The "Universal TCXO" - better stability for the Kenwood TS-590, TS-570, TS-480 (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 relatively inexpensive (US$18 at the time of writing) and has a stability of 0.25ppm - which is likely better than the original TCXO offered by the manufacturer - and likely less expensive as well.

This page describes not only the installation of the ProgRock 2 in a Kenwood TS-590, but also in the TS-570:  These two radios use very different frequencies, but the ProgRock 2 is easily programmed to whatever is needed!

Any weird frequency

Square waves are OK   The ProgRock 2 produces a square wave output, rich in (odd-order) harmonics - but is this OK for use in a radio?   It should come as no surprise that modern radios use the reference oscillator - the function being provided by a TCXO - as the basis for timing (possibly) every other frequency-determining module - and this is done digitally.  Somewhere along the line, the signal being output by the reference oscillator - whether it's a TCXO, an ordinary crystal oscillator or even the ProgRock - will be converted to a digital signal, anyway.   In many cases, the TCXO - if not the original oscillator - already produces a square wave which means that the rest of the circuitry is, by default, happy with the square wave output from the ProgRock 2. While not directly applicable here, if a square wave were fed to circuitry via, say, a long, poorly-terminated coaxial cable, the square wave might get badly distorted and the circuitry in the radio may not be able to properly trigger on it - but since the ProgRock's output is being fed directly into the radios circuitry, this is not going to be a problem.

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 - more than 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 somewhere near 300 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 0.25ppm 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 0.25ppm which amounts to as much as 2.5 Hz at 10 MHz (or 7.5 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 a web cam - 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, with Spectran 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 or just do a bit of math to compensate for the slight difference in the audio card's sample rate.

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 of the carrier:  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.

Note:  If you have a known-accurate reference oscillator of your own (such as a GPS Disciplined oscillator, Rubidium oscillator or similar) by all means, use it!

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 the (optional) 1pps input on the ProgRock 2

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 0.25ppm) 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.

* * *

A ProgRock 2 in the Kenwood TS-570

The (older) Kenwood TS-570 (all variants) can also be retrofitted with a ProgRock 2 in lieu of the Kenwood "SO-2" TCXO - and it's also pretty easy.  Using the same steps as above, program the ProgRock 2's "Clock 0" for 20000000 Hz (20 MHz exactly).  I modified my own TS-570 for the same reason that my friend modified his TS-590:  The original oscillator would audibly drift in frequency with temperature and it was over 100 Hz high on 10 meters  (approx. 25 Hz on 40 meters) once it warmed up, causing the occasional complaint that I was off-frequency.  I do not use this radio for digital modes like FT-8, but if I had, I'm sure that I would have made this modification some time ago!

Figure 6:
The ProgRock 2 installed in place of the original Kenwood
SO-2 TCXO in the TS-570.  The wires through the
board were bent and soldered to the V+ and three
ground pins.  The output of the ProgRock 2 is
connected to the "out" pin on the board via a 47 ohm
and 1000pF capacitor in series.
Click on the image for a larger version.

Via online search, you can find the instructions for installing the SO-2 TCXO module and these show how the PLL board (the one in the bottom of the radio) may be removed:  Be careful with the flat ribbon cables!

Rather than solder in the TCXO, cut five short pieces of tinned wire (20-24AWG, 0.6-0.8mm dia) to be about 3/4" (20mm) long and solder them in the five holes into which the original TCXO was soldered and re-install the board.

On the board itself you'll notice that two of the holes are marked - one for power and one for the "out" pin of the TCXO into which we will feed our 20 MHz clock from the ProgRock 2:  The other three pins are ground.  First, the ProgRock 2 is "dry fit":  It is placed on the circuit board (with a piece of foam or cardboard underneath to space it slightly away - perhaps 1/8" to 3/16" or 3-5mm) and the wires that we soldered bent around to the contact pads and trimmed, taking care that they not touch the pads on the back side of the board or anywhere else that they shouldn't.

As can be seen in Figure 6, the "V+" pin was wrapped around and soldered to the "V+ pin (which carries 5 volts) on the ProgRock 2 (the one in the lower-right corner of the ProgRock board in Figure 6) while two of the the three wires for the ground connect to top-side "GND" pads on the ProgRock while the third is soldered to the top of the USB connector.  As the ProgRock 2 is very light, these wires are more than adequate to hold it into place - just be sure to keep the board height low enough to avoid interfering with the shield when it is replaced.

