Tuesday, July 10, 2012

Prop Controlled Receiver - Success

In the past I have blogged about the QRSS, WSPR, and OPERA Beacons that I have created with the Propeller Microprocessor (Prop). The Processor generates the RF signal that is passed through a LowPass Filter to remove Harmonic, and then connected to an Antenna. For some experiments a Homebrew Power amplifier was used to increase the processor generated RF signal from 7 milliwatts to 150 milliwatts. I have received many reports (or Spots) from many stations around the world. I think the farthest was from Australia at about 12,000 miles. I consider my Propeller Beacon a fun and very successful project.

And Now,  . . .

I have been playing with the idea and trying to receive signals with the Propeller. A simple direct conversion receiver using a SA612 mixer would require a Local Oscillator at the received frequency. The Propeller can generate RF frequencies from; 0 to 128MHz, with an amplitude of about 3Volts Peak-to-Peak. Which is a little more than needed by the SA612, but it can be attenuated with simple resisters if needed.

My original plan was to construct a simple direct conversion receiver daughter board for the Prop. But, Jeff - KO7M pointed out that the N3ZI Double Conversion Receiver using two of the same SA612 chips, was available as a kit (this was actually several months ago). I put the kit on order, and it has been setting around waiting for my User Interface (UI) project to be completed (or at least working). The receiver kit needs two RF signals, one for the VFO and one for the BFO (about 11.055MHz). The BFO is normally supplied by a Crystal. I plan to replace the BFO Crystal with another RF signal from the Prop.

For the High Band (above IF) and Upper Side Band, the injected frequency is the BFO frequency (IF) plus the intended (displayed) receive frequency. For example, for USB: the VFO = BFO + Received Freq, for LSB: VFO = BFO - Received Freq.

For the Low Band (below IF) and Lower Side Band, the injected frequency is the BFO frequency (IF) minus the intended (displayed) receive frequency. For example, for USB: VFO = BFO - Received Freq, for LSB: the VFO = BFO + Received Freq. (I think that is correct?)

Obvious the VFO should be adjustable so the receiver can be tuned to the desired received frequency. But, maybe not obvious, is that small adjustment of the BFO can be used to adjust the band width of the received signal. In both cases a handy way of adjusting either or both frequencies is necessary. And that is where my UI comes in, it has knobs and buttons that can be programmed to control two Prop RF signals which will be supplied to the receiver.

Note: Sadly, I think the N3ZI Receiver Kit is no longer available.

Prop Controlled Receiver - Left
Prop Transmitter - Right
Today's Success !!

I have one Prop running ko7m's Keyer Program and another controlling the double conversion receiver. The test distant between the transmitter and receiver was only about 12 feet, but it works !

With the receiver connected to an antenna, I have received other signals with this set up; WWV and other Amateur stations.

The Receiver is mounted under the Prop on the left, the two SMA coax cables connect the VFO and BFO. Two of the corner holes of the receiver match the hole pattern of the Prop board (nice). The Keyer Transmitter is on the Right.

This weekend at Salmoncon, Jeff and I will demonstrate some of our Prop projects. Maybe, we will have time attempt to do real QRP DX'ing with our Props.

Many more experiments and improvements are planned.

For now, . . . it all just, fun and games, with the Prop !!



  1. Hoping to get some time to try this with my raspberry pi. It just occurred to me the other day that these processors are so fast you can generate rf directly. How is the frequency stability?

  2. Doug,

    In general, a Crystal is a 50ppm device, and accuracy is dependent on the circuit in which it is connected, and stability is dependent on temperature. The RF Freq that can be directly generated with a Microprocessor is dependent on the Crystal and any Calibration Correction that you apply. At 50ppm and 10MHz RF, that can be 500Hz off freq. With a normal Crystal, temperature is critical and will normally foil your attempts even with a computed Calibration factor.

    For the most accurate and stable RF, I have started using a 10MHz TCVCXO (at 2ppm) as the master clock for the Prop Processor. I am sure something like that can be used with the PI. See my blog and search for TCVCXO.

    Eldon - WA0UWH