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Old 28th Dec 2009, 6:39 pm   #5
G8HQP Dave
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Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
Default Re: Homodyne/Direct Conversion

I'm not sure what people mean by the audio image, in this context. An AM signal has two sidebands each carrying exactly the same information. The issue with the homodyne is getting the reinserted carrier exactly at the right frequency and phase. This needs either a very narrow filter and precise tuning, or a phase locked loop. Either would be very expensive with 1940's-60's valve technology. Modern ICs can do it cheaply, but still not as cheaply as a simple diode envelope detector.

The image is a problem with SSB reception, but then you don't need a PLL as the frequency doesn't have to be exact and the phase doesn't matter at all. So you save money by losing the PLL, but instead need a good filter or an image-cancelling mixer (otherwise known as phasing SSB).
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