The top-right corner pad on the ProgRock 2 in Figure 6 is the "CLK 0" that we programmed - but like the TS-590, it must be capacitively coupled to the clock input on the TS-570 and this is done with a series capacitor:  I used a 1000pF capacitor for this, but anything between 470pF and 0.1uF would be fine for this radio.  On the schematic I noted that there is a 10pF capacitor to ground in the TS-570 on the "out" pin so I also included a 47 ohm resistor in series with the capacitor just in case the output of the synthesizer would be "unhappy" with capacitive loading - and also to reduce the amount of RF drive into the '570's clock input.  This resistor may not have been necessary, but hey, it's just a resistor so why not play it safe?  The final steps are to cut the two resistors, R503 and R504, seen to the right of the ProgRock 2 board:  This necessary step disconnects the power and the output lead of the original oscillator circuit.

Upon reassembling the TS-570, I tuned in WWV on 5 MHz and switched between LSB and USB (with the RIT set to zero) and heard no discernible change in pitch during a part of the transmission with the tone indicating that the radio was "dead on" frequency.  As the ProgRock 2 is rated for 0.25ppm stability, it should stay within 5-8 Hz on 10 meters, worst-case - about 1/20th as much drift as with the original oscillator!

While I could have done so, I chose not to add the resistors to permit the external application of a GPS-based "1pps" input to "lock" the ProgRock 2, as was described above for the TS-590.

From start to finish, it took me about an hour to program and install the ProgRock 2 in my TS-570 - but your mileage may vary.

* * *

A ProgRock 2 in the Kenwood TS-480

Figure 7:  The "option" board in the TS-480.

If your TS-480 drifts around a bit, you can use a ProgRock 2 in it as well and for this radio, the ProgRock2 would be programmed for 15.6 MHz.  Unlike the TS-590 and TS-570, the spot for the TXCO is not quite large enough to accommodate the ProgRock 2 - at least not without a bit of modification.  Figure 7 shows where the TCXO is mounted.

Looking carefully, you'll see electrolytic capacitor C973 and to the left of it, there's a mounting screw:  Both of these interfere with the mounting of the ProgRock 2.  Fortunately, this "sub-board" is easily removed with the removal of the three screws and "un-clipping" it from the white connectors which allows C973 - which is used for filtering the DC power to the TCXO - to be removed:  As the TCXO itself has its own power supply filtering, this capacitor is unnecessary.  With the board removed, mounting the ProgRock 2 is also made much easier.

Figure 8:  The ProgRock 2 in the TS-480

Figure 8 shows the mounting of the ProgRock 2 in the location for the TCXO.  The 5 volt supply for the ProgRock 2 is from the mounting pin closest to C973 (the lower-right hand hole below the one with the white circle) while the RF output from the ProgRock 2 - via the red capacitor just visible in the upper-left corner of the board (a value between 1000pf and 0.1uF is OK) is connected to the lower left-hand hole, below and to the left of the "o" in the word "option" ("TCXO Option") in Figure 7:  The "ground" for the ProgRock 2 is from the upper-left pin of the original TCXO - or it could be connected to any part of the ground plane on this PCB.  In Figure 8, you can also see where a piece of tinned copper wire was run from the center "ground" pin of "Option Filter 1" and tacked to the top of the USB connector for additional mechanical support.

The "downside" of this is that the TCXO blocks access to one of the screws.  While we could have installed "flying leads" on the board  which allowed the ProgRock 2 to be moved around a bit to get access to the screw, we chose to leave out this mounting screw - but it was glued to the PC board just in case we changed out mind.  The other downside is that if we wanted to install Option Filter 1, we would have to re-think the mechanical installation of the ProgRock 2 - perhaps tilt it at an angle.  Of course, if one wished to have just one optional filter, it can be installed in the "Option Filter 2" position, instead.

* * *

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, TS-480 or TS-570.   The ProgRock 2 draws a modest amount of current (40-60mA) so its addition will likely not be consequential in power consumption on "desktop" 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 pad or glue) or some sort of "zip tie" to hold the ProgRock 2 board into place.  If possible, be sure to install it such that the ProgRock 2 may be moved so that its USB port may 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 ever 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 radios have an 8, 9 or 10 volt supply as well) that will work nicely or one could use an appropriately-chosen series resistor (likely in the 47-82 ohm range for a 12 volt supply - but please do your own measurements) to drop its supply by a few volts.

