The Soft Rock 40 is a low cost 40 meter receiver by Tony Parks (KB9YIG) that implements a simple Quadrature Sampling Detector (QSD, see A Software Defined Radio for the Masses from QEX Jul/Aug 2002") when connected to a computer sound card and software such as PowerSDR. The receiver board is tiny, about 1.5 inches square, yet when paired with a computer and software makes quite an impressive and diminutive receiver.
Note:The pictures and schematics on this page are from an early version of the Soft Rock 40. The basic principles and circuits apply to the AmQRP level of the Soft Rock 40, however specific component names, and board layout is different.
Some PDF's of the circuit:
The basic Soft Rock 40 is centered on 7.054 MHz and can tune +/- 24 kHz from there in software. Pretty neat trick for a crystal controlled receiver. A picture of my built Soft Rock 40 is below.
When I built my Soft Rock 40, I wanted to do some experiments and pair it up with a American QRP DDS Daughter Card to give it more tuning range, so I replaced some of the components on the Soft Rock board with headers to go to a daughter board. Specifically, I replaced the Q1 pass transistor with a 78LO5 5v regulator and did not populate R18 and R1. I also replaced R2 that powers the crystal oscillator, and C8 that feeds the output of the oscillator to the comparator with headers. I also mounted headers at R18 (for input power to the 78L05) and at C2 (shorted together for ground). In the basic daugther board setup shown below R2 and C8 are mounted on the daughter board so the receiver functions as a standard Soft Rock 40.
The basic version works great. IQ amplitude and phase balance of the down converted signal looks good and is easily balanced using the PowerSDR software. A modified version of PowerSDR tunes +/- 24 khz using the DttSP receive oscillator. I also looked at the IQ output of the board from 6.9 MHz to 7.2 MHz and the amplitude and phase response look fairly linear in this range, so tuning wider ranges 96 and 192 khz sampling rate sound cards should be quite workable.
I had a NJ QRP DDS card and an Atmel AVR Butterfly based VFO using KD1JV's AVR Butterfly controller software already built. Figured this would make a good VFO to try and drive the Soft Rock 40 with, so I lashed them all together using a Butterfly carrier board from ERCOS Technology.
In this lash up, the Butterfly and KD1JV's software is used to control the NJ QRP DDS card and the output of the DDS card is fed (via a 47 pF cap) to the SoftRock 40 through the header I'd installed in place of C8. The header at R2 is left open, disabling the oscillator on the SoftRock 40 board.
Results of the lash up are quite good. Using the DDS card to drive the QSD allows the tuning range of the Soft Rock 40 to be moved all over the 40 meter band.
Further integration of a DDS and a micro controller with a Soft Rock 40 would allow for an inexpensive full band coverage SDR type receiver. Integration with a USB based sound chip, and a USB capable micro controller would allow for an SDR with 1 connection to the computer, and one to an antenna. With a little tweaking of the input filters one could potentially cover both the 30 and 40 meter bands.
This has been a fun project to put together. Thanks to Tony Parks (KB9YIG) for the Soft Rock 40 circuit and board design. Thanks to Gerald Youngblood (K5SDR), Bob Mc Gwier (N4HY) and Frank Brickle (AB2KT) for the GPL licensed PowerSDR and DttSP software. Thanks to Steve Weber (KD1JV) for publishing his Butterfly DDS code, and to the NJ QRP folks for the DDS card. Also thanks to all the Flex Radio Friends on Team Speak that tossed out ideas on building a simple SDR receiver.
Comments to: Bill Tracey (kd5tfd@ewjt.com)
Last Updated 10 July 2005
Copyright © Bill Tracey 2005