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Build to Learn - SoftRock Lite

What is the SoftRock Lite?

The SoftRock Lite is Tony Parks' $10 SDR receiver. It receives a signal, in our case on 40 meters, breaks it down into its "in phase" and "quadrature" (I/Q) components, amplifies and then feeds them into the stereo input of a sound card. This is the door prize we have been handing out at the monthly meetings.

How does the SoftRock Lite work?

The easiest way to think of a SoftRock Lite is as two Direct Conversion receivers working together. With a direct conversion you mix the received signal with the signal from an oscillator in a component (mixer) like an NE602/612. The output is the addition of the two signal and the difference of the two signals. For instance if you have an oscillator at 7.040 MHz and you mix it with and incoming RF signal, a signal at 7.041 MHz will be output as 1 KHz (7.041 - 7040), which you can hear, as well as, 14.081 MHz (7.041 + 7040), which you won't worry about. The SoftRock Lite will produce two of these, but the second one will be 1/4 of a phase (90 degrees) off. Producing an I signal and a Q signal is the heart of the Softrock. Just like the the Direct Conversion receiver, the SoftRock Lite will amplify these signals and output them to a phone jack.

But here is where the similarities with a Direct Conversion receiver end. The DC signal is a monaural signal going to a speaker or headphones and filtered to several hundred hertz bandwidth. The SDR signal is a stereo (I/Q) signal going to a soundcard, which, if it is good enough will process 96 KHz bandwidth. Through the magic of Digital Signal Processing if you have the I and the Q signals any information can be decoded from them, be it CW, SSB, AM, FM, PSK or whatever. You just write the program to do it - well you download it.

Building the SoftRock Lite

We are now going to build a SoftRock Lite for 40m stage by stage. At each stage we will understand what that stage does, identify the components to be populated, and test the stage once it is built. (As I build, test and photograph each stage I will turn the bullet into a link to the stage page. Better be done by the October meeting.) The stages we will build are:

SRLSchematicC.jpg