As noted above, you must be sure to keep the DC on the output terminal 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]

An alternate PA transistor for the QRP Labs Ultimate 3S beacon for more output power

The QRP Labs Ultimate 3S beacon (kit) is a reasonably-priced, compact and self-contained unit.  Despite its apparent simplicity and low price it is capable of transmitting in a variety of modes, such as WSPR, CW, Hellschrieber to name but a few on any amateur band from 2200 meters through the U.S. 222 MHz band.

Sort of.

Actually, it's not really that simple:  For operation on 160 through 10 meters, the construction is rather straightforward but for the higher (VHF) and lower (LF, MF) frequency bands, a few "mods" have to be made - at the very least, some "custom" low-pass filters need to be made and modifications to the power amplifier.  On the bands 160 through 10 meters the design of the circuitry means that at best, only a few hundred milliwatts of RF is possible with the parts supplied, the output power dropping off as one goes up in frequency.  For WSPR, even a few 10s of milliwatts will usually yield the desired results (e.g. detection of band openings) but there are instances where more power

The power amplifier:

Figure 1:
The front panel of my Ultimate 3S beacon, WSPRing away on
20 meters.  You can tell that I live in the U.S. by the position of
the "power" switch!

Click on the image for a larger version.

To minimize the cost, the power amplifier section (Q1-Q3) of the Ultimate 3S beacon uses BS170 N-channel low-power MOSFET transistors.  These devices are capable of dissipating about 1/3-2/3 of a watt each and there is room for three of these devices.  If an efficiency of about 50% can be obtained, it should be possible to safely get between 0.1 and 0.5 watts out of the beacon on the lower bands (e.g. 160-30 meters) - plenty for modes that allow very weak signals to be detected such as WSPR.

But, there is a problem.  The BS170 is not an RF transistor, but designed for low-power switching such as level conversion, turning on LEDs, small motors and relays.  At low frequencies - up to several MHz - it actually works quite well, capable of about a watt if three devices are installed, but by the time one gets to 10 meters it is, in this application, rather challenging to get more than about 100 milliwatts from the Ultimate 3 without a bit of tweaking.

Besides getting the available power amplifier kit for the beacon, one of the options the builder can choose is whether to wire the PA transistors for 5 volts or connect it to a higher-voltage power supply.  In general, using a higher-voltage supply - say, 12-15 volts - will enable somewhat higher RF output power, but this also means that the same amount of bias current at 5 volts will result in higher power dissipation and finding the best value - without blowing up the transistors - is a bit of a delicate dance.

The problems:

The problems with this device at higher bands such 10/12 meters (and up) include:

• Device capacitance.  There are a number of parasitic reactances involved - including the input and Miller capacitance.  All of these conspire to make it more difficult achieve a wide voltage swing and/or to turn the FET on and off quickly - something that needs to be done to amplify higher frequencies efficiently

• The drive capability is rather limited.  The power amplifier section of the Ultimate 3 beacon is driven directly by the synthesizer which, for older units (mostly the non-"S" version) could be a DDS board, but the more recent versions use the Si5351 synthesizer chip which has a somewhat lower output level.  Neither of these devices produce enough output to "fully" drive the FET's gate.

The upshot is that while the rather simple amplifier circuit works pretty well at low frequencies, it drops off rapidly as one goes up.

One of the methods to deal with limited drive signals is to bias the transistor slightly.  Because it - like any similar FET - takes a volt or three to start turning on, biasing the transistor toward "on" with a fixed DC voltage means that the limited RF drive signal doesn't have as "far to go" when it comes to driving the device.

Adding this bias works well - but only to a point:  Eventually, the transistor is conducting so much DC current that it is dissipating heat at/near its maximum rating and increasing the bias even more to further increase its effective gain is not an option.  One option is to add heat sinking (by gluing the transistors to a piece of aluminum or copper) to keep them cool, but this is of limited utility.

The Ultimate 3 beacon has the capability of using up to three of these transistors in parallel and while this can improve the power output at lower frequencies (maybe) the limited drive capability of the synthesizer - plus the fact that each transistor has its own capacitance - doesn't necessarily help.  One other factor often overlooked is that FETs are notoriously inconsistent in their DC characteristics:  Unless one goes through pains to sort and match individual FETs - even devices from the same lot - when several are placed in parallel and biased, one is inevitably going to pull more drain current than the others.  This means than when several parallel devices are used, one or two are going to be doing most of the work and under stress while the other two (or one) will be doing comparatively little.

All of this would seem to be an argument to use a single, more capable amplifying transistor to obtain more output power.

