of the RX-888 (Mk2) was measured, using the signal strength at 1500 kHz as a reference:

Frequency (kHz)	Attenuation (db) Unmodified unit	Attenuation (db) Bias-Tee inductor removed
1500 (Reference)	0	0
1250	0.3	0
1000	0.4	-0.1
750	1.0	-0.2
500	2.6	-0.3
475 (630 meters)	2.9	-0.3
400	4.5	-0.2
300	7.4	-0.2
250	10.9	-0.2
200	19.9	-0.2
150	17.9	0
137 (2200 meters)	14.7	0
100 (Loran C)	9.7	0.3
75  (DCF77 approx.)	7.5	0.7
60  (WWVB, JJY)	6.8	1.1
50	6.7	1.9
40	7.4	5.3
30 (Submarine comms)	11.3	7.4
25 (Submarine comms)	12.8	10.5
20 (Submarine comms)	17.5	15.0
15	22.5	22.7
10	30.7	32.8
7.5	36.4	40.0
5	45.5	49.5
2.5	61	60
1	80	87

Table 1:  Attenuation measurements at 1.5 MHz and below using both an unmodified RX-888 and the same one after the bias-Tee inductor was removed.

Comments about the frequency response of the unmodified unit:

As can be seen from the table above, the "stock" RX-888 is flat within about 2 dB or so across the AM broadcast band (520-1700 kHz) but it falls off precipitously between 100 and 300 kHz with a bit of "rebound" in the 40-150 kHz area, likely due a very low "Q" resonance of inductance and capacitance of the aforementioned components (inductors, transformers) in the signal path.

In the "VLF" range (30 kHz and below) the unmodified receiver may be somewhat usable when using an active antenna to overcome losses, but at 20 kHz and below the response drops off like a rock and, as the chart shows, it's pretty much unusable below 5-10 kHz.

The factors above conspire to prevent a flat frequency response at lower frequencies - say, those below 1.5 MHz.  For the table below, my reference amplitude and frequency is 1.5 MHz as it seemed to be more or less representative of the amplitude response above this, in the HF range - and it seemed to be comfortably above that at which the aforementioned high-pass effects of the components were having a significant effect.

Figure 2:
The red arrow points to the location of the
10uH bias-Tee inductor.  As seen in
Table 1, its removal can significantly improve
MF and LF performance.
Click on the image for a larger version.

Can anything be done to improve LF/VLF response?

YES, it is possible to modify the RX-888 to improve the low and frequency response by removing the Bias-Tee inductor from the HF port and as can be seen from the above data there is a dramatic difference in usable sensitivity at frequencies below 1 MHz - particularly below 400 kHz.

This is the easiest modification as it entails the removal of a single component and Figure 2 shows the location of this inductor.  It may most easily be removed with a hot-air rework tool, but it should be possible to carefully use a solder-wetted iron to heat it and remove with a pair of tweezers (temporarily remove any thermal pad below that portion of the board if it's present) or a very sharp pair of diagonal flush-cut pliers to remove it (perhaps destructively) as well.

There are other ways by which the low frequency response may be improved, including:

• Replacing the coupling transformer.  The transformer used in the RX-888 is likely specified for a low-end frequency response of 1 MHz or so, so it's not surprising that this may be the worst offender (once the bias-tee inductor has been removed) in low-frequency roll-off.  Replacing it with a different unit with larger inductance (a commercial or hand-made unit) would certainly help.  It may also be possible to simply replace the transformer with coupling capacitors (say, 0.1uF) - but this would be at the expense of sensitivity and performance across the entire frequency range, something that might be acceptable if one's primary interest was in the MF/LF/VLF spectrum.  As the inductance of the transformer's primary is known to be about 125uH, we can see that this is likely the main cause of attenuation below 60 kHz.
• Increasing value of coupling capacitors.  The coupling capacitors in series with the signal path are likely not ideal for coupling VLF frequencies.  A value such as 0.1 uF or larger would be suggested.

For VLF use (30 kHz and below) if you have interest in this frequency range you may be better off not trying to use the RX-888 - at least directly.  Some possibilities include:

• Use a VLF up-converter.  Converting the frequencies 0-30 kHz to a higher frequency range will put this spectrum within the useful range of the RX-888 and practically any other modern receiver.  There have been a number of VLF up-converter units for sale in the past, but I don't have a specific recommendation.  If this up-converter is clocked from the same source as the RX-888's clock (e.g. using its onboard 27 MHz oscillator, or both from a common, external clock) then frequency drift could be minimized.
  
• Use a sound card.  A modest computer sound card with a 192 kHz sample rate and a 16 to 24 bit A/D converter is perfectly capable of ingesting frequencies up through at least 80 kHz and down (nearly) to DC.

Having a receiver capable of VLF (3-30 kHz) or ELF (300-3000 Hz) is one thing, but having an antenna system capable of this is a different matter altogether.  There are many available E-field active whips that will work well down into the 10-20 kHz region, but below that frequency you are into the realm of specialized gear - and listening at "audio" radio frequencies in all but the most rural areas devoid of power lines and other forms of civilization can be fraught with frustration and disappointment due to the likely pick-up of mains-related energy and its harmonics.

Here are a few links related to equipment for LF/VLF reception.  Note that I have not necessarily built, bought or used the equipment described below, so your mileage may vary.

• The BBB-4 Natural Radio Receiver.  This is a simple circuit is intended for "Audible" RF frequency ranges of a few hundred Hz to a few 10s of kHz and is suitable for detecting "spherics" like whistlers and chirps.
• Some thoughts on E-Field Whistler Receiver Design.  This article describes design aspects of ELF and VLF receivers
  
• An up-converter for receiving long and very long waves.  This article describes an upconverter that may be built.
  
• AMRAD low-frequency up-converter.  This is another upconverter design for VLF/LF use.  The low-end frequency response is limited by T1 in this design.
  
• Heros VLF-LF-MF up-converter.  This is a commercial LF/VLF upconverter. 
• The AMRAD Active LF Antenna.  This article - from the September, 2001 QST - describes a high-performance antenna capable of reception down to at least 10 kHz.  In lieu of the CP666 transistor, one may also use the Russian KП903 (a.k.a. "KP903") power JFET which may be found on EvilBay and other sites.
  

The effective reception of signals in the LF, VLF and ELF frequency range is highly contingent on having a "quiet" receive site, largely free of local noise sources and also on scrupulous attention to detail when it comes to decoupling the feedline (going to the "noisy" chassis of the receiver) from the antenna to prevent unwanted signals from being conveyed - but that's a topic of its own!

See the article A (semi)-typical suburban E-field whip receive system for the 630 and 2200 meter amateur bands - link. for a few details on how this might be done.

