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Re: Dumb Question



At 06:53 AM 1/18/00 -0500, Mark Turnauckas wrote:
>Just when you think 'you got it'...
>I thought I understood sideband well enough to use, but now I'm lost again.  
>Can some kind soul point me to a site that has a primer on this subject?  
>Before I read Paul's response, I would have bet a jelly doughnut that sideband, by definition, meant lower OR upper sideband.

I don't know of a primer, but maybe I can clear up the issue.

When you switch the receiver from one sideband to the other (and re-tune so the signal is back in the passband), what happens to the output is that you've inverted the audio frequencies. That is, the low frequencies become high, the high frequencies become low, all flopped around a nominal center frequency that you chose when you re-tuned. 

I'm not sure exactly how to explain why that is without some pictures and some math, but if you want to understand it more deeply I bet there's something in the ARRL Handbook. Look under "superheterodyne" in the index.

Anyway, flopping the frequencies like that makes voice unreadable. The human ear/brain uses frequency ("pitch") information, and the information in the low frequencies is different from the information in the high frequencies. Thus your understanding that only one sideband will work is very correct for SSB voice.

However, the PSK modulation used on the Pacsat downlinks is much simpler (mathematically) than a voice. In particular, it is symmetrical around its center frequency. So, if you flop the frequencies, you haven't changed anything! So it doesn't matter which sideband you use.

73  -Paul
kb5mu@amsat.org


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