Monday, February 27, 2012

A True Software Defined Transmitter

The last few weeks; Julian - G4ILO, Jeff - KO7M and myself have been working on building WSPR, QRSS, and OPERA Beacons. For that, we have built everything around the Parallax Propeller Microprocessor. The processor, the protoboards, and associated peripherals (i.e, Rotary Encoder, LEDs) are generic and could be used for any microprocessor control project. Even the CW Key jack is generic and is connected to normal data lines, and therefore it could be used for other projects, such as, stereo audio output for a robotic sound system.

Stated another way, Nothing on the system could be considered Transmitter specific. This project is only a Transmitter because of the software that is running. Therefore it is a 100% Software Defined Transmitter, an "SDT".  I do not think there are many that can make the same claim.

A True 100%
Software Defined Transmitter
Only the optional add-on Output Low Pass Filter , Power Amplifier, and Antenna could be considered specific to a Transmitter. The raw processor provides about 7mW of RF power, the amplifier raises that to about 100mW. But contacts have been made with just the low power of the bare bones generic hardware as an SDT.

We have used the SDT as time scheduled WSPR, QRSS and OPERA Beacons and have successfully received reports from many places around world. With the addition of Jeff's Keyer Program, and my UI program (to take advantage of the LCD and Knobs) the SDT can be used interactively with a CW Key. As more software is developed, more modes and capability will be implemented.

The SDT In Action
Internet Based Map Report on the Left
Text Reports on the Right
The only new thing I plan to add to the collection of peripherals for the microprocessor protoboard is an external high precision master clock to help thermally stabilize the RF output frequency (parts are on order).

So far, I am very pleased with the results we have obtained.

NOTE: If you try this, make sure you use an output Low Pass Filter, as the Propeller output is a square wave with many harmonics. The FCC requires harmonics to be down by 43db.

If you are a HAM and interested, buy a Prop, join the fun.

(Note: I am not an agent or associated with Parallax)



  1. Do you plan to release the source code and schematic(s) under an open source licence?

    Also, is this design tied down to any particular features of that Parallax development PCB shown in the photos?

  2. Yes and No.

    Yes, I plan to release the code and schematic for anyone to use.

    No, it is not tied to a particular of Prop. My implementation uses the proto area of the Propeller USB Protoboard, but any Prop can be used. Each of the devices have assigned pins in the CON section of the Spin Files.

    I am busy setting up a GitHub account for easy download. So far it seems easy, but it will take some time.

    Eldon - WA0UWH