Real-world observations

At the Northern Utah WebSDR - where there are, at the time of writing, full-time WSPR receivers - it so-happens that there are currently some KiwiSDR and RX-888 based receivers sharing the exact, same signal path.  The KiwiSDR - which is capacitively coupled (e.g. you can hear "tinny" audio from the receiver tuned to 0 Hz and you apply the source to the antenna connector) has quite good response well into the VLF range.

Compared to the RX-888, the KiwiSDR performs noticeably better on the 2200 meter amateur band (137 kHz) in decoding WSPR and FST4W signals in which the '888 is about 15dB down.  As the '888 based system can't hear the 2200 meter signals as well, this indicates that signal levels feeding the '888 are a bit too low for it to "hear" the noise floor of the antenna system - but it also indicates that, perhaps, a few dB of boost in the signal path may remedy this:  This RX-888 has NOT had its bias-Tee inductor removed - but that's on the "to do" list:  After the bias-Tee inductor is removed I expect that it will perform comparably to the KiwiSDR at 2200 meters and I'll update this web page after having done so.

As the "LF/VLF" antenna system at the Northern Utah WebSDR is separate from that of the HF signal path - being combined in a special filter/amplifier module - boosting only the LF/VLF path would be the most beneficial as it wouldn't compromise HF reception by potentially overloading the A/D converter as would boosting everything.

The Highs

The RX-888's specifications state that it contains a "60 MHz" low-pass filter - but the precise nature of its response is not noted.

Comment about sample rates and aliasing - and the need for additional low-pass filtering

The use of the 60 MHz low-pass filter implies that the designers intended an A/D converter sample rate of more than twice that frequency - and since the RX-888 will happily sample at more than 130 MHz, this fits the need.  Many users do not operate their RX-888 at 130 MHz, however, as their interest does not extend beyond HF and operate it, instead, at around 65 MHz to reduce CPU and power loading.

A bit of warning here:  With a 65 MHz sample rate, the '888 will happily respond to signals above the Nyquist frequency (half of the sample rate, or 32.5 MHz) and these signals - spectrally "inverted" - will naturally appear at lower and lower frequencies as the original source signal's frequency increases.  Since the '888s low-pass filter is set at around 60 MHz, it will do nothing to prevent this:  The far right column of Table 2, below, shows the aliases of the test frequencies.

What this means is that users of the RX-888 using it at a sample rate lower than 130 MHz should be using an outboard low-pass filter.  With a sample rate of 65 MHz, a good-quality 30 MHz Low-Pass filter is strongly recommended and will suppress aliased signals that would otherwise appear above Nyquist.  Such filters may be found online via the usual retailers, but do not overlook an old 30 MHz transmit-type low-pass filter of the sort used to prevent interference to analog TV by an HF transmitter - often found at amateur radio swap meets or on EvilBay for cheap.

 The amplitude response, relative to 30 MHz, is shown below:

Frequency (MHz)	Attenuation (dB)	Alias frequency (MHz) @130 MHz sample rate	Alias frequency (MHz) @65 MHz sample rate
30 (Reference)	0	-	-
40	0.8	-	25
50	3.8	-	15
54	5.5	-	11
60	8.5	-	5
64	10.5	-	1
70	14.1	60	5  (double alias)
75	17.7	55	10  (double alias)
80	21.7	50	15  (double alias)
85	27.1	45	20  (double alias)
90	32.5	40	25  (double alias)
95	37.8	35	30  (double alias)
100	42.8	30	30  (triple alias)
105	48.0	25	25  (triple alias)
110	52.9	20	20  (triple alias)

 Table 2:  Sensitivity response of the RX-888 relative to 30 MHz

Table 2 shows the amplitude response of the RX-888 (Mk2) relative to 30 MHz.  The third and fourth column show the resulting aliased frequencies at sample rates of 130 and 65 MHz, respectively.

"Could I intentionally use aliases to receive higher frequencies than my sample rate would allow?"   After reading this, you might ask yourself "If I operate at a sample rate of 65 MHz, could I intentionally do this to receive spectrally-inverted 6 meter signals between 15 and 11 MHz?"   The answer is yes, you could - and as the chart above shows, they would be only 3.8-5.5dB down from the "real" signals across that same 15-11 MHz range.  Intentionally allowing aliases to occur is often done to allow the detection of signals well above the sample rate.  The caveat here is that one would want to sharply filter the source of the "above Nyquist" frequencies to limit them to the band of interest as well as prevent noise on the aliased frequency (15-11 MHz in this example) by filtering those frequencies as well.   Doing this works just fine as long as proper filtering is done to keep out the "unwanted" signals (at the higher and lower frequencies) along with appropriate amplification make up for losses.   In the example above, the lower part of 6 meters would appear just above the 20 meter band - but if one adjust the sample rate, the alias could be moved farther away from 20 meters and, with proper filtering, one could receive both 6 and 20 meters on the same receiver hardware.

What the above table above also shows is that the 60 MHz low-pass filter isn't very good:  By the time you get to the bottom of the FM broadcast band (88 MHz) we know that the attenuation is only around 32 dB.  Here in North America it's common for an FM broadcast station to have many 10s of kilowatts of ERP which means that if you live anywhere near such a station - even if you are using an antenna that wasn't designed to receive FM broadcast frequencies - you may experience some interference around the alias frequencies noted in Table 2.

No matter the sample rate at which you operate your RX-888, it's recommended that you carefully check for aliased responses of FM transmitters.  If you find them - and even if you don't - I'd recommend a separate FM broadcast band blocking filter be installed to quash ingress from strong signals:  Without it you'll probably get some leakage of moderate-to-strong signals in the 22-42 MHz range (frequency-inverted) if you are running at a 130 MHz sample rate or in the 23-32 MHz range if you are running at a sample rate of 65 MHz.

Figure 2 also demonstrates why - if you operate the '888 with a sample rate of 65 MHz - you should really be using a good 30 MHz low-pass filter with it:  Any signals above 30 MHz - including noise - will be attenuated only to the extent shown in the table and will interfere with the desired 0-30 MHz signals.

* * * * *

Other RX-888 related posts at this site:

• Measuring signal dynamics of the RX-888 -  This page discusses the gain distribution of the RX-888, its apparent sensitivity and steps that one should take to maximize performance when used for simultaneous "all of HF" reception.
• Improving the thermal management of the RX-888 (Mk2) - The internal power dissipation of the RX-888 exceeds its ability to get rid of the heat that it produces, reducing reliability - particularly in environments with elevated temperature.  This page discusses what to do to remedy this.
• Using and external clock with the RX-888 (Mk2) - Although the RX-888's TCXO is pretty good, you may wish to use an external reference to provide very high frequency accuracy and stability - and this page gives advice and warnings about doing so.
• Repairing a dead RX-888 (no A/D converter clocking) - While external clocking of the RX-888 (Mk 2) is desirable, it must be done with a bit of care to protect the circuitry involved.  If you do manage to damage your '888, this page may be helpful in its repair.
  

