Software Defined Radio
The quick & dirty way
After some failing attempts at building a 612 based shortwave receiver with stuff I had in the junk box, I decided to make a quick and dirty I-Q mixer. The output is to be processed with PC-based software, and the audio input to serve as narrow band ADC. The circuit is quite straightforward and similar to numerous other designs found on the web. The clock source is provided by a Philips PM5716 pulse generator.
The clock is converted into the I-Q counter signals by means of a 7474 dual flipflop. I used an AS type, but HC or HCT works as well. One of the outputs is exported, so it can be used to clock an oscilloscope or for a frequency counter.
The I-Q counter controls the switches in the 74HC4052, which connect the RF input to the NE5534 opamps. The oppamps integrate the sampled RF signals into a mixed down I and Q signals, which are fed to left and right audio channel in the PC.
The bandwidth of the system is determined by the ADC in the PC, which usually is 22kHz or so. The base frequency is half the clock from the pulse generator.
The circuit has been implemented on a breadboard. I added some power supply stabilisation, based on zener and transistor, to get rid of rumble and other artifacts.
The antenna is a set of wires and a copper waterpipe is used as a ground connection. The PM5716 pulse generator is used as clock and a lab PSU as dual power source (+/- 8V).
The result with WinAmp can be seen below, where the receiver is tuned to a AM broadcast station.
The proper way