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December 1st 06, 11:50 AM
posted to rec.radio.shortwave
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AM recption notes.
In article ,
"Brenda Ann" wrote: "Michael Black" wrote in message ... ) writes: wrote: I found this web page while looking for a nifty audio filter I found last year. At the very least it gives food for thought. http://www.radiointel.com/phil/phils_radio_tuning_tricks.pdf Terry Eh, the author confuses DSB and AM. I wouldn't put much faith in his/her analysis. Huh? IN what way? I glanced at it and maybe missed something, but DSB is AM. And he certainly says it at the outset, and when he's talking about the components he's talking about 2 sidebands and a carrier. Now, "DSB" often has fallen into the meaning of "DSB with no carrier", but technically one should specifically define that there is no carrier. Some of the problem with AM reception discussion is that it was defined in a certain set of terms, for decades and then even for beginners up till recent times (and maybe even today). So they'd define AM as a signal that is amplitude modulated, and that sets things up for the vision that the carrier amplitude goes up and down. Then when SSB became commonplace, instead of going back from the beginning and redefining it all, a separate set of definitions gets tacked on. This leaves people thinking they AM and SSB are two different things, when they are basically the same. Then when discussion of "low distortion AM detectors" comes along, it isn't even clear what people are talking about. Because one is not using a certain type of detector for AM (ie 2 sidebands with a carrier), and a different type for SSB. The talk of "Amplitude Modulation" invokes a vision of a detector that is following the voltage variations of the signal. But that's not the case at all. The carrier mixes with the sideband in the "envelope detector" and that beating is what brings the modulation back down to "baseband". It's just not a good mixer. Listen to an SSB signal without a carrier or BFO. That's the sound of the envelope varying according to the modulating signal, and there's no way to make sense of it without a carrier. No differing loads on the dioded detector, no precision half wave detector (with the diode in a feedback loop), no forward biasing of the diode, can ever make up for the lack of carrier. The carrier of an AM signal is needed to beat with the sidebands and get it back to audio. If the carrier fades in comparison with the sidebands, you start hearing things like that SSB with an "envelope detector", because the carrier is no longer strong enough to mix the sidebands down to audio, and the "envelope detector" is actually following the envelope of the signal. The basic concept of demodulation is no different whether the signal is AM (with carrier), DSB (with no carrier) or SSB (with no carrier). They all need the carrier, or a locally synthesized equivalent, to beat the sideband(s) down to audio. If things were spoken of that way from the beginning, then there'd be less of a leap to the "synchronous detector". No only would a universal set of concepts be applied to all modes, but the point of a synchronous modulator would become clear. A single diode is a lousy mixer. On the other hand, since the carrier of an AM signal comes in with the sidebands, there's no reason for having a second and isolated input for that carrier. But, long ago, people would mess with "exalted carrier reception", which would be the first step up from those "envelope detectors". They'd turn on the Q-multiplier, which had a narrow peak but a wide skirt, and that would boost the incoming carrier in reference to the sidebands, so there was a stronger carrier feeding into the "mixer". It seems that only when SSB came along, and there were design reasons to go to better mixers for the demodulation, that two input mixers started being used, commonly called "product detectors". There were design reasons for going to those, but the basic concept of a locally generated carrier did not require anything more than the single diode "envelope detector". Indeed, the concept had been there back in the days of regen receivers, and every superhet that could be used for CW had a BFO that would feed into the "envelope detector", to give to provide a beat with the incoming signal. I should point out that when the synchronous detector was described in CQ magazine in the late fifties, the actual mixers were single diodes. But once you had product detectors, that opened things up. The notion of boosting the incoming carrier for better mixing action became more clear. I've said before, there was an article in QST about an advanced receiver in the fifties, and it had two parallel IF chains. One wide for voice, the other narrow for CW. But, it also allowed the output of the narrow chain to feed the product detector, and there was the "quasi-synchronous detector" before anyone came up with the name. For AM (with carrier), you had two choices. You could strip off the extra sideband and carrier, then the incoming signal was the same as an SSB signal, and then demodulate it as an SSB signal. This saw a lot of useage in the sixties, when SSB only ham rigs hit the market, and yet AM was still common. People needed a means of demodulating the AM signals, and that worked. While I think it got discussed in the fifties as a better means of AM demodulation, nobody in the sixties was talking that way. It was just a means of demodulating AM signals when there was no means of doing so. (Not only did the SSB-only receivers have narrow IF filters, but often there was no way of turning off the BFO, and the product detectors were a type of mixer that required having that second signal at the second input; without it, you'd get little or no output even with an AM signal that brought it's own carrier.) But if you didn't want to do that, you had to deal with getting the "locally generated carrier" in the right place. Not just so it wouldn't beat against the incoming carrier (when it was strong enough) but if it wasn't placed in the right place, the sidebands would not be translated back to audio in the same places. (So if you sent a 1KHz tone, and the "locally generated carrier" was not right in the middle between those sidebands, one sideband would translate down so that 1KHz tone was 1010Hz while the other would translated down to 990Hz, which would obviously clash with each other.) That's what the "synchronous" bit is about. It's about putting the same sort of BFO that you'd use with SSB (which would feed the same sort of product detector used for SSB), with the addition of circuitry to synchronize the BFO with the carrier of the incoming AM Signal. If the discussion had started with the AM detector as a mixer, then there'd be little magic about "synchronous detectors". The whole process is simply about getting the "carrier" strong in reference to the sidebands, so good mixing happens in the detector. The "synchronous" bit is only a secondary thing, a need because you want the locally generated carrier in the right place. There have always been various means of getting better mixing action at the demodulator. But the important thing has always been about doing that. Michael Damn good explanation, Michael. And 100% accurate. He always writes good posts. -- Telamon Ventura, California |
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