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Crystal phasing & single signal reception
On Sat, 24 Aug 2013, gareth wrote:
I wonder if some more experienced and senior (in the literal sense) amateur could explain to me one difficulty that I have in understanding single signal reception with the crystal phasing control? AIUI, the phasing control is adjusted so that the frequency that would give the audio image is phased out by being in the notch. When I had an SP-600, I don't remember using the phasing control much. I'd leave it somewhere, I'd set the BFO and all was generally fine. When that type of filter came along, it didn't replace anything, it added something (so there was a wave of commercial receivers where you could get two models, one without the filter, the other with the filter). The single signal was because it suddenly did get rid of the audio image. Tghe phasing control doesn't really change that, it allows some level of control and you also get a notch at the same time. DOn't forget the crystal gave really quite high selectivity, the phasing was simply to balance out the capacitance of the crystal holder. Since this added some control, you'd see the phasing control on the front panel. Most of those filters had a selectivity switch, which would vary the amount of loading of the output of the crystal filter, which allowed for varying selectivity (though since it was only one crystal, the skirt selectivity wasn't that great). The wider the filter was, the less effect the phasing control would have on it. Which is likely why I never saw much use in the phasing control on that SP-600, I'd generally keep it at 3KHz MIchael VE2BVW If that is true, how is it then possible to adjust the BFO to a pleasant-for-you tone, because such adjustment will alter the CIO frequency? Surely the BFO has to be set in advance to be halfway between the serial resonant frequency and the notchable parallel resonant frequency? This is a query stimulated by my current project which is to build a boatanchor style RX, with a flywheel-loaded slide rule dial like that of the Eddystone EA12 |
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Crystal phasing & single signal reception
"Michael Black" wrote in message
news:alpine.LNX.2.02.1308241914010.27526@darkstar. example.org... On Sat, 24 Aug 2013, gareth wrote: I wonder if some more experienced and senior (in the literal sense) amateur could explain to me one difficulty that I have in understanding single signal reception with the crystal phasing control? AIUI, the phasing control is adjusted so that the frequency that would give the audio image is phased out by being in the notch. When I had an SP-600, I don't remember using the phasing control much. I'd leave it somewhere, I'd set the BFO and all was generally fine. When that type of filter came along, it didn't replace anything, it added something (so there was a wave of commercial receivers where you could get two models, one without the filter, the other with the filter). The single signal was because it suddenly did get rid of the audio image. I think that it can only do that if the CIO / BFO is half way between the peak and the notch? Tghe phasing control doesn't really change that, it allows some level of control and you also get a notch at the same time. If the notch is variable, then it will be some other audio frequency which would be notched out (clearly you'd go for the most troublesome interference, but that would not necessarily be the audio image) TNX FER contribution OM |
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