Over at NBitWonder, we have been working to develop a software-defined radio for a few months. Last weekend, the radio received its very first signals. There is a short writeup and demonstration video at our website.
This is quite similar to Jeri Ellsworth’s design in that is uses a Tayloe detector and some DSP to recover a signal. This design, though, achieves frequency agility using a DS1085 programmable oscillator module — it can be set from 8kHz to 133MHz in 10kHz steps. A CPLD performs clock division, level conversion, and quadrature clock generation, and feeds a high-speed bus multiplexer that actually does the demodulation. After some simple subtraction performed by a high-bandwidth op-amp, the in-phase and quadrature signals are passed to a PIC32 via a low-pass anti-aliasing filter. Inside the PIC, an FFT is used to filter the signal to any of several possible bandwidths, then an IFFT is taken and demodulation is performed in the time domain. Finally, an audio signal is reconstructed via PWM, low-pass filtered, and sent to the output.
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