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.


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

Controlling The Si5351 Signal Generator From HDSDR

I’ve mentioned a few times that my preferred interface for software-defined radios (SDRs) is HDSDR. I like the way it’s layed out and I especially like how it allows for control of different hardware by simply dropping in a new control DLL.

Ever since I built the previously described Si5351 signal generator, it’s been on my list of things-to-do to write an HDSDR DLL to control it. That only made sense since the whole reason for building the signal generator was to use it as a source when experimenting with various receiver designs. And since I recently received an email asking if just such a DLL was available, I decided it was time to do something about it.

So What Is This Hardware Interface You Mention?

As described in the HDSDR FAQ, HDSDR communicates with hardware through a control module referred to as the External Input Output Dynamic Link Library (ExtIO-DLL). The nice thing about this is the interface for the ExtIO DLL is publicly available. If you have a new piece of hardware you’d like to use with HDSDR, just write a DLL that provides the proper interface, drop it into the HDSDR program directory, and you’re good to go.

Writing the DLL…

If you’ve ever written a DLL for the Windows operating system you won’t have any trouble producing an ExtIO-DLL. The interface description can be found in the specifications available on either the original Winrad page or I2PHD’s WeakSignals page. Of 19 possible entry points, only eight (InitHW, OpenHW, StartHW, StopHW, CloseHW, SetHWLO, GetStatus, and SetCallback) are required.

I had already written a Python library to control the Si5351 so this was the logical place to start. I ported the routines to C and integrated them as appropriate within an ExtIO-DLL project skeleton. Since the required entry points are sufficient to control the Si5351, they’re the only ones I implemented. The source code and precompiled binaries are available from the GitHub repository as a Visual Studio 2008 project. If you’re going to rebuild from souce you’ll also need the libusb-win32 library.

And Using The Hardware

The signal generator uses an Adafruit Trinket with appropriate firmware as a USB-to-I2C interface for the Si5351. When you plug the signal generator into the USB port the first time, Windows will attempt to load a driver once the Trinket gets past its bootloader. The driver can be obtained as part of the original I2CStick project upon which the Trinket firmware is based. Download the zip, extract the driver, and install it as you normally would. The file includes versions for both 32 and 64 bit but I’ve only tested the 64 bit version on Win7.

At this point, drop the ExtIO-DLL from the Visual Studio project (it can be found in the Release or Debug directory, depending upon which you want) into the HDSDR program directory. Start HDSDR as normal and choose the Extio-Si5351 DLL when prompted.


With the DLL as written, the signal generator will produce I and Q signals at the HDSDR local oscillator (LO) frequency on the Si5351 clocks 0 and 1. Since there’s only control for one frequency in the Extio-DLL interface I didn’t bother with clock 2.

Also, even though you can go down to a 0 Hz LO in HDSDR, the Si5351 won’t work that low. It bottoms out somewhere between 1 and 2 MHz so if you’re going lower than that you’ll need some sort of divider circuit.

Finally, while this project was written for the Windows operating system, it could be modified to work under Linux with Wine. This is actually the next improvement I’m looking at since, at least for me. using HDSDR with a Linux native library to control the hardware would definitely be the best of both worlds.

An S/PDIF Sound Card Using The PCM2906

When S/PDIF became available in the Teensy Audio Library I thought this might be the solution to ground loop problems I’d been having when interfacing projects to my PC. However, I quickly realized I didn’t have any sound cards with an S/PDIF interface.

In the belief that I’d rather build than buy I decided to update one of my previous projects, a PCM2904 based sound card, to include an S/PDIF interface. The update was a cinch because TI has a pin-for-pin replacement for the PCM2904 (the PCM2906) with an S/PDIF interface. All I had to do was replace the audio jacks with fiber-optic transmitter/receivers connected to the appropriate pins.

The resulting schematic and prototype are shown below. (If you’re interested, the boards are available through OSH Park)

SPDIF Sound Card Schematic

SPDIF Sound Card Prototype

There’s not a lot to it. It’s simply the reference circuit from the PCM2906 data sheet with portions I don’t expect to use eliminated.

Testing The Interface

To test the interface I looped the transmitted fiber optic signal back to the receiver. When plugged into my Linux laptop it registered as a Texas Instruments PCM2906C Audio Codec as shown below.

Bus 004 Device 003: ID 058f:6366 Alcor Micro Corp. Multi Flash Reader
Bus 004 Device 002: ID 8087:0024 Intel Corp. Integrated Rate Matching Hub
Bus 004 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 003 Device 004: ID 1bcf:2c02 Sunplus Innovation Technology Inc. 
Bus 003 Device 003: ID 413c:3016 Dell Computer Corp. Optical 5-Button Wheel Mouse
Bus 003 Device 002: ID 8087:0024 Intel Corp. Integrated Rate Matching Hub
Bus 003 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 001 Device 013: ID 08bb:29c6 Texas Instruments PCM2906C Audio CODEC
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

Starting up the PulseAudio Volume Control and playing back a file using VLC, I could see the input and output audio levels varying with the signal from VLC.

If you want to actually hear the looped back audio you can use the PulseAudio loopback module.

  1. Open a terminal and execute the command shown below. The result will be a list of names associated with alsa modules, sources, and sinks.

    pactl list | grep Name:
  2. Look for the alsa analog stereo output to identify the sink. On my system it takes the form alsa_output.pci-0000_00_1b.0.analog-stereo.

  3. Look for the PCM2906 input to identify the source. Ony my system it took the form alsa_input.usb-BurrBrown_from_Texas_Instruments_USB_AUDIO_CODEC-00.analog-stereo.

  4. Open a terminal and execute the command:

    pactl load-module module-loopback source=<source_name> sink=<sink_name>

    where <source_name> and <sink_name> are those identified above. Please note the source and sink names are case sensitive.

You should now hear the looped-back audio through your system speakers.

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