Work-arounds:

The BS170 is quite popular in QRP transmitters because it is cheap, but it can be made to work "less badly" and the best way to do this is to strongly drive its gate with an RF signal.  Often, high-speed CMOS gates are used for this such as a 74AS04 or equivalent with multiple sections wired in parallel.  Doing this "brute force" drive technique can greatly improve the output capability of this otherwise low-frequency device and if done correctly, a DC bias is unneeded, saving a lot of hassle.  Unfortunately, the Ultimate 3 beacon doesn't have a device like this in its signal path, instead connecting the output of the synthesizer (more or less) directly to the gate of the output transistor(s), but one could hack the circuit and wire such a device into the circuit.

Another work-around would be the use of a transistor specifically designed for RF use.  While there are many such devices available, most are quite expensive or hard to find.

One such device is the RD16HHF1 made by Mitsubishi and recommended for the optional 5 watt PA board available from QRP Labs, but this transistor is becoming increasingly difficult to find.  Taking into account the fact that it may difficult to use the original (small!) holes for the BS170, using this device should work well - provided that it is operated within the capabilities afforded by the limited ability to dissipate heat.

The RD16HHF1 is also a favorite for counterfeiters that take an ordinary FET's die, put it in a package and label it as the real thing:  This fakery may work on lower bands, but it falls apart at higher bands for the same reasons that the BS170's efficacy drops off.  Some counterfeiters don't even bother to mount a fake die, instead take an ordinary power FET and label it as an RD16HHF1:  Because the drain and source connections of the RD16HHF1 is "backwards" from "normal" FETs, a device like this will simply short out the power supply!  The only way to be absolutely sure that one has a genuine RD16HHF1 is to put it into a "component tester" - those inexpensive (<$20) devices that will identify practically anything - and see if its pin-out is correct and that its gate capacitance is in the 60-100pF area - and then try it in circuit.

The PD85004:

In perusing the catalogs I determined that a likely candidate device was the PD85004, made by ST Microdevices and available from a number of vendors such as Mouser Electronics.  This device, designed to operate from 13.8 volts, is rated to output several watts at 900 MHz, so it should surely be coaxed to work at HF, right?

This device is a bit more expensive than the original, in single quantities costing about $3.25 each as opposed to about $0.50 each for the BS170 - but the expense isn't very onerous, and it is probably cheaper than a genuine RF16HHF1 - and it is also rated for operation at 13.8 volts.

Figure 2:
The PD85004 mounted to an EvilBay SOT-89-4 carrier.  Short wire
leads go in holes 1, 2 and 3 to effectively turn it into a through-hole
device.  The heat sink was not yet added when this picture was
taken.  As described in the text, I eventually used a 10 ohm
resistor for the gate lead (position 1) to prevent circuit instability.
Click on the image for a slightly larger version.

One complication with the use of this device is that it is available only in a surface-mount package.  Fortunately, I had on hand some SOT-89-4 "carrier" boards (readily available on EvilBay - search for "SOT-89 adapter board") to which I soldered the device, effectively turning it into a leaded device that can be wired into the original FETs' board locations.  These boards cost anywhere from $0.03-$0.20 each, if you buy 10 or more - and that price often includes shipping!

To improve device dissipation a piece of copper flashing was carefully soldered to the tab of this device (which is the grounded source lead) after it was mounted to the carrier (see Figure 3.)  While the rated dissipation of this device is 6 watts, the mechanical layout of the Ultimate 3 beacon significantly limits the size of the heat sink as well as how much heat can be radiated/conducted to its surroundings.

Modifying the U3 for use with the PD85004:

Comment:

Earlier versions of the U3S specified a simple inductor ("L1") for the drain circuit of the power amplifier while later versions depict either a simple series inductor or a bifilar-wound transformer designated as "T1" - the latter being capable of somewhat more power with the original BS170.  My U3S is of the earlier version with "L1".  I've only tested it with the simple inductor, but this modification should work well with the "T1" configuration as well - but since it is untested, one should be particularly wary of instability.

Initially, I simply wired a PD85004 in place of a BS170 - but owing to the fact that the new device was designed to operate near 1 GHz - and that the layout of the U3 circuit board and interconnects didn't look to be particularly "VHF friendly" - so I expected that there could be some problems.  Before powering it up for the first time I turned the bias potentiometer all of the way down and pre-set my bench supply to current-limit at 500 milliamps - just in case I'd miswired something or I'd managed to turn the bias control all of the way up, instead.

