WSPR implements a protocol designed for probing potential radio wave propagation paths using low power transmissions. A set of algorithms encodes the sender's callsign, Maidenhead grid locator and transmitter power into a data packet that is transmitted by the sender at defined time intervals. WSPR Software at the receiver sites can decode these signals with a signal noise ratio (S/N) as low as -28db in a 2500Hz bandwidth and forward reception reports to a central database at WSRPnet.
The aim of this project was to build a dedicated WSPR Beacon transmitter that satifies the following criteria:
- use of low cost COTS parts
- low power consumption
- autonomous operation 24/7
- network interface
- frequency hopping capability
Previous exposure to Raspberry Pi boards made the selection of the computational engine easy. The Raspberry Pi is a low cost credit card sized computer with many of the standard interfaces of a larger system and user programmable I/O pins. User programmable I/O pins of the RPi are used to control the DDS signal generator and lowpass filter board.
The choice for the DDS was an AD9850 based module sourced online from eBay. Using the AD9850 serial data input made it an easy task to interface with Raspberry Pi.
A HF Projects LPF100 relay controlled lowpass filter board, a left-over from an earlier project, is utilised to provide filtering of the RF output signal. An ULN2003 seven darlington arrays chip provides the interface between the RPi and lowpass filter.
A Humble Pi prototyping board mounts the DDS module, ULN2003 interface, RF buffer amplifier and power supply onto the RPi.