 

This page stolen from ka7oei.blogspot.com

[END]

The "Universal TCXO" - better stability for the TS-590 (and other radios) using the QRP Labs ProgRock 2

Figure 1:
The TS-590G into which the ProgRock was installed.

A useful accessory for many amateur transceivers is a TCXO - a device, often offered as an option, that improves the absolute frequency stability and accuracy of the radio.  When in current production, the TCXO is available from the manufacturer - and possibly from third parties - but long after the radio has been made, a TCXO may be difficult to find.

One option for addressing this issue is the use of the QRP Labs ProgRock 2 - LINK.  This unit is pretty inexpensive (US$18 at the time of writing) and has a stability of 0.5ppm - which is likely better than even the original TCXO offered by the manufacturer.

Any weird frequency

While it would be convenient if radios had a nice, easy frequency like 10 MHz as their main oscillator, that is rarely the case - and this was true for a friend's TS-590G which wanted 15.6 MHz.  This radio, which he purchased second-hand, did not come with a TCXO and based on his experience during June Field Day and winter Field Day (in January) it drifted excessively - a few 10s of Hz on 10 meters - enough that he would occasionally get complaints about him being "off frequency" - even if it was he that was calling CQ!

Although an aftermarket unit was available, he was intrigued by the idea of using the ProgRock 2 as this same device could be programmed for any frequency between about 3.5 kHz and 200 MHz with a resolution of 1 Hz.  Additionally, the ProgRock 2 allows the use of a 1 PPS (1 pulse-per-second) output from a GPS module to "discipline" the oscillator with even greater stability - but more on this later.

Prepping the ProgRock 2

Using the ProgRock 2 is pretty easy:  It has a micro-USB connector onboard and when plugged into a computer, it can appear as a serial port - refer to the manual for the appropriate driver.  Using a serial terminal program - like PUTTY - one simply enters the frequency, to the nearest 1 Hz, hit the "S" key to save it to memory and you are pretty much done.  The ProgRock will allow the output of more than one frequency if needed (the manual has more detail) but we will be using output #1, which is also the one into which we'd program the needed frequency, setting the others to zero (e.g. "off").

Figure 2:
ProgRock 2 with the 3.9 and 10k resistors mounted to allow
the external application of a 1pps signal from a GPS module
to stabilize the frequency further.  The bottom side of the
ProgRock 2 is shown.
Click on the image for a larger version.

Having said that, there's a bit more to it in that it needs power, ground, and the signal output needs to get into the radio - but more on that in a moment.  

As my friend wished to experiment with using a 1 PPS source to nail it down to frequency, a 3.9k series resistor was added to the "1pps" pin along with a 10k resistor to ground to keep the pin from "floating" around in voltage when nothing was connected to it.  Figure 2 shows these resistors mounted on the "bottom" side of the board:  The upper resistor is the 3.9k connected to the 1pps pad with the lower, 10k resistor connected to a ground pad.  The junction of the two (with the yellow piece of insulating tubing) is where the 1pps input would be connected.

The use of the 3.9k resistor is described in the ProgRock 2's documentation which notes that the onboard microcontroller operates from 3.3 volts - but placing this resistor in series (the value of which isn't particularly critical) limits the current into the logic pin, allowing it to be safely driven by a 5 volt - or even 12 volt - 1pps pulse. 

Figure 3:
The Progrock 2 mounted to the original TS-590 TCXO board
using short, insulated jumper wires.  The top side of the
ProgRock 2 is shown.
Click on the image for a larger version.

As noted in the ProgRock 2's documentation, as long as the 1pps pin is held low, it's ignored and the unit will operate based on the frequency set by its onboard oscillator, but when it sees the 1pps pulses, it measures the time between their rising edges to determine how far off the internal clock is from ideal, making slow, incremental changes.  If the 1pps signal were to later disappear, it would simply "hold" that frequency until the ProgRock 2 was power-cycled at which point it would revert to the internal clock unless/until it was again presented with a 1pps signal.

There's a place for it!

While the "stock" TS-590 did not come with a TCXO, there was a small "daughter" board adjacent to the portion of the circuit board with the stock oscillator on which the user is expected to solder a TCXO in the form of a "crystal can" oscillator module - or, in the case of some after-market units - replace that board entirely.  As the ProgRock 2 is roughly the size of a postage stamp (it will fit within an HC-6 crystal can!) it could be wedged on this same board - which is convenient as this board also carries 5 volt power for the original TCXO, so a bit of pretty easy "micro" surgery was undertaken.

Figure 4:
A hand-drawn diagram showing the connections
on the top side of the TS-590's TCXO board and
the ProgRock 2 board.
Click on the image for a larger version.

Figure 3 shows how the ProgRock 2 board was mounted on the original TCXO board.  Fortunately, all of the needed connections are there:  +5 volts to run the original TCXO, ground, and the signal output.  Figure 4 shows a hand-drawn diagram showing the original TCXO board (top) with its pin locations while a representation of the ProgRock board (with the USB connector oriented on top) is in the lower drawing along with its connections.

Using small gauge, insulated wire liberated from a scrap of CAT5 Ethernet cable, short-as-possible jumpers were run between the TCXO board and the ProgRock.  In Figure 3, the "ground" connections were made using green wire - one of them utilizing the body of the USB connector - while the output signal used blue and the power used orange:  In the upper-right corner of the ProgRock 2 board - just above the USB connector - you can just see the yellow insulating tubing of the 1pps connection.

There is JUST enough room - if one scrunches the edge of the ProgRock 2 board against the TCXO board's white connector (and by routing wires such that they are not between the ProgRock 2 board and the connector) so that it will fit in the original location within the TS-590 as can be seen in Figure 5, below.

Comment:

It was noted - during testing of the TS-590 - that  the combination of 10 meters at 100 watts while using the built-in tuner - seemed to "glitch" the ProgRock for reasons unknown, although it's suspected that magnetic fields from the PA/Tuner board are finding their way through the aluminum chassis from the opposite side.  Simply tipping the ProgRock 2 board from being flat against the original TCXO board to more of an angle and adding another ground wire jumper to the TCXO board seemed to fix this.

One important consideration is that you MUST be sure that there's a blocking capacitor somewhere between the output of the ProgRock 2 and the input of the circuit that it's driving.  As it turns out, the stock TS-590 TCXO board has such a blocking capacitor - but if your application does not, or you are not sure if it does, simply use a 0.001 to 0.1uF capacitor in series with the output - and this capacitor may also serve in lieu of a jumper wire in connecting it to the radio.