Powering up the beacon and temporarily disabling transmit (easily done in WSPR mode by disconnecting the GPS antenna that is used for timing) I noted the current consumption - about 350 mA, much of that being the LCD's back light - and carefully adjusted the bias to cause a 100mA increase in current consumption.  With the antenna output of the beacon connected to a dummy load via a wattmeter I then reconnected the GPS antenna, readjusted the power supply current limiting and waited for the unit to come online and cycle through the various amateur bands while listening, in turn, to each frequency on a local receiver - using it as an oft-overlooked piece of useful test equipment that most amateur operators already own!

The result, not unexpected, was that I was able to get a power reading on each band, but on some bands - 160 through 40 meters - I heard a loud "hiss" +/- about 20 kHz from the transmit frequency instead of a CW note while the higher bands, 30 through 10 meters, sounded normal.  This just goes to show that at these frequencies this GHz-rated device may need some "taming" to prevent the apparent low-frequency instability and that a wattmeter alone is not necessarily useful for determining if an amplifier is working properly!

To tame the amplifier, I did several things:

• I installed a 0.1uF between the wiper of the bias adjustment potentiometer R5 and ground.
• I placed a 220 ohm resistor in parallel with R6 from the bias supply.  This, along with the added capacitor, helped "swamp" the drive signal and provide some lower impedance, low-frequency termination of the device's gate.
• I replaced the wire lead on the SOT-89 carrier that provided the gate connection with a 10 ohm resistor - pin "1" on the device carrier board in the pictures.  This added resistance helps to break up effects of spurious reactances that can cause the transistor to behave badly in-circuit.
• Update:  I decided wire in a series-connected 0.1uF capacitor and 1.5k resistor between the gate (pin 1) and drain (pin 3) to quell a suspected VHF/UHF instability under some conditions, manifested by the drain current changing slightly on some bands when I touched the grounded(!) heat sink.
• Update:  I had to take the U3 apart for another reason and while I was at it I connected a 470 ohm, 1/2 watt resistor across L1 - between V+ and the drain - for good measure.  See notes below about this change.

In testing with the dummy load I found that the amplifier section was now stable.

I then proceeded to carefully adjust the bias and watch the power meter.  Turning up the bias to several hundred milliamps I observed that I could get 2-3 watts of "clean" RF on on every band - but this much power was too great for the heat sink - especially in a "closed up" case - but had an "infinite" heat sink been possible I'm certain that I could have safely operated at this power level.  Monitoring the temperature of the heat sink I found that I could safely get about 1.5 watts out on 160-20 meters, dropping to about a watt on 10 meters, but erring on the side of caution I backed this off a bit to 0.5-0.75 watts on 10 meters which correlated with about 50mA of idle current.

Figure 3:  
The installed PD85004 with heat sink in the Ultimate 3S beacon
for initial testing.  A piece of copper was soldered to the tab of the
transistor to allow it to dissipate a couple watts of heat.  Unfortunately,
there isn't really an easy way to make the heat sink capable of dissipating
significantly more heat without mechanically complicating things.
Not visible in this picture are the added 0.1uF capacitor between the wiper
of R5 (the pot visible to the left of the transistor) and ground
and the 220 ohm resistor in parallel with R6.
  Note that the connections from two of the three original
BS170 positions were used to wire this transistor into the circuit.  Behind
the transistor is the "OCXO" version of the synthesizer and I could have
soldered the heat sink to its case to improve dissipation, but this may risk
reducing the OCXO's own thermal and frequency stability.
This picture was taken before the 10 ohm resistor was put in series
with the transistor's gate to quell instability, replacing the wire lead
at position "1" on the carrier and before the addition of the series
1.5k and 0.1uF capacitor between the gate and drain..
Again, the PA section of my beacon is wired to operate from 13.8 volts.
Click on the image for a larger version.

On the air testing:

My "main" HF antenna is a "lazy loop" of about 225 feet (approx. 70 meters) circumference at an average height of around 30 feet (about 10 meters) feed with 450 ohm window line with a 1:1 balun in the shack - designed to be connected to an antenna tuner.  Because I am not using the tuner with the U3 this means is my antenna it is not resonant (at 50 ohms) on any particular frequency, typically having a VSWR of greater than 5:1 on most bands.  While this may sound bad, the window line itself contributes negligible loss of its own and a reasonably-designed power amplifier should be able to tolerate such a mismatch.  I've run it this way for months with the BS170 finals without a problem and I've gotten reasonable signal reports.

When I connected the modified Ultimate 3S beacon to this antenna, everything worked fine - until I got to 40 meters, at which point I'd hear a loud "click" on the local receiver and the display would go blank.  Apparently, the bad (reactive) termination of the antenna caused the amplifier section to "take off" into some sort of mode of instability and somehow crash the beacon's processor.

