Saturday, 26 September 2009

Early Development Prototype - CW84

I'm calling it the CW84 because it's a refinement of an 80-metre cw transceiver I made about four years ago, which I called the CW80. Not only is it four years younger, but the true wavelength of a 3.550MHz signal is 84 metres. It's a direct-conversion receiver, very straightforward in design, with an equally simple transmitter tacked onto it. A 700Hz offset is added to the transmitted signal by adjusting the control voltage fed to the tuning varactor, which is a forward-biassed LED.

Today, I achieved some satisfactory results with an initial development prototype. It's cobbled together on stripboard, and the picture shows the diagram of the circuit as it stands. There is as yet no RF input to the SA602 mixer / oscillator, and I haven't tried to recover any audio from the thing. It does, however, tune it's oscillator sweetly between 3.550 and 3.583MHz, which is exactly the 30kHz I wanted; this little band covers the 3.555 and 3.564 slow Morse code (QRS) 'sandpits', and the 3.560 low-power (QRP) centre of activity.

The front end may be a simple bandpass filter and a 1k-ohm pot attenuator, or I may include a FET preamp as I did in the CW80. I'll try to avoid complexity wherever possible. The whole idea is one of simplicity; very little setup, few wound components and even fewer trimmer capacitors. Today, I eliminated a padder from the oscillator tuning, and that's the kind of change which I welcome. There will be no AF gain pot. The original CW80 doesn't use one, simply because the RF attenuator does all the gain-setting required. It also stops a novice operator from setting the RF gain too high, backing off the AF gain and wondering why the mixer is getting swamped with powerful signals. The SA602 is a lovely device, if you respect the fact that Gilbert cell mixers are easily overloaded, but have good conversion gain and hence work well at low input levels. Another simplification is in the AF output stage. The LM386 is noted for its low ancillary component count, but I've taken the liberty of doing without the bypass on pin 7 and the Zobel network on the output at pin 5. If they prove necessary, I'll add them; but only if.

Tomorrow (hopefully), I'll add a front end and a pair of 'phones. I may even try listening to my gate-dip oscillator later tonight. Whatever happens, and whenever I get around to doing it, I'll post it right here. Remember; as always, although I assert copyright on my work, please feel free to try these things for yourself. I intend to market kits for these projects, and support them with this blog and other web-based materials, but use your junk-box stock or the shopping-lists which I publish with the designs.

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