In a previous blog posting I have demonstrated an extremely simple transmitter / keyer implementation on the propeller platform. The next logical step was to build a receiver to go with it.
What I have implemented is a very simple direct conversion receiver based on the NE602 Gilbert Cell mixer and a LM386 audio amplifier. The circuit is based on the famous sudden receiver design from the Rev George Dobbs G3RJV many years ago and looks something like this:
My implementation removes all the tuned circuit stuff from pins 6 and 7 of the NE612 (NE602 in my circuit) and injects the RF generated by the propeller via a blocking capacitor directly into pin 6 to set the receive frequency.
Here is a photograph of the lash-up with the receiver built manhattan-style on a little 5x7 cm PCB scrap.
Right now I am sitting here listening to a 75 metre net running on 3.98 MHz LSB (The Oregon Emergency Net).
Obviously, I have a lot of work to do on user interface code...
Now as you might well imagine, there are a number of problems with playing at RF frequencies with bits of wire lashing all the bits and pieces together stuck into protoboards. For example, I had to move the RF pin as far from the encoder pins as I could in order to not confuse the encoder code. The frequency kept changing on its own until I turned off the RF. There is a fair amount of "hash" in the receiver audio being generated by the LCD controller chip. But as far as a proof-of-concept impelementation, I could not be happier.
With this success, I plan to put together a complete (simple) all-band transceiver using a propeller as the controller as well as RF generation component for both the transmitter oscillator as well as the local oscillator for the receiver.