Finally, don't forget to disable the original oscillator of the radio into which you are installing the ProgRock 2.  In the case of the TS-590, there are two jumpers that must be removed - one to cut power to the original oscillator and the other to disconnect its output - these black jumpers are just visible to the right of the orange connector on the jumper cable to the TCXO board on Figure 5.  In some radios the TXCO replaces the original oscillator entirely so there's no need to "disable" it.

Figure 5:
The TCXO + Progrock 2 boards, installed in the TS-590.
There is enough wire length to connect the USB to program
the ProgRock in-situ if the mounting screw is removed.
Click on the image for a larger version.

Checking the calibration

You might notice that the TS-590's TCXO board is connected with a short, 4-wire jumper (the red, black and green wires in Figure 5) and this is long enough to allow connection of the ProgRock 2 board to a USB cable and a computer to allow the frequency to be adjusted "live", while the radio is in operation - this requires removing the single mounting screw to permit the board to "hang loose".

Simply setting the ProgRock 2 to 15.6 MHz exactly in the configuration menu resulted in the TS-590 being within 2 Hz of the correct frequency when checked against the 10 MHz WWV/H signal - this difference likely because the ProgRock 2's onboard 25 MHz oscillator was very slightly off, but well within the 0.5ppm tolerance.

But what if you wanted it to be closer?  Keep in mind that the frequency tolerance of the ProgRock 2's own TCXO is 0.5ppm which amounts to as much as 5 Hz at 10 MHz (or 15 Hz at 30 MHz) so absolute accuracy over a wide temperature range is unrealistic - but "dialing it in" at the typical room temperature (or that of the radio's interior after it has been on for a while) is quite reasonable - although there's a caveat to this if you plan to use the 1pps input as we'll soon discuss.

Dialing it in

If you have an ultra-precise frequency reference such as a GPS-disciplined oscillator or a Rubidium reference, by all means use it - but if you don't, you can use an off-air frequency reference like WWV, WWVH, CHU, BPM, or whatever else is near you that is KNOWN to be very precise - but the higher the frequency, the better.

Using 15 MHz WWV as an example, tune the radio USING THE KEYPAD so that it is exactly on frequency:  Note that the TS-590 can tune smaller than the 10 Hz steps shown on the display, so turning the dial doesn't guarantee that you are on the "zero Hz" frequency step.  Without bumping the main tuning knob and knocking it off by less than a 10 Hz step listen for the WWV transmission to hear the portion when they are transmitting the 500 or 600 Hz tone (this step won't work if they are not transmitting this tone) and switch between USB and LSB:  If you hear any difference in tone, you may wish to tweak the ProgRock's frequency up or down as appropriate.  If the tone on USB is slightly lower than that on LSB, the ProgRock's frequency needs to be set slightly lower.

An alternative method to setting the frequency is to use a spectrum analysis program - "Spectran" by I2PHD (LINK) is probably the easiest to use.  In this case, one would tune Spectran for a 1 kHz tone and configure it to pick up the audio via the computer's microphone - or using a direct audio connection such as a rig interface or audio cable from the radio.  If you are using WWV/H for this, it's suggested that you first listen using AM and verify that your sound card's sample rate is accurate, showing precisely 500 or 600 Hz during the periods when WWV/H is transmitting those tones.  If you find that it's not showing exactly 500 or 600 Hz (to within a Hz or so) you may wish to try a different sound card/computer combination.

Using USB on the TS-590, tune exactly 1 kHz below WWV/H (e.g. 14.999 kHz) using the keypad and measure the frequency:  If the tone frequency measures slightly high when using USB, the ProgRock's 15.6 MHz frequency can be increased slightly - but remember that it may be done only in 1 Hz steps.  Remember that 1 Hz at 15.6 MHz will cause a frequency shift of about 0.6 Hz at 10 MHz and almost 2 Hz at 30 MHz as the effect will be proportional to the radio of the reference frequency (15.6 MHz in this case) and the frequency to which the receiver is tuned.

Comment about tuning step size.

Many modern transceivers tune in 10 Hz steps or finer - but note that these steps are often not exactly what they may seem.  For example, some radios' 10 Hz steps aren't exactly 10 Hz each - some being a bit more, some being a bit less - but that they will average 10 Hz steps.  The same goes for the smaller step sizes as well.

Keep this in mind when you are attempting to set/measure a given radio exactly to frequency as this slight difference in step size may result in some frequencies being slightly different from what is expected and this difference may vary by seemingly random amounts.

Using (optional) the 1pps input

As noted earlier, the ProgRock 2 can take a 1pps input from a GPS receiver module, using this to make gradual corrections of the frequency.  Doing this if the GPS signal is reliable will result in the frequency being very stable over a wide temperature range, but there are two caveats to this:

• The ProgRock 2 doesn't (yet?) have in its firmware a means by which one can input an offset of its 25 MHz TCXO frequency.  As the onboard 25 MHz TCXO is not likely to be exactly correct, this means that if you set set the frequency at room temperature - and the oscillator is slightly off - when you apply a 1pps input the frequency will then be shifted assuming a 25 MHz clock frequency.  The reason for this is that the 1pps will set the frequency as if the onboard 25 MHz TCXO were 25 MHz, exactly - but since it probably isn't (remember - it's rated to be within 0.5ppm) a frequency shift will result.
  
• In other words, if you want your radio to be precisely on frequency with a 1pps input, you will have to "dial it in" with 1pps applied and expect it to be slightly off when no 1pps signal is present.
  
• If you ever do apply a 1pps signal - even briefly - the Progrock 2 will "remember" that offset even when the 1pps is removed until the unit is power-cycled.  If the 1pps is removed, the oscillator will now be free to drift with temperature. 
  

• The frequency step corrections as a result of the 1pps input are not infinitesimally small.  What this means is that with 1pps applied, every second the frequency will shift slightly, typically hovering above and below the target - but the magnitude of these corrections may be set in the configuration of the ProgRock 2.
  