Adding a "wee bit" of attenuation:

The work-around was to add an (approximately) 1.5dB resistive pad in series with output antenna connection.  Consisting of two 3.9 ohms resistors and a 220 ohm resistor in a "Tee" arrangement, this prevented the return loss as seen by the beacon from ever exceeding about 3dB, or a VSWR of about 6:1.  This little bit of padding reduced the transmit power only a fraction of an "S" unit, but with its added 3dB of return loss was sufficient to keep the amplifier stable on all of the bands.

Figure 4:
Typical "T" type resistive attenuator that can be useful for preventing
instability and/or damage to the final transistor in the event of a
poor match to 50 ohms.  These resistors were wired/mounted
at the RF output connector, after any low-pass filtering.
For Ra I used the standard values of 3.9 ohms and for Rb, 220 ohms,
resulting in approximately 1.5dB of attenuation.  For the 2.5 dB
attenuator on the WA7X beacon, Ra was 6.8 ohms and Rb was 180
ohms.  Neither of these sets of values are precise 50 ohm matches,
but they are more than "good enough"!  At up to a watt of RF, 1/4 watt
resistors may be used for Ra but a 1 watt resistor (or multiple,
lower-power resistors) should be used for
Rb so that it can tolerate very high mismatch conditions.

The addition of a bit of attenuation on a transmitter like this isn't necessarily a bad thing as it can offer a bit of protection - both in terms of VSWR and things like lightning strikes, offering both a DC discharge path and act as a bit of a "sponge" for induced spikes and excess power.  Even with this bit of attenuation I can safely coax about 1 watt of RF output on 160 through 20 meters, dropping to a bit over 0.5 watts on 10 meters - about 10dB better than I'd managed with a single BS170 on that band.  Because of the mismatch and commensurate losses I've currently set the WSPR beacon to report half this amount of power on some bands, but will increase it again when (?) I get around to putting up a multi-band matched antenna system.

The use of a resistive pad on the output of the transmitter had a precedent.  Upon installing another Ultimate 3S for the WA7X beacon at a remote cabin - this system operating exclusively on 10 meters - we discovered, the hard way, that the optional 5 watt amplifier (using an RD16HHF1)  didn't like it when the 10 meter vertical was temporarily detuned due to snow, causing a mismatch that resulted in the coincident failure of the output transistor.  In that case we added a 2.5 dB resistive pad (5 dB added return loss) to prevent the beacon from ever seeing worse than a 4.5:1 VSWR (even if the antenna connection were accidentally removed) and instead of 5 watts, the beacon is now operating at "2 watts" and is pretty "bullet-proof", reliability being very important for a remotely-controlled beacon at a remote location.

If that hadn't stopped the instability...

If the amplifier hadn't been adequately stabilized by the aforementioned modifications, there would have been two more things that I would have tried:

• (Now done - see note above) - Add a 220-470 ohm, 1/2 watt resistor across T1 (or L1), the output transformer (coil) between V+ and the output transistor(s) drain(s).  This resistor can help "Q-spoil" a low frequency resonance on the inductance of  L1 that may cause similar oscillations.  This sort of instability is quite common, often due to the fact that RF devices can have tremendous gain at very low frequencies and the interaction with the rather large amount of inductance of the coupling transformer.  My U3 is a bit older and uses "L1" which is simply an inductor between the drain of Q1-Q3 and V+.  If you have chosen a bifilar/trifilar transformer (e.g. "T1") then this resistor would be wired between V+ and the drain.
• (Already done - see note above) - Add a series 1k resistor and 0.1uF capacitor between the output transistor drain and gate.  This degenerative feedback will also help quell spurious oscillations.

Ultimately, I hope to connect the beacon to an antenna (perhaps a trapped vertical) that is resonant on at least most of the bands on which the beacon operates, but in the mean time, this seems to be working out pretty well.

Final comments:

I started out this blog entry with the mention of bands above 10 meters, which naturally brings up the question:  Will this same modification work on 6 meters and higher?

The answer is yes, probably.

While I can imagine that it should be possible to obtain, perhaps, 0.25-0.5 watts on 6 meters with this same device and using similar techniques, going up much higher in frequency and getting some RF power will probably require a bit of modification as the board layouts and interconnects start to get a bit "iffy" at VHF and higher, requiring special care to avoid excessive harmonic content and other spurious signals.

[End]

Stolen from ka7oei.blogspot.com