Posting Comment for "ARNSW Homebrew Group 2011 Challenge"

*Author Name
Email Address
Website URL

About Posting a Comment

Comments are moderated before they will appear on the website, this is a manual process and may take some time. Please be patient.

Author Name is a required field.

Email Address is optional, but without one I won't be able to contact you back. It is never shown or linked on the website. You can always just email me if you'd rather not post a public comment. I generally reply in-line with a comment rather than email you back, unless I want to discuss something in private or off topic. Please check back to see when I reply.

Website URL is optional, if supplied the Author Name will be hyperlinked to this URL.

You may use wikitext in the body, preview may be handy here. Don't worry if you can't figure them out, just give me a hint what you want linked to what and I'll do it during moderation. Wikitext is not BBcode!

Spammers: Please don't bother wasting your time scripting up posts to this form. Everything is moderated, your post will never be seen on the web even transiently, there is no way to even view it by its internal ID, it will never be indexed. I will simply delete your post in the moderation interface. If I'm your target audience you're really on the wrong track; I'll never click on a URL in your garbage. The post content is not emailed to me (and I don't use a Win32 mail client anyway), I view the posts in plain text in the moderation interface so no clever tricks of any kind will make anything you type be interpreted by anything other than me, a human. Just give up and go elsewhere please!

18th December 2011 15:22

Alan Yates wrote...

Bert,

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.

Regards,

Alan

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...

Hi,

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

Regards / Per