• For most modes on HF - including FT8, FT4, PSK31, CW, Sideband or even many digital modes - these small "sub-Hz" shifts would likely be inconsequential. 
• If you are using a digital mode where fractional-Hertz frequency shifts are important, you may want to carefully consider using 1pps at all, weighing the pros and cons of having seemingly random small frequency shifts.  Modes where this may be important would be WSPR, FST4W (particularly the modes longer than 2 minutes), coherent CW, during an FMT (Frequency Measurement Test) or any other instance where small frequency steps may be disruptive.
• If you are in a situation where the continual frequency correction is an issue but you want the frequency to be closer than what the TCXO onboard the ProgRock will allow you might consider manually applying the 1pps signal intermittently to occasionally recalibrate the frequency.  This would allow the frequency to drift slightly with temperature between calibration intervals.
• While one may configure the adjustment size in the ProgRock 2 and likely minimize the size of the frequency adjustment steps, remember that it must be capable of correcting for the normal and expected frequency changes related to temperature.  This need sets a minimum correction size that will be practical and the varying environments with differing temperature and its stability will affect this.
  
• If you are using a 1pps input on a radio that operates in the VHF/UHF and/or microwave frequencies, these small frequency shifts will be proportionally larger and may even be noticeable on SSB and/or as slight "clicks"in received audio - possibly making the radio unusable for digital modes altogether.  It may be possible to configure the ProgRock 2 to mitigate this somewhat by reducing the magnitude of the corrections, but they will always be there.

Using the ProgRock2 in other radios

As the ProgRock2 can be programmed for about any frequency you like, it can be used in radios other than the Kenwood TS-590.   The ProgRock 2 draws a modest amount of current (40-60mA) so its addition will likely not be consequential in power consumption on "desk" and "mobile" radios - but it may be significant on a QRP or portable radio.  It's likely that most radios do NOT have a handy board onto which the ProgRock 2 may be easily mounted like the TS-590, but the unit is small enough that it will likely fit in/near the location intended for the oscillator/TCXO.

Be sure to use as short as leads as practical and it will likely be necessary to use some sort of adhesive (foam or glue) or some sort of "zip tie" to hold the ProgRock 2 board into place.  If possible, be sure to allow the ProgRock 2 to be connected to a  computer to allow final tweaking of frequency once it is installed - at least before it is secured into place:  Once the frequency has been "dialed in" it's unlikely that you'll need to readjust it any time soon.

The ProgRock 2 is also rather flexible in its power supply, but even though it is rated to 12.0 volts, I would NOT recommend allowing more than 10 volts to be applied to it - and the input voltage can be as low as around 4 volts meaning that it's likely that if the radio itself has an already-existing supply rail (5 volts like the TS-590 - many have an 8, 9 or 10 volt supply as well) that will work nicely.

As noted above, you must be sure to keep the DC of the output of the ProgRock from being shorted to ground (via a transformer or inductor to ground) or to another voltage source (such as a bias network of an amplifier/buffer) as it has no blocking capacitor of its own.  In the TS-590 the original TCXO board had its own blocking capacitor - but if your intended circuit doesn't have such - or if you don't know if it has one - simply add a 0.001 to 0.1uf (value not critical) series blocking capacitor of your own.

Most "recent" radios (e.g. those made since the early-mid 90s) have a single frequency reference for their synthesizer - but ones prior to this (and a few after) may have more than one master oscillator that determines the precise frequency.  It's worth noting that the ProgRock 2 can output more than one frequency at a time (three if you are not using the 1pps input - just two if you are) and it may be possible to program one of the ProgRock's other outputs to another useful frequency.  One possibility is for very old analog radios that sport a 100 kHz crystal calibrator or similar:  The ProgRock 2 would be excellent for this purpose.

In some cases, these "other" frequencies may include the radio's BFO (Beat Frequency Oscillator) or HFO (Heterodyne Frequency Oscillator) in which case you may need to be more creative - but it's worth noting that the ProgRock has up three "digital" inputs that may optionally be used allowing up to eight separate frequency combinations to be produced - possibly allowing one to replace impossible-to-find crystals in vintage radios - but this is a possible topic of another article.

* * * * *

This post stolen from ka7oei.blogspot.com

[END]

Hiking and POTA (Parks On the Air) operation from Arches National Park (US-0004)

Figure 1:
Double-O Arch
Click on the image for a larger version.

Earlier this month I had the opportunity to spend nearly a week in Arches National Park in south eastern Utah.  As the name implies, there are a lot of natural arches scattered throughout the area, the result of erosion occurring over millenia, the semi-porous sandstone eventually yielding the the inexorable forces of wind and water.

These trips to red rock country are not new to me:  I've been visiting this part of the state for more than 30 years now, having spent more well over six months cumulative camping, hiking and backpacking in this austere land.  On most of these trips, I have been in the company of other amateur radio operators - and that was the case here, as well.

Compared to other national parks in Utah, Arches is fairly small - on a few 10s of miles long and narrower than this in width.  Compared to some places I go, it's a bit of a "tourist" park meaning that it's fairly crowded with comparatively few developed trails concentrated in a few areas.

Figure 2:
Fins and more fins, backgrounded by the La Sal mountains
as seen along the "primative" trail.
Click on the image for a larger version.

When going to such places, I tend to do about as much hiking as I can - but Arches is comparatively limited, but one of the longer trail systems is that associated with Landscape Arch and Double-O arch.  On this hike I took the "primative" trail, separating me from the madding crowds - a much longer route over occasionally rugged terrain, occasionally requiring a bit of scrambling up or down slick rock:  Just the way I like it!

Over the course of a few hours I made my way from the campground to Double-O Arch where I met the rest of my group who'd taken the other trail where we stayed for a while before splitting again and heading back.  Altogether, I managed about 10 miles (16km) or so by the time I got back to camp.

With temperatures in the mid 80s (about 30C) I set up my radio in the shade of my tent and shade and started operating.

* * *

Equipment:

Antennas:

The evening before, I had a bit of extra time around dinner and I took that opportunity to set up my portable antennas in the cooling evening air.  For this POTA operation, I eventually set up two antennas - the first one being my JPC-7 loaded dipole.

Figure 3:
Operating CW in the shade, on a portable table, using
a cast iron frying pan to keep the paddle in place.
Click on the image for a larger version.

I've discussed the JPC-7 antenna on this blog before (LINK) - and have used it for several POTA operations already with good results.  Since the last POTA operation I'd rewound the loading coils, replacing the original stainless steel wire with silver-plated copper to reduce the losses - I discuss the details about this HERE.  It's difficult to estimate how much improvement this change made, but it's likely in the general area of 3dB or so - only 1/2 "S" unit or so, but it's certainly worth a bit of hassle to improve efficiency on an already-small antenna.

A day after setting up the JPC-7, I also set up the JPC-12 vertical antenna (described here).  This antenna, too, has been refitted with a silver-plated loading coil as well:  With a few extra mast sections, a top-hat and resonant, elevated radials it also makes for an excellent portable antenna - albeit a bit more complicated to set up than the loaded vertical, particularly when changing bands.

