Ganymede M0IFA: Let's play home made WSPR

Thursday 4 June 2020

Let's play home made WSPR

A couple of years ago I wrote an Arduino sketch to generate WSPR on 7040.1kHz. This could be loaded on my general purpose Si5351 signal generator (see this blog, 2018 August) which has three outputs (only one CLK0 is used), a rotary encoder for frequency and/or any other functions, and an OLED display. The design is straightforward,

It also includes a Real Time Clock DS3231 chip on the I2C bus to give the accurate timing required by WSPR and here is the DATE_TIME_SET sketch to set the date/time.

To drive the display I use the superb library u8g2 and an Oled header I wrote which defines some functions like 'display message' or 'number' in different sizes, or draw a 'bar graph', etc Using this header gives all my projects a uniform display GUI. The frequency can be adjusted over the narrow 40m WSPR band, initially it is set to mid-band at 7040.1kHz. The WSPR signals are encoded using a WsprMessage class library.

Back to the WSPR hardware.
The SIGGEN output is quite low, around +7dBm (5mW, 0.5Vrms) or so and not enough, unless you have a superb antenna system, to transmit WSPR. At least +25dBm (3-400mW, 4-5Vrms) is needed. Come-in the two stage amplifier I described below. This has a gain of +40dB so needs an input of around 10-20mV for full output of around 375mW.

So I have used a small attenuator to bring the SIGGEN output down to the amplifier input level.

The Si5351 SIGGEN produces a square wave output with strong harmonics, so a good Low Pass Filter is needed to feed the antenna and not radiate interference. The set up then becomes,

I checked this out by receiving the signals on my ELAD SDR tuned to 7038.6kHz USB with the audio fed to the WSJT software, which decoded it very well. (I can run this either using the SDR software FDM-SW2 -> VAC -> WSJT under Windows using Parallels on the Mac or directly using WSJT on MacOS with data outputs from the ELAD SDR, which has a built-in soundcard).