Back in 1999, not too long after the first appearance of PSK31, I decided that I wanted to construct a beacon transmitter that would operate using this mode but at the time the only practical means of generating PSK31 was with a computer, a sound card and an SSB transmitter. Not wanting to tie up that much gear for this purpose I set about to use the then-popular PIC16C84 microcontroller which was popular among the homebrew builders.
At the time the AM broadcast band had (relatively) recently been expanded up to 1705 kHz but very few stations occupied the new 1605-1705 kHz segment. In perusing the FCC rules I noted that FCC part 15 section 219 had been modified to allow low-power experimental operation in this new segment and I decided that with the lack of activity in this frequency range that it was time to put up a "MedFER"(Medium Frequency Experimental Radio) beacon.
The balanced modulator method
Upon investigating various methods of producing a PSK31 signal I experimented with the generation of a bipolar baseband signal that could be applied directly to a balanced mixer. While this method worked well it had the problem than it required that all following stages be linear.
A diagram of the prototype of that transmitter may be seen in Figure 1. For this transmitter a crystal-controlled oscillator is constructed using two transistors (Q1, Q2) and the output is buffered by U3, a 74HC00 quad NAND gate. The frequency used for this circuit was unimportant as it was a "proof of concept" and I (think) used an NTSC "Colorburst" crystal which operated around 3.58 MHz. Following the first U3 NAND buffer the remaining sections are used to provide a two phase signal with the output split 180 degrees which fed to a very simple balanced modulator consisting of just two diodes, a few capacitors and resistors.
To provide modulation, a PIC16C84 was used to provide a 32-step staircase modulation using PWM techniques. This PWM output, using a frequency of 1 kHz which is exactly 32 times that of PSK31's 31.25 Hz baseband frequency, is then filtered with a two stage R/C low-pass filter network consisting first of a 4.7k resistor and 0.1uF capacitor followed by a second stage with a much higher impedance consisting of a 150k resistor and 0.033uF capacitor. The result of this filtering was that the majority of the 1kHz energy was removed, leaving a fairly clean 31.25 Hz baseband signal.
This signal was then buffered and split into two signals, one of them inverted, and these were applied differentially via simple R/C networks across the two diodes: If the baseband signal were to go positive, the other side would go negative and turn on one diode, but it if were to swing the other way, the other diode - fed with the RF signal that was 180 degrees out of phase with the first - would be turned on. The end result was a fairly nice, linear BPSK envelope and baseband waveform when viewed on a receiver with an oscilloscope.
While it worked to prove a concept, this signal has a few shortcomings. First, the RF signal from the oscillator and buffer is not likely to have a precise 50% duty cycle which means that a bit more RF energy would be available in one phase than the other, resulting in a somewhat "lopsided" BPSK amplitude envelope. The other problem has to do with two NAND gates being used to provide the 180 degree phase shift in that the addition of the inverting gate has a few 10s of nanoseconds of propagation delay. While this doesn't sound like much, it does amount to a significant number of degrees of RF phase even at low HF frequencies and the end result is that the "Phase Diagram"(see Figure 2) is slightly distorted.
While I could have gotten this method to work (e.g. used a bandpass/lowpass filter to get a nice, clean sine wave and a transformer to get the 180 degree phase shift) it does how a down side: All subsequent stages would need to be linear. While not a great technical problem, it did mean that for the LowFER transmitter, which has a 100 milliwatt input power limit, a linear final amplifier would have at best around 75% efficiency which would mean that I'd lose about 1dB of signal. While this may not sound like much I figured that I could do better with a more efficient amplifier scheme.
The Amplitude Modulator Method
Having proven the ability to produce a reasonable quality PSK31 waveform with a lowly PIC I decided to try a different approach: Apply high-level modulation to the output amplifier stage. What's more, this amplifier stage need not be linear at all: It could be a conventional Class C stage which would boost the efficiency to something around 80%, but I decided on going a step farther to a Class-E amplifier.
I first became aware of the Class-E amplifier more than a decade earlier when my friend Mark, WB7CAK, designed one for his LowFER (Low Frequency Experimental Radio) beacon that operated in the 160-190 kHz "experimenter's" band authorized by part 217 of FCC part 15, and as with the MedFER operation, the input power was also limited. After a bit of number crunching and fiddling on the workbench Mark came up with a simple circuit and a few basic equations that described how such an amplifier could be built, publishing an article in the Western Update - a small publication tailor for both LowFER and MedFER enthusiasts. Because this publication may be difficult to find, I have reproduced it, with permission from the author, and it may be found here: (Link).
