Raspberry Pi WSPR Transmitter

Sometimes several things come together that make a really easy project...

Having read several copies of the Official Raspberry Pi Magazine, I was intrigued that one of the issues this magazine actually gave away a Raspberry Pi Zero on the front cover in the same way that computer magazines in the 90's used to have CD-Roms and floppies on the front!

The current incarnation of the Raspberry Pi Zero W has onboard WiFi and Bluetooth networking and with the addition of a Raspberry Pi Stem becomes a complete OTG (On the Go) USB stick - ideal for working away in Lancashire without the need to cart a mouse/keyboard/monitor up to your hotel room!  You can even get a 3D printed case online for a few pounds...

Then I found WsprryPi - a WSPR transmitter that uses the Raspberry Pi's onboard frequency generator to transmit WSPR and uses NTP (Network Time Protocol) to discipline the oscillator frequency - all that is needed is an external Low-Pass Filter and you have a little transmitter.

Enter Language Spy with a neat little filter module.  I managed to get one for 17m (another one of my favourite bands) before the stock ran out.  

The Raspberry Pi output pin is a square-wave switched from 0V to 3.3V and therefore is approximately 1.65V RMS, there are obviously losses in the low-pass filter as the higher harmonics that make up the square wave are lost, however I measured the output at approximately 0.9V(RMS) into a 50Ω load

This works out to approximately 10 mW (or 10 dBm) so QQRP.

Unfortunately a recent upgrade to Windows 10 appears to have broken the raspberrypi.local shortcut, so whilst the board still works, the OTG capability doesn't and a mouse and keyboard are needed once more.