Tuesday, February 28, 2012

Propeller Receiver

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.


  1. Now that is very nice! I have been trying to work some some of the cheap controller boards into ham projects. Inspiring!!!

    Mike KD5KXF

  2. I guess I am a little late looking at this, but can you provide an overhead shot of the circuit. I can only see one inductor and it looks like a torodial and I am assuming it is L2. Is that correct? Next question is what size wire did you use for the inductors? And where is L1 in the photo? I am assuming it is an air-core?

    Anyways, I am a novice and this is my first receiver build and would really like it to work out. Any help will be appreciated. Thank KG5ARI

  3. Hi Aaron,

    No in this lash-up L1 is an axial-lead lump sum device. It just looks like a resistor. It is what I had in the junque bin. I would not recommend it to you however as the Q is pretty low. I would probably either air-wind or us a toroid if I was to do it again. However, it worked and allowed me to prove the proof of concept.

    The wire was 24 gauge, again what I had in the bin. I use what I have and measure the result with an LCR metre.

    I apologize for the late response. How did your build work out? it is a trivially simple circuit and lots of fun to build. Here is a pointer to a set of build instructions for a kit that has the schematic and board layout if you would find that helpful. http://www.gqrp.com/suddenbuildingyourkitbooklet40m.pdf

    Here may be a better example for you as it is the instructions to a build-a-thon at FDIM (Four Days In May) hamfest in 2009. Great instructions here.


    Best of luck and let me know if I can assist in any way. I will try to be more responsive. You can reach me directly by email at ko7m at arrl.net.