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#24
January 19th 07, 03:45 AM
posted to rec.radio.shortwave
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Sync detectors and fading
Carter-k8vt wrote: wrote: Why do you insist that the atmosphere treats the carrier differently from the rest of the signal? Because it does. See below... Geez. You have a spectrum produced by modulation. If the modulation is AM, then a carrier is present. Now you are saying the atmosphere is sucking out the narrow band carrier and leaving the wideband spectrum untouched. Fiction at best. Bzzzzt. Wrong! Yes, the "atmosphere" [ionosphere] DOES "suck out a narrow band" or even a single frequency. Ask any amateur radio operator that has used RTTY (radio teletype). If the carrier is effected, the odds are signals nearby (i.e. the audio) will be effected. I still stand by the statement that the only advantage to sync demod is that you have cut the bandwidth in half. I never bought the "fresh carrier" story. The RTTY "modulation mode" used is FSK or frequency shift keying. At any given instant, the transmitter is sending either a "mark" or "space", essentially two carriers if you will, 170 Hertz apart that represent the 5-level Baudot code as used in ham RTTY. As an aid to tuning, an oscilloscope is used as a tuning indicator; the mark signal from your RTTY decoder is connected to the horizontal plates of the 'scope, the space signal to the vertical plates. On the screen of the CRT (due to the persistence of the CRT phosphors and your eyes), this shows what appears to be a "+" sign, also known as the classic "cross display". When you see the cross on your screen, you know you are tuned in properly. So, "What does this have to do with the discussion above?" you ask. Remember, you are looking at essentially two "carriers", 170 Hz apart, one on the horizontal axis and one displayed on the vertical axis. During disturbed ionospheric conditions, many times you will see one signal or the other disappear; i.e., the cross turns into a single line, either a "-" or a "|", depending if the mark or space faded--and yes, sometimes both fade, but it is more common to see one or the other disappear. This phenomenon is known to hams as "selective fading", is quite common and is interesting to observe. So, yes, the ionosphere CAN suck out one signal separated from another by as little as 170 Hz. Carter K8VT If the selective fading is as tight as you indicate, then there would be "holes" in the audio spectrum of the recovered AM, much like a comb filter. Sync demod won't fix that problem. There are too many people that think sync demod will cure everything. It's just not true. Now if you have a nearby signal bleeding into the desired signal, then pick the sideband the furthest away from the interfering signal. Here, sync works great. If you have fading, you can narrow band the signal by using one sideband. It helps a bit, but the signal will still fade. All this assumes your sync is decent, and not a growler. Otherwise, all bets are off. |
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