While the maths behind the derivation of the operation of a Class-E amplifier can be somewhat involved, the concept is quite simple: When the drive signal to the transistor - typically a power MOSFET at LowFER frequencies - goes low the transistor shuts off and it does this quickly so that transistor spends as little time as possible "partially" conducting. When this happens, the voltage on the drain rises at is pulled up by the choke in the drain, but then falls again due the "ringing" of a resonant circuit on the output tank. Precisely at the time that the drain voltage hits zero, the output transistor is switched back on. The result of these two events is that the FET is either completely on or off which means that little or no power is dissipated in it and when it is turned back on, one does so when the voltage is zero, anyway, practically eliminating any losses that would occur at that instant due to the resistance of the FET and the tank circuits being "shorted out".
The result of all of this is an RF amplifier that (exclusive of the drive signal) is demonstrably capable of 95%-98% efficiency. In the MedFER and LowFER world this means that with our power level being limited on the input, we will have, for all practical purposes, all of our input power at our disposal rather than, say, 70-80% of it as would be the case with almost any other amplifier type.
The obvious problem with a Class-E amplifier is that the drive signal must be a square-shaped wave which means that amplitude modulation of that drive signal is not easily managed if efficiency is to be maintained. What one can do is to modulate the power supply feeding this amplifier instead.
Remembering that a PSK31 signal consists of two parts - the amplitude modulation and the phase shift - we can split these two signals in the modulator. The first part, amplitude modulation, may be done by modulating the supply voltage of the output amplifier stage. The second part, phase modulation, may also be done early in the process simply by flipping the phase of the RF signal under computer control. In order to keep the signal "clean" all we really need to do is to time the flipping of the phase with the amplitude being brought to zero so that we don't transmit the broadband "click" that would otherwise occur when we did this abrupt phase shift: The schematic of the transmitter is depicted in Figure 3.
In this circuit the frequency-determining crystal oscillator operates at four times the transmitter frequency, or around 6.8 MHz in the case of the MedFER transmitter. During construction it was observed that at around 1.7 MHz it was was easier to achieve Class-E operation at this power level with a drive waveform that had a 25% duty cycle so a 74HC4017 counter was used, wired as a divide-by-four but giving two 25% duty cycle outputs, 180 degrees apart. To select which of these signals were to be used a simple MUX was constructed using four NAND gates, this time being designed so that the same amount of propagation delay would occur during either phase to eliminate the upside-down "Vee" seen in Figure 2.
The PWM signal was generated using simple R/C filtering in the same way as it was for the balanced modulator circuit, but this time op amps were used to set the offset and gain (or "span") so that the baseband waveform could be precisely adjusted in both amplitude and so that when the baseband signal went to zero, the output power from the Class-E circuit would as well, compensating for the voltage offset of the series modulating transistor, emitter-follower Q4. The output transistor, Q3, is a low-power MOSFET wired into a simple L/C "tank" circuit that is tuned to result in the coincidence of the zero crossing of the drain voltage and the transistor being turned back on by the 25% duty cycle drive signal. Multiple taps are provided on the tank coil making it easy to set both the output power and match it appropriately to the load.
For modulation the PIC produces a semi-sine waveform that looks very similar to one "cycle" on the double-frequency output of a full-wave diode rectifier and when this waveform amplitude is taken to "zero" another output of the PIC causes a phase switch to occur. It is in this way that the BPSK modulation is broken into two parts - the phase change and the modulation envelope - and we are able to use a non-linear amplifier for the output.
After constructing this I later learned that a similar scheme was applied to some of the earlier OSCAR amateur satellites. In order conserve precious power, the linear transponders were constructed using the "HELAPS"(High Efficiency Linear Amplifier using Parametric Synthesis) system where the amplitude and phase components of multiple signals in the satellite's passband were converted into their phase an amplitude components allowing both energy-saving class-C RF amplifiers and DC-DC switching converters to be used, the end result being a faithful, amplified reproduction of the input signal with a lower power budget that would have otherwise been required.
Where is it now?
This beacon was mounted in its enclosure on the roof of my house in 1999, using a rather large loading coil (see Figure 6) to match its output impedance to the top-hatted 3 meter vertical antenna - and it is there to this day. While not regularly used, it still works - provided that the tuning of the loading coil is checked before use! Since the beacon was constructed more stations have taken to the air in the "new" AM segement, but its operating frequency - nominally 1704.965 kHz - is just below the top edge of the band, as far away from any QRM as possible.
In the past the BPSK31 signal from this beacon was copied during the daylight at a distance of 75 air miles (approx. 120km) and it had been copied in various places in the western U.S. at night. This beacon has since been modified to so that it may be on-off keyed so that "QRSS3"(low-speed Morse with a 3 second "dit") could be sent in addition to PSK31 allowing even greater distances to be spanned under more diverse conditions.
