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18th December 2011 15:22

Alan Yates wrote...


Glad it works for you.

Indeed, it is a direct-conversion receiver, so unless the local oscillator is perfectly synced (frequency and phase) to the transmitter you will get annoying heterodyne effects. This is particularly distracting with AM transmissions which have the strong carrier and both sidebands to alias not quite correctly right on top of each other... For SSB transmissions this is not an issue.

A high-pass filter will help a bit, but as you approach zero-beat the slowly drifting phase difference makes amplitude varying effects which are annoying. The folding of both sidebands over each other gets less audible when the frequency error is only a few hertz.

An alternative is a receiver which can injection lock the carrier of strong signals, like the 30 Metre Autodyne Receiver or Emitter Follower Regenerative Receiver. Both are quite simple and are capable of receiving AM and SSB. There is a similar circuit I've been considering for one of the 2011 Advent Calendar projects which I've designed specifically for shortwave reception.

Super-heterodyne receivers are a bit more complex, but can avoid this problem by amplitude demodulating the IF after amplification, rather than getting most of their gain at audio frequencies after product detection. It is also quite easy to offer both product and amplitude detection in a superhet by having different detectors, or a switchable but fixed-frequency BFO before the amplitude demodulator. In principle you could build a synchronous AM demodulator which injection locks the IF and product detects it. (This would be even better than a simple envelope one, or an "infinite impedance" JFET circuit - the locally generated carrier tends to reject distortion associated with propagation effects on the transmitted carrier.) This wouldn't be too different to the autodyne RX above, just setup at the IF with a mixer in front to convert the frequency of interest to the IF. I might build such a radio in the future, it would have the advantage of being and CW/SSB receiver with AM compatibility and quite resistant to the annoying phase distortion and fading you get on shortwave paths.



18th December 2011 14:39

Bert wrote...

Hi Alan,

I have built this receiver and it works very fine.

However, is it possible to receive AM transmission using this receiver? I found that it is very hard to centre the tuning exactly onto the carrier of the received signal. A slight offset from the carrier results in a very low frequency tone that distorts the audio output severely. Have you dealt with this problem before? I guess the problem will be relieved by adding a 100Hz high pass filter or by adding a frequency stabilizer to the local oscillator?

6th May 2011 21:40

Per wrote...


really nice sounding rx despite its simplicity. I must try to build one myself for 40 meters.

Regards / Per