The Beginnings Of A Simple Transmitter For Amateur Digital Modes

With the increased capabilities of laptops, desktop computers, and even embedded systems, amateur digital modes are typically generated via software. This includes modes like WSPR and JT-65 as well as classic modes like RTTY.

For the last couple of months I’ve been working on a low-power transmitter intended to be used with these modes. At its most basic the problem is a simple one. The digital signal is generated at audio frequencies and the transmitter simply needs to translate that signal to the desired operating frequency.

In practice, the problem is a little more complicated. I considered a number of different ways to accomplish it but ultimately fell back on an old standard, a phasing style SSB transmitter. While it’s still a work in progress, I’ve gotten far enough along that I thought it was time I started documenting it.

Initial Design

The approach I decided upon is split into two parts; signal generation and frequency translation. My intention is to have the baseband audio signal generated in software using one of the generally available packages that produce a dual-channel I/Q audio stream. This simplifies the hardware design by eliminating the need for the phase shift network normally found in phasing style SSB transmitters.

The hardware for frequency translation is based upon W1TAG’s Low Frequency Phasing Exciter. The first time I saw this method used was in a September 1990 article by PA0DEN (van Graas, 1990), who referred to it as “The Fourth Method” of generating SSB signals. W1TAG’s design was originally intended for use in the LF bands but it wasn’t difficult to update for use in the HF bands.

I started by replacing the 4 4066 SPST FET switches in the original design with a single CBT3253 4-to-1 multiplexer. I also replaced the external oscillator and ring counter used to generate the switch waveforms with a single Si5351 that has clocks 0 and 1 configured to operate at the same frequency but 90o out of phase. Since I intend for this hardware to only function as a SSB exciter I eliminated the transistor output amplifier. Finally, I replaced the TL072/074 op-amps with AD8051/8052 capable of operating at HF. The updated circuit is shown in the figures below.


Phasing Transmitter Phase Shifter

Phase Transmitter Phase Shifter

Phasing Transmitter Mixer

Phasing Transmitter Mixer

You may notice there is no controller shown for the Si5351. I intend to include one eventually but for now I’m just using the Si5351 USB controller I described in one of my previous posts.

Testing The Hardware

For initial testing I used G3PLX’s software IQ transmitter. The Si5351 was tuned to a convenient frequency. The software’s Transmit mode was set to USB and modulation to 1 kHz tone as shown below. These settings produced a 1 kHz tone of approximately 2V peak-to-peak at the soundcard output.


SDRTx Software Configuration

With the soundcard connected to the transmitter input I could hear (and see, using HDSDR) the expected tone approximately 1 kHz above the Si5351’s tuned frequency.

Looks Good So Far

So that’s where things stand. It’s by no means ready for prime time. I’m getting about 20 mW into a 50 ohm load. Even at that low level, without a filter on the output I wouldn’t transmit anywhere but into a dummy load and I’m on the lookout for a good metal case to provide some shielding. Even so, it’s a nice start and I’ll post updates as it progresses.

References

Van Graas, D. H. “The Fourth Method: Generating and Detecting SSB Signals.” QEX (Sept 1990): 7-11.

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