A Small, Low-Cost “Software Defined Radio” Receiver Kit For 40-Meters, Used With Powerful Software On The Pc For Multi-Mode Operation

The SoftRock-40 is a small, low-cost, good-performing 40-meter “software defined radio” receiver that plugs into a computer USB port and delivers I-Q audio signals to the computer’s sound card. It was designed by Tony Parks, KB9YIG and Bill Tracey, KD5TFD as an “SDR sampler project” for hams everywhere to easily try out software defined radio. This way a great many hams can see for themselves the incredible performance that can be achieved by having just a little inexpensive hardware working in conjunction with some powerful (free) software running on the PC. Receiver performance is simply amazing … MDS readings of of -128 dBm are typical, with the 1 dB compression level at about -20 dBm!

SoftRock-40 Kit assembled

The SoftRock-40 Receiver Kit is a 1.1″ x 2.4″ circuit board that plugs into the PC’s USB port (for 5V power). The RF coming in from the antenna cable is sampled in a “quadrature sampling detector” which produces two audio outputs (in-phase and quadrature) that are fed to the line-in connector of the computer’s sound card for subsequent processing. Axial-lead components on the top side of the pcb, and six SMT-packaged IC’s on the bottom side make this kit a quick-and-easy assembly. No hardware tuning or adjustments are required!

PowerSDR Console software running on the PC

The PowerSDR Console (free open source software from FlexRadio) running on the PC performs final tuning, filtering, AGC and demodulation of the I & Q quadrature audio signals coming from the hardware. The extensive “front panel” provides many features usually found in high-end radios: spectral display modes, dual VFO operation, multiple filters, frequency memories, programmable offsets, DSP filtering at the IF, and more!


The hardware portion of the SoftRock-40 could not get much simpler. The hardware downconverts and quadrature samples a 48 kHz swath of RF that is fed to the soundcard of the computer. Referring to the block diagram shown below, a crystal-controlled oscillator generates a 28.224 MHz reference signal that gets buffered and divided in half twice to produce reference clock frequencies at 14.112 MHz and 7.056 MHz. These clocks are fed to a simple-yet-effective circuit called a Quadrature Sampling Detector, or QSD for short, which samples the bandpass-filtered RF signal coming in from the antenna. As a result of the sampling, the QSD outputs two signals at audio baseband frequencies representing the downconverted RF signal, where the two signals have the same frequency components but are related in quadrature to each other, meaning they have a 90° phase difference. The in-phase signal ‘I’, and quadrature signal ‘Q’ are amplified and are then delivered as audio input to the line-in input on the PC for processing by the PowerSDR software.


The software part of the SoftRock-40 project turned out to be rather straightforward, building on the solid foundation of the PowerSDR Console software program created by FlexRadio for their SDR-1000 transceiver hardware. The PowerSDR Console software performs final tuning, filtering, AGC and demodulation of the I & Q quadrature audio signals coming from the hardware. All that was needed was some fairly simple changes to the PowerSDR code to handle the SoftRock-40 board. Most of these changes were in the area of tuning and hardware control.


System Requirements

Most modern desktop computers will have more-than-adequate power to handle running an SDR receiver like the SoftRock-40 with its PowerSDR Console software. The only solid requirement is that you must have a stereo ‘line-in’ connector to the sound card in the computer. Some real old desktop computers, and most modern laptop computers only have a mono input to the sound card; but his will not do since we have two audio signals (I and Q) needing to be processed by the computer’s sound card.

The PowerSDR Console software will run just fine on a low-end Dell computer with a 2.2 GHz Celeron processor. The CPU is only about 35% utilized on this box that Dell now sells for less than $300 with software and monitor. We have also run the SoftRock-40 on some older and slower computers with good results. Also, by lowering the frame refresh rate to about 8 fps, the load on the CPU is significantly reduced, which tends to help get acceptable performance on the older machines. Or for even slower machines, you can turn off the spectral display altogether and still have a great receiver!

The SoftRock-40 is designed to plug into a USB port. The only reason for this (currently) is to derive 5V power. No data is coming to/from the USB port. So if your computer does not have a USB port, you can externally supply 5V to the board at that connector. The current requirements are very light (~20 ma), so a 78L05 3-terminal regulator could easily convert down a conventional 12V power source, or batteries.

Speaking of sound cards, you don’t need a high-performance one to make this little SDR receiver work. Even with a common, off-the-shelf, plain-ol-vanilla soundcard, users will be very impressed. Even those sound cards that come built into the computer motherboard work real well. We’ll have audio clips online shortly demonstrating this aspect. But in general, the better the soundcard is, the better the performance will be. The current top-end/recommended sound card is the Delta 44, available as an item from the

SoftRock-40 pcb plugged into USB port on a desktop computer

The little SDR receiver is powered from the USB port. One cable supplies the audio output to ‘line-in’ on the computer’s sound card, while the other cable connects to a 40-meter antenna.

Feature article in Homebrewer Mag #6

Be on the lookout for the SoftRock-40 as a full feature cover article in Homebrewer CD Magazine issue #6, due to subscribers in September, including an hour-long audio interview with the two designers.

Kit Assembly Service” available from KB9YIG

Now that the SoftRock40 v4.0 kits are starting to appear in our mail boxes let me again state that I would be happy to build your kit. Kit build time is at least three hours or more depending on the persons kit building experience. In the last several months I have built over forty SoftRock-40 receivers and presently have a stock of 12 built/test units ready to send out. I would be happy to trade your AmQRP SoftRock-40 kit plus $15, ($18 for Europe), for one of my built/tested units. The $15, (or $18 for Europe), includes the postage cost back to your QTH. (If you are not happy with the built unit, I will return your kit, the $15 building fee and postage costs in exchange for the built unit you received.)

If you would like your kit built for you, just send your kit and a $15 check to me and I will immediately respond with a built/tested unit in the mail that day. (This assumes of course that my stock of built/tested SoftRock-40’s can stay ahead of any demand for kit build.)

As a co-designer of the SoftRock40, I hope that everyone finds the little receiver to be fun as well as a low cost introduction to SDR and the PowerSDR software.

Tony Parks, KB9YIG
1344 E 750 N
Springport, IN 47386

Thanks & Acknowledgement

This has been a fun kit to put together with designers Tony Parks (KB9YIG) and BIll Tracey (KD5TFD). Their initiative and vision of the benefit for many that could be derived from having an SDR sampler was truly inspirational. The AmQRP is very proud to be working with them to help bring about a greater awareness of the technology and an enjoyment by the many new users of their project.

Thanks also to all the Flex Radio Friends who collaborated with the designers along the way on TeamSpeak, tossing around ideas on building a simple SDR receiver.

And thanks especially to Gerald Youngblood (K5SDR), Bob McGwier (N4HY) and Frank Brickle (AB2KT) for the GPL-licensed PowerSDR and DttSP software, and for the pioneering they did in developing the SDR-1000 transceiver, upon which the SoftRock-40 is based.

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