I haven't done much with the code for this transmitter other than add a few features when it was ported to the (then) newer PIC16F84. Needless to say, there are more modern devices available that contain hardware that would have simplified the design such as that to generate a much higher frequency and higher resolution PWM signal and perhaps one day I'll investigate their use.
For more information on this and related projects - including schematics, various applications, more pictures and some source code, visit the "CT Medfer Beacon" web page - link and related pages linked from there.
[End]
At the time the AM broadcast band had (relatively) recently been expanded up to 1705 kHz but very few stations occupied the new 1605-1705 kHz segment. In perusing the FCC rules I noted that FCC part 15 section 219 had been modified to allow low-power experimental operation in this new segment and I decided that with the lack of activity in this frequency range that it was time to put up a "MedFER"(Medium Frequency Experimental Radio) beacon.
 |
| Figure 1: The "Balanced Modulator" (Baseband) version of the PSK31 transmitter/exciter. Built to test a concept, it has a few flaws, but it did work. Click on the image for a larger version. |
The balanced modulator method
Upon investigating various methods of producing a PSK31 signal I experimented with the generation of a bipolar baseband signal that could be applied directly to a balanced mixer. While this method worked well it had the problem than it required that all following stages be linear.
A diagram of the prototype of that transmitter may be seen in Figure 1. For this transmitter a crystal-controlled oscillator is constructed using two transistors (Q1, Q2) and the output is buffered by U3, a 74HC00 quad NAND gate. The frequency used for this circuit was unimportant as it was a "proof of concept" and I (think) used an NTSC "Colorburst" crystal which operated around 3.58 MHz. Following the first U3 NAND buffer the remaining sections are used to provide a two phase signal with the output split 180 degrees which fed to a very simple balanced modulator consisting of just two diodes, a few capacitors and resistors.
To provide modulation, a PIC16C84 was used to provide a 32-step staircase modulation using PWM techniques. This PWM output, using a frequency of 1 kHz which is exactly 32 times that of PSK31's 31.25 Hz baseband frequency, is then filtered with a two stage R/C low-pass filter network consisting first of a 4.7k resistor and 0.1uF capacitor followed by a second stage with a much higher impedance consisting of a 150k resistor and 0.033uF capacitor. The result of this filtering was that the majority of the 1kHz energy was removed, leaving a fairly clean 31.25 Hz baseband signal.
 |
| Figure 2: Phase diagram of balanced modulator circuit in Figure 1. The propagation delay of the gates result in a rather imprecise 180 degree phase shift causing the upside-down "Vee" in the phase diagram. |
While it worked to prove a concept, this signal has a few shortcomings. First, the RF signal from the oscillator and buffer is not likely to have a precise 50% duty cycle which means that a bit more RF energy would be available in one phase than the other, resulting in a somewhat "lopsided" BPSK amplitude envelope. The other problem has to do with two NAND gates being used to provide the 180 degree phase shift in that the addition of the inverting gate has a few 10s of nanoseconds of propagation delay. While this doesn't sound like much, it does amount to a significant number of degrees of RF phase even at low HF frequencies and the end result is that the "Phase Diagram"(see Figure 2) is slightly distorted.
While I could have gotten this method to work (e.g. used a bandpass/lowpass filter to get a nice, clean sine wave and a transformer to get the 180 degree phase shift) it does how a down side: All subsequent stages would need to be linear. While not a great technical problem, it did mean that for the LowFER transmitter, which has a 100 milliwatt input power limit, a linear final amplifier would have at best around 75% efficiency which would mean that I'd lose about 1dB of signal. While this may not sound like much I figured that I could do better with a more efficient amplifier scheme.
The Amplitude Modulator Method
Having proven the ability to produce a reasonable quality PSK31 waveform with a lowly PIC I decided to try a different approach: Apply high-level modulation to the output amplifier stage. What's more, this amplifier stage need not be linear at all: It could be a conventional Class C stage which would boost the efficiency to something around 80%, but I decided on going a step farther to a Class-E amplifier.
 |
| Figure 3: Diagram of the "AM" version of the transmitter using separate amplitude and phase modulation paths, allowing a non-linear but highly efficient Class-E output amplifier to be used. Click on the image for a larger version. |
While the maths behind the derivation of the operation of a Class-E amplifier can be somewhat involved, the concept is quite simple: When the drive signal to the transistor - typically a power MOSFET at LowFER frequencies - goes low the transistor shuts off and it does this quickly so that transistor spends as little time as possible "partially" conducting. When this happens, the voltage on the drain rises at is pulled up by the choke in the drain, but then falls again due the "ringing" of a resonant circuit on the output tank. Precisely at the time that the drain voltage hits zero, the output transistor is switched back on. The result of these two events is that the FET is either completely on or off which means that little or no power is dissipated in it and when it is turned back on, one does so when the voltage is zero, anyway, practically eliminating any losses that would occur at that instant due to the resistance of the FET and the tank circuits being "shorted out".
