Tuesday, 6 January 2009

A simple direct conversion receiver

Back to my roots - simplicity
I tried a little experiment. I hooked-up my Poundshop receiver (Google it...) to my netbook, and fired-up the wonderful SAQrx Panoramic Receiver software. I found I could use SAQrx as a digital signal processing AF subsystem, and tune-around inside the Poundshop's gigantic audio bandwidth. I was satisfied with the results, but the instability of the Poundshops's 7088 chip meant it drifted too quickly for comfort when the passband was narrowed. I needed crystal control, and a fixed reference.

Why use batteries?
Most of my projects run from 12V lead-acid gel-cells, or 9V alkalines if low-powered. The current project has two features; a single active device with a 4.5V - 8V supply range, and a close association with a small computer. Why not use the power from a USB port? It's 5V, and capable of supplying 100mA (more if care is taken). So, I found a redundant USB cable, an old earbud cable (the kids tread on the earpieces!) and scribbled out a plan on the Schematic Editor.

My favourite chip
It had to be a SA602 mixer-oscillator. These have poor big-signal handling, but are very easy to use. I attached a 7030kHz crystal, a smattering of capacitors and a loop antenna (one metre length of four-core cable), and hooked-up the computer power and signal cables. It worked as soon as I'd turned the SA602 over; a blunder which deserves it's own paragraph...

A creature of habit
Nearly all my projects are produced in 'ugly' style, where chips are turned-over, and used legs-up. So, when I came to use a 'familiar' chip in a piece of stripboard, I reversed all the connections. Happily, the SA602 survived this abuse; I simply desoldered it and mounted it in the solder-side of the board. Success!

A little deaf, but it works
It's a starting point. I may try to improve the antenna, and / or add another SA602 as a RF amp stage. Because the antenna and mixer inputs are balanced, this is an ideal choice. I get an extra 15 - 18dB of gain, controllable by applying a bias voltage to the oscillator emitter pin (6). This works well. I have a home-brew guitar amplifier which uses this scheme as a tremolo effect, with pin 6 fed from a sine-wave generator via a 'level' pot.