VFO
The input is USB, this provides both power +5V, and the input commands to the FT232R convertor, which converts the USB signals to the serial bus I2C signals for the frequency generator CY27EE16ZE. The VFO has a 10MHz xtal and the x4 tuning frequencies are derived from this.
The VFO is followed by a divide by 4 "johnson" counter which generates four phase signals to drive the mixer. (0, 90, 180, 270 deg)
MIXER
The mixer uses a standard CMOS switch 74HC4066 to sample the four clock phases of the input antenna signal. The samples are integrated during the 0, 90, 180 and 270 deg phases on the 2n2 capacitors. The I and Q audio signals are obtained by mixing the outputs of each pair of switches in a couple of op-amps.
This is the bare, and very simple, bones of an SDR receiver. It has to be used with a PC with a good sound card (ideally 24bit/96kHz capability for a tuning range of +/-48kHz around a chosen frequency), and software for driving the VFO, displaying the received signals and decoding them as AM, FM or SSB, such as the SDRadio software for a Windows PC or the DSP Radio software for the Mac running OS X.
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