 |
| Figure 4: The constructed MedFER beacon transmitter, built on the bottom of a weather resistant outdoor enclosure to be mounted at the base of the antenna. |
The obvious problem with a Class-E amplifier is that the drive signal must be a square-shaped wave which means that amplitude modulation of that drive signal is not easily managed if efficiency is to be maintained. What one can do is to modulate the power supply feeding this amplifier instead.
Remembering that a PSK31 signal consists of two parts - the amplitude modulation and the phase shift - we can split these two signals in the modulator. The first part, amplitude modulation, may be done by modulating the supply voltage of the output amplifier stage. The second part, phase modulation, may also be done early in the process simply by flipping the phase of the RF signal under computer control. In order to keep the signal "clean" all we really need to do is to time the flipping of the phase with the amplitude being brought to zero so that we don't transmit the broadband "click" that would otherwise occur when we did this abrupt phase shift: The schematic of the transmitter is depicted in Figure 3.
 |
| Figure 5: The phase diagram of the signal produced by the "Amplitude Modulator" MedFER PSK31 beacon transmitter. The phase shift is precise and the intermodulation products are well within the tolernaces dictated by good operating practice. |
The PWM signal was generated using simple R/C filtering in the same way as it was for the balanced modulator circuit, but this time op amps were used to set the offset and gain (or "span") so that the baseband waveform could be precisely adjusted in both amplitude and so that when the baseband signal went to zero, the output power from the Class-E circuit would as well, compensating for the voltage offset of the series modulating transistor, emitter-follower Q4. The output transistor, Q3, is a low-power MOSFET wired into a simple L/C "tank" circuit that is tuned to result in the coincidence of the zero crossing of the drain voltage and the transistor being turned back on by the 25% duty cycle drive signal. Multiple taps are provided on the tank coil making it easy to set both the output power and match it appropriately to the load.
 |
| Figure 6: Loading coil used to match the transmitter output to the feedpoint impedance. This coil is wound using 3/8" copper tubing and uses a variometer inside the coil to provide a low-loss means of adjusting the inductance. |
For modulation the PIC produces a semi-sine waveform that looks very similar to one "cycle" on the double-frequency output of a full-wave diode rectifier and when this waveform amplitude is taken to "zero" another output of the PIC causes a phase switch to occur. It is in this way that the BPSK modulation is broken into two parts - the phase change and the modulation envelope - and we are able to use a non-linear amplifier for the output.
After constructing this I later learned that a similar scheme was applied to some of the earlier OSCAR amateur satellites. In order conserve precious power, the linear transponders were constructed using the "HELAPS"(High Efficiency Linear Amplifier using Parametric Synthesis) system where the amplitude and phase components of multiple signals in the satellite's passband were converted into their phase an amplitude components allowing both energy-saving class-C RF amplifiers and DC-DC switching converters to be used, the end result being a faithful, amplified reproduction of the input signal with a lower power budget that would have otherwise been required.
Where is it now?
This beacon was mounted in its enclosure on the roof of my house in 1999, using a rather large loading coil (see Figure 6) to match its output impedance to the top-hatted 3 meter vertical antenna - and it is there to this day. While not regularly used, it still works - provided that the tuning of the loading coil is checked before use! Since the beacon was constructed more stations have taken to the air in the "new" AM segement, but its operating frequency - nominally 1704.965 kHz - is just below the top edge of the band, as far away from any QRM as possible.
In the past the BPSK31 signal from this beacon was copied during the daylight at a distance of 75 air miles (approx. 120km) and it had been copied in various places in the western U.S. at night. This beacon has since been modified to so that it may be on-off keyed so that "QRSS3"(low-speed Morse with a 3 second "dit") could be sent in addition to PSK31 allowing even greater distances to be spanned under more diverse conditions.
I haven't done much with the code for this transmitter other than add a few features when it was ported to the (then) newer PIC16F84. Needless to say, there are more modern devices available that contain hardware that would have simplified the design such as that to generate a much higher frequency and higher resolution PWM signal and perhaps one day I'll investigate their use.
For more information on this and related projects - including schematics, various applications, more pictures and some source code, visit the "CT Medfer Beacon" web page - link and related pages linked from there.
[End]