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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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27th Jan 2020, 7:40 pm | #1 |
Octode
Join Date: Sep 2017
Location: Bristol, UK.
Posts: 1,657
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FM de-emphasis network
Can anyone explain how this circuit (Grundig Elite-Boy 500) does the FM de-emphasis? Surely it must be done between the output of the ratio detector (junction of C526 & R532) and the FM/AM IF input switching to the audio amplifier just after MP11. There are only three components between those two points – a 1.8KΩ resistor, a 4.7 nF capacitor, and a 220 pF capacitor. An online calculator gives a time-constant of only 8.5 µs for that network. Surely there must be something else going on…
Mike |
28th Jan 2020, 8:05 pm | #2 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,087
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Re: FM de-emphasis network
I agree!
May be a typo... 1.8k perhaps is actually 18k in circuit? |
28th Jan 2020, 9:19 pm | #3 |
Octode
Join Date: Sep 2017
Location: Bristol, UK.
Posts: 1,657
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Re: FM de-emphasis network
Thanks Kalee. I have checked the value in the set, and it's clearly marked as 1.8K, 5%. It measures 1.776K (only about 1.5% off spec). I've also measured the capacitor (the decimal point isn't that clear) and it's marked 4,700, and measures 4,574pF (2.75% low). So component values seem to be on-spec. Haven't checked the 200pF as that value is trivial in relation to the tolerances.
I'm still puzzled, and wonder if Papa Grundig just meant it to be that way. There is some sibilance on audio, which doesn't start to disappear until a 10nF is substituted for the 4.7nF. The ratio detector circuit isn't one I've seen before- no third winding. Perhaps the unknown circuit impedances and inductances are significant enough to explain the peculiar component values in the de-emphasis circuit. Mike |
28th Jan 2020, 9:20 pm | #4 |
Nonode
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,944
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Re: FM de-emphasis network
I think that one also needs to consider the load into which the demodulator and de-emphasis look when FM is switched in. That load – presumably essentially resistive - is effectively in parallel with the de-emphasis capacitor. Depending upon its value, it may be the dominant factor in setting the de-emphasis time constant, rather than the combination of the demodulator output resistor that is in series with the de-emphasis capacitor(s). De-emphasis circuits can be hard to figure out at times – they are not always simply a low impedance source feeding a simple R-C network that in turn is looking into a (relatively) very high impedance that has but negligible effect. I discovered that aeons ago when trying to figure out the de-emphasis circuit of the Quad FM tuner. (In that case the load presented by the control unit was part of it.)
Cheers, |