Radio and power:

The radio - an older Yaesu FT-100 (with the CW filter from an FT-100D) which was powered by a 100 amp-hour Lithium-Iron Phosphate battery using a paddle from cwmorse.us - (link).  I've used this particular paddle ("Outdoor pocket double paddle with magnets") for several POTA activations and as before, I've used the same cast-iron fry pan for all of them to keep the paddle from sliding around - often ending up with a bit of soot on the side of my hand and wrist!

Figure 4:
The antennas - and solar panel.
There was no audible interference from the
now-modified solar controller.
Click on the image for a larger version.

Operating (mostly) on 20 meters I managed to make about 285 contacts - all but four of them CW with 277 of them counting as POTA contacts.  The operating position was almost as POTA as one gets:  Sitting in a chair, under a shade, surrounded by sand and red rock.

Mixing antennas with solar - with no QRM!:

 Figure 4 shows the "antenna farm".  In the foreground - just left of center - is the JPC-7 loaded dipole, using a studio tripod for support while in the background - to the right of center - can be seen the JPC-12 vertical with tophat.

Also in the foreground is a 200 watt solar panel - but you may be wondering if this would cause QRM (interference) from its controller:  The answer is NO - but this is only true because I've done previous work to add extra filtering to it.  Even with the antenna (particularly the JPC-7) right next to the solar panel with its controller, I could not hear any interference at all - but this is by design as I have taken steps to make it quiet, and you can read about the details to accomplish this HERE in a previous blog entry.

At this camp site there were two other PV systems in operation located some distance away from the antenna, but I could hear those.  For the one closest, I happened to have an FT240-43 toroid on hand and I was able to cram five turns (with connectors) of the cables from the two panels feeding it:  Predictably, this reduced the QRM somewhat (1-2 S-units) - but as noted in the blog entry noted above, ferrite alone will not likely solve such a QRM issue!

Figure 5:
Red and green auroras backgrounding the big dipper.
Click on the image for a larger version.

The "other" PV system - which was even further away - caused minimal interference so nothing was done about it - but since I'd used my only FT240-43 toroid, I wouldn't have been able do anything about it, anyway.

Red Rock + Aurora = More red!

As it happened, the sun did a bit of burping in the days leading up to and during this trip, the result being the repeated appearance of a visible aurora, the first appearing on October 7 when very visible red pillars appeared in the northern sky:  Scrambling to the top of a nearby bluff, we could see a bit of red and green in the sky along with the Big Dipper.

For the next few days we noticed something else:  On the first night, the sky was spectacularly dark - the Andromeda Galaxy being visible - but on the night of the first aurora and for a few nights thereafter it seemed as though we lost a lot of the "deepness" of the sky.  We also noticed that despite the lack of moonlight, we could see the surrounding landscape and make out large objects on the ground without needing additional light.

Figure 6:
Sky glow, lighting up the camp and environs.
Click on the image for a larger version.

We eventually realized that what we were seeing was sky glow.  In other words, the entire sky was glowing dimly:  Not bright enough to be perceived as color, but the cumulative glow of the entire sky was enough to illuminate the landscape in that odd way.

A few days later the aurora was clearly visible again - and that's when the photo in Figure 6 was taken, showing a bit of red behind the clouds to the north and some green glow on the northern horizon.

* * * * *

This page stolen from ka7oei.blogspot.com

[END]

Neon bar-graph VSWR/Power meter using the ИН-13 (a.k.a "IN-13") "Nixie" - Part 2

Figure 1:
Power/VSWR meter using ИН-13 neon bar-graph
indicators.
Click on the image for a larger version

In Part 1 (link) I laid out the requirements of the ИН-13-based neon bar-graph VSWR/power meter.  Admittedly, this is a "buy cool, old tech and figure out what project might use it" scenario - but having one tube always showing the forward power and the other tube showing either reverse power of calculated VSWR was the goal.

In the previous installment we talked about how to generate the high voltage (130 volts or so) for the bar-graph neons, the means to drive precise amounts of current through the tubes using precision current sink circuits, and the "Tandem" coupler to detect forward and reflected power.

 

Mounting the tubes

 

Figure 2:
ИН-13 tubes in the raw.
It is up to the constructor to determine how best to mount
these tubes - and how to connect them to the circuit.
Figure 3 shows how flexible wires were attached as the
wires on the tubes themselves are very easily broken!
Click on the image for a larger version.

In looking at Figure 1 you can see that the ИН-13 tubes are mounted to pieces of clear acrylic, but a quick look at Figure 2 shows that they don't really have a means of mounting, leaving the method to the imagination of the user.

In preparing the tubes for mounting I trimmed the wire leads and soldered flexible wires to them, covering them with "hot melt" (thermoset) adhesive to passivate the connection, making them relatively durable:  The original wires will NOT tolerate much flexing at all and are likely to break off right at the glass "pinch" - which would make the tube useless.   Figure 3 shows how the leads were encapsulated - the thermoset adhesive being tinted with a permanent marker - mainly to add a bit of color.

Laser-cut sheets and markings

Figure 3:
Close-up of the "hot-glue" covered wire
attachments for the ИН-13 tubes.  Also visible
are the black wire loops holding them in place
and the laser-edged markings on the acrylic.
Click on the image for a larger version.

In looking at Figure 1 and 3 you will also notice that there are scales indicating the function and showing scale graduations and the associated numerical values.  I'm fortunate to have a friend (also an amateur radio operator) who has a high-power laser cutter and it was easy to lay out the precise dimensions of the acrylic sheets and also have it cut the holes for the mounting screws in the corners as well.

While it takes a bit of laser power to cut the sheets, a far lower power setting will ablate the surface, yielding a result not unlike surface engraving and when lit from the edges, these ablations will light up with the rest of the sheet remaining pretty dark:  A total of four sheets were cut and "engraved" in this way:  The front sheet for "VSWR" and its markings, the middle sheet for "Reverse Power" and the rear acrylic sheet for "Forward Power".  It was possible to arrange the lettering so that only "VSWR" and "Reverse Power" were atop each other but in subdued light - and with a bit of darkened plastic in front of the display - the markings on the un-lit sheet are practically invisible.  The fourth sheet mentioned was left blank, being the protective cover. 

Edge lighting

Edge-lit displays go back decades - and the idea likely goes back centuries where it was observed that imperfections in glass (later, plastic) would be visible if the substrate was illuminated from the edge.  Since the early-mid 20th century, one could find a number of edge-lit indicators - usually in some sort of test equipment of industrial displays - but they occasionally showed up in the consumer market - usually acrylic or similar with the markings engraved with a rotary tool or - as may be done nowadays, a laser.

