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Voltage Controlled Oscillators & FM demodulator


TedM
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hi folks,

i am trying to find reasons why is good linearity a desirable in VCOs?

ok so i know the frequency and voltage are inversely proportional but that's not really an answer..freq increases voltage decreases.


and on the other side factors which affect the linearity of the demodulator?

apart from noise (and i thought FM was pretty robust against noise?)  are there any other factors?


thanks for any help.
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  • 1 month later...

If the VCO tuning response is not linear, the center frequency will not be deviated by equal amounts by the modulating signal, if it is a pure sine wave for example. When this signal gets demodulated  this will show up as distortion in the recovered sine wave.

FM detectors with some of the best linearity is called a pulse-count detector and was invented by Pioneer corp.

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Most single stage varactor RF VCO's will also suffer from modulation sensitivity. That is you need to apply more modulation level for the same required deviation as you increase the tuning voltage (higher frequency) to the VCO.

Example: you will need much more modulation for the same loudness (listening volume) on 108MHz than at 88MHz.


This problem can be fixed by using a dual section VCO. One varactor for tuning the RF and another biased and lightly coupled to the tank to handle the modulation. With this method modulation sensitivity is constant over a wide tuning range. C3 is very small because only a small variation is needed to deviate the carrier by 75kHz needed for wide band FM transmission

Figure shows a typical arrangement, with another section added.

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Mathematical analysis of the usual Foster-Seeley and ratio detectors reveal a problem. To have very good reproduction of the modulating signal the split secondary winding should have equal properties. This is very difficult, if not almost impossible to achieve in real life. The slightest mismatch will introduce some distortion. The pulse-count detector eliminates this problem.

Later quadrature detection also tried to solve this problem by having only a single tuned circuit, but created another problem that a unwanted 2nd harmonic gets generated.

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