While incandescent lamps would have been used in the past, LEDs are the obvious choice these days and for this I selected some "high brightness" LEDs to light the edges of the engraved acrylic sheets.  For the "Forward Power" sheet - which would be that which was always illuminated in use - I chose white while using Green for VSWR and Blue for Reverse Power.  I'd considered Yellow and Red, but discarded the former as it might appear too much light the white under some conditions and past experience has reminded me that - particularly in a dark room - the human eye can't see or focus on fine detail on red objects very easily.

Figure 4:
Six LEDs are epoxied to the edge to evenly light the laser-
etched markings in the acrylic sheet.  The faces of the LEDs
were filed flat to facilitate bonding and improve efficiency.
Click on the image for a larger version.

Figure 4 shows some details as to how the edge lighting is accomplished.  Six equally-spaced LEDs were epoxied to the bottom edge of the display, arranged to be nearly the width of the engraved text.  In writing this entry I observed that photographing edge-lit displays such as this is nearly impossible owing to the variations in illumination (e.g. it's difficult to take pictures of very bright objects in the dark!) but the effect is very even as viewed by the human eye.

The six LEDs were connected as two series strings of three LEDs:  As each LED requires about three volts - and I have only a 12 volt power source - doing so requires only a bit more than nine volts to power the LED arrays.  As the green and white LEDs are also silicon nitride based as well, they take similar voltages.

Not readily apparent from Figure 4 is the fact that the LEDs were modified slightly.  As we are trying to interface a standard T1-3/4 LED to the flat edge of a plastic sheet, it's apparent that the rounded, focused lens makes this physically difficult.  To mitigate this, the top of the LED was flattened with a file and the clear epoxy was removed to just above the light emitting die.  The result of this is that a flat surface is mated to another flat surface for a physically stronger bond and a more efficient coupling of light and a bit of the LED's original directivity in the form of the "lens" is removed from the equation. 

Just prior to mounting the acrylic sheets in the "stack up" some black electrical tape was applied.  This tape was put on both sides of the sheet, extending just above the bottom edge, to reduce the glare from the LEDs and to minimize the possibility of this light coupling into the adjacent sheet.

Mounting the tubes and sheets

As can be seen from Figure 3, the tubes are held in place with loop of solid-core insulated wire - the holes mounting them also "drilled" with the laser.  The "stack-up" of acrylic sheets and the tubes - both of which were mounted on "VSWR" acrylic layer - is held together using 6-32 brass machine screws and spacers with a piece of 1/4" (5.2mm) plywood covered with black felt for the back to provide contrast.

The box and base

As can be seen from figure 1, the entire unit is in a wooden base:  The same friend with the laser cutter also had some scraps of red oak and a simple base was made, decorated with an ogee cut around the perimeter with the router while atop it a simple box with mitered corners - facing at a slight upward angle - in which the display and electronics reside.  On the base itself are two buttons:  One switches between VSWR and Reverse Power and the other between peak and average readings.  These switches have other functions as well, which will be discussed in the third installment when the final circuit and internal workings of the software is discussed.

* * * * *

This page stolen from ka7oei.blogspot.com

[END]

Neon bar-graph VSWR/Power meter using the ИН-13 (a.k.a "IN-13") "Nixie" - Part 1

Figure 1:
Power/VSWR meter using
ИН-13 (a.k.a. "IN-13") neon bar-graph indicators.
Click on the image for a larger version.

(Be sure to read Part 2 of this article - LINK).

 

Several years ago I bought some Soviet-era neon bar-graph displays - mainly because I thought that they looked cool, but I didn't have any ideas for a specific project.  

 

After mulling over possible uses for these things for a year or so - trying to think of something other than the usual audio VU meter or thermometer - I decided to construct a visual watt/VSWR indicator for amateur radio HF use.

 

* * *

 

I actually bought two different types of these bar-graph tubes:

• The ИН-9 (a.k.a. "IN-9").  This tube is 5.5" (140mm) long and 0.39" (10mm) diameter.  It has two leads and the segments light up sequentially - starting from the end with the wires - as the current increases.

• The ИН-13 (a.k.a "IN-13").  This neon bar-graph tube is about 6.3" (160mm) long and 0.39" (10mm) diameter.  Like the ИН-9 its segments light up sequentially with increasing current but it has a third lead - the "auxiliary cathode" - that is tied to the negative supply lead via a 220k resistor that provides a "sustain" current to make it work more reliably at lower currents.

Note:  It would be improper to refer to these as "Nixies" as that term refers to a specific type of numeric display - which these are not.  Despite this, the term is often applied - likely for "marketing" purposes to get more hits on search engines.

Figure 2:
A pair of ИН-13 neon indicator tubes.  These tubes are
slightly longer than than the ИН-9 tubes and have three leads
Click on the image for a larger version.

For a device that is intended to indicate specific measurements, it's important that it is consistent, and for these neon indicators, that means that we want the bar graph to "deflect" the same amount anytime the same amount of current is applied to it.  In perusing the specifications of both the  ИН-9 and  ИН-13 it appeared that the  ИН-13 would be more suitable for our purposes.

This project would require two tubes:

• Forward power indicator.  This would always indicate the forward RF power as that was that's something that is useful to know at any time during transmitting.
  

• Reverse power/VSWR.  This second tube would switchable between reverse power, using the same scale as the forward power display, and VSWR - a measurement of the ratio between forward and reverse power and a useful indicator of the state of the match to the antenna/feedline.

Driving the tubes

  

"Because physics", gas discharge tubes require quite a bit of voltage to "strike" (e.g. light up) and these particular tubes need for their operation about 140 volts - a "modestly high" voltage at low current - only a few milliamps (less than 5) per tube, peak.

Figure 3:
Test circuit to determine the suitability of various inductors and transistors
and to determine reasonable drive frequencies.  Diode "D" is a high-speed,
high-voltage diode, "R" can be two 10k 1 watt resistors in parallel and
"Q" is a power FET with suitably high voltage ratings (>=200 Volts)
and a gate turn-on threshold in the 2-3 volt range so that it is suitable
to be driven by 5 volt logic.  V+ is from a DC power supply that is
variable from at least 5 volts to 10 volts.  The square wave drive, from a
function generator, was set to output a 0-5 volt waveform to
make certain that the chosen FET could be properly driven by a 5 volt
logic-level signal from the PIC as evidenced by it not getting perceptibly
warm during operation.

Generating high voltage from a low is one of the aspects that I tackled in a previous project on this blog when I built a high voltage power supply for the Zenith Transoceanic:  You can read about that here - A microcontroller-based A/B Battery replacement for the Zenith TransOceanic H-500 radio, with filament regulation - link.

 
The method used for this project and the aforementioned Zenith radio is  boost-type converter as depicted in Figure 3.  The switching frequency must be pretty high -  typically in the 5-30 kHz range if one wishes to keep the inductance and physical size of that inductor reasonably small.

As in the case of the Zenith Transoceanic project, I used the PWM output of the microcontroller - a PIC - to drive the voltage converter with a frequency in the range of 20-50 kHz.  For our needs - generating about 140 volts at, say, 15 milliamps maximum, I knew (from experience) that a 220uH choke would be appropriate.  Figure 4, below, shows the as-built boost circuit.

Figure 4:
The voltage boost converter section showing the transistor/inductor, rectification/filtering and
voltage divider circuitry.

Description:

 

Q301 is a high-voltage (>=200 volt) N-channel MOSFET - this one being pulled from a junked PC power supply (the particular device isn't critical) which is driven by a square wave on the "HV_PWM" line from the microcontroller:  R301, the 10k resistor, keeps the transistor in the "off" state when the controller isn't actively driving it (e.g. start-up).  L301, a 220uH inductor, provides the conversion:  When Q301 is on, the bottom end is shorted to ground causing a magnetic field to build up and when Q301 is turned off, this field collapses, dumping the resulting voltage through D301, which is a "fast" high voltage diode designed for switching supplies - a 1N4000 series diode would not be a good choice in this application as it's quite "slow".

 

R304, a 33k resistor, is used to provide a minimum load of the power supply, pulling about 4.25 mA at 140 volts:  This "ballast" improves the ability of the supply to be regulated as the difference between "no load" (the neon bar-graphs energized, but with no "deflection") and full load (all segments of the tubes illuminated) is less than 4:1.  The resistive divider of R302 and R303 is used to provide a sample of the output voltage to the microcontroller, yielding about 2.93 volts when the output is at 140 volts.  The reader will, by now, likely have realized that I could have used R304 as part of the voltage divider - but since the value of this resistor was determined during testing, I didn't bother removing R302/R303 when I was done:  Anyway, resistors are cheap!

 

Setting the current:

 

Having the 140 volt supply is only the first part of the challenge:  As these tubes use current to set the "deflection" (e.g. number of segments) we need to be able to precisely set this parameter - independent of the voltage - to indicate a value with any reasonable accuracy.  For this we'll use a "current sink".

 

Figure 5:
The precision current sinks that drive the neon tubes precisely based on PWM-derived voltage.
Click on the image for a larger version.

 

Figure 5, above, shows the driving circuits for the two tubes using the "precision current sink".  Taking the top diagram as our example, we see that the inverting input of the op-amp (U401c) is connected to the junction of the emitter of Q401 and resistor R406.  As is the wont of an op amp, the output will be driven high or low as needed to try to make the voltage (from the microcontroller) at pin 10 match that of pin 9 - in this case, based on feedback from the sense resistor, R406.

What this means is that as the transistor (Q401) is turned on, current will flow from the tube, through it and into R406 meaning that the voltage across R406 is proportional to the voltage on pin 10.  It should be noted that current through R406 will include the current into the base - but this can be ignored as it will be only a tiny fraction (a few percent at most) of the total current.  It's worth noting that this circuit is insensitive to the voltage - at least as long as such current can be sunk - making it ideal for driving a device like the ИН-13 (or ИН-9) in which its intended operation is dependent on the current rather than the operating voltage.

At this point it's worth noting that the driving voltages from the microcontroller ("FWD_PWM" and "REV_PWM") are not plain DC voltages, but rather from the 10 bit PWM outputs of the microcontroller.  The use of a 10k resistor and 100nF (0.1uF) capacitors (R405 and C406, respectively) "smooth" the square-ish wave PWM into DC.

 

Q401 and Q402 were, again, random transistors that I found in scrapped power supplies, but since there's at least 70 volts drop across the tube, about any NPN transistor rated to withstand at least 80 volts should suffice.  It's also worth noting the presence of R407, which provides the "sustain" current on the "auxiliary" cathode.

Figure 6:
An exterior view of the tandem coupler module.
Visible is the top shield and the three feedthrough
capacitors used to pass voltage and block RF.
Click on the image for a larger version.

RF sensing

For sensing forward and reflected power I decided to use an external "sensing head" that was connected inline with the radio, on the "tuner" side of the feedline.  

For sensing power in both directions I chose the so-called "Tandem" coupler which consists of a through-line sampler in which a short length of coaxial cable carrying the transmit power (T1 in the diagram of Figure 7) passes through a toroidal core - using some of the original cable's braid grounded at just one end as a Faraday shield.  An identical transformer (T2) is connected across the first (T1) for symmetry.

When carefully constructed this arrangement has quite good intrinsic directivity and a wide frequency range.  Figure 6 shows the diagram of this section.

Figure 7:
Schematic diagram of the "Tadem" coupler.  A bidirectional coupler sends power to
separate AD8307 logarithmic amplifiers - one for forward and the other for reverse.
The outputs, expressed in "volts/dB" are sent to the microcontroller.
Click on the image for a larger version.

The RF sensing outputs of the second tandem coupler (T2) then goes through resistive voltage dividers (R606/R607 for the reverse sample and R603/604 for the forward sample) to a pair of Analog Devices AD8307 logarithmic amplifiers - one for forward power and the other for reverse - to provide a DC voltage that is logarithmically proportional to the detected RF power.  This voltage is then coupled through series resistors (for both RF and DC protection) R605/R608 and to the outside world using feedthrough capacitors.

The use of a logarithmic amplifier precludes the need to have range switching on power meter as RF energy from well below a watt to well over 2000 watts can be represented with only a few volts swing.  Looking carefully at Figure 6 one can see a label that notes that the response of the AD8307 is about 25 millivolts per dB - and this applies across the entire power range of a few hundred milliwatts to 2000 watts.

All of this circuitry is mounted in a box constructed of circuit board material and connected to the display unit with an umbilical cable that conveys power and ground along with the voltages that indicates forward and reflected power.

Figure 8:
An inside view of the Tandem Match (sense unit) showing
the coupling lines, internal shielding and AD8307 boards.
Click on the image for a larger version.

Figure 8 shows the as-built "sense unit" and the two coaxial sense lines are clearly visible.  As can be seen, the "main line" coupler is physically separated and shielded from the secondary sense line, using PTFE ("Teflon") feedthrough lines to pass the signals.

The AD8307 detectors themselves can be seen at the left and right edges of the lower half of the unit, built on small pieces of perfboard.  All signals - including the 12 volt power and the DC voltages of the output pass through 4000pF feedthrough capacitors to prevent both ingress and egress of RF energy which could find its way into the '8307 detectors and skew readings.

* * * * *

In a future posting (Part 2) we'll talk about the final design and integration of this project.

This page stolen from ka7oei.blogspot.com

[END]