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Mc3361 Fm Ic AFC control problem

dr pepper

Advanced Member level 1
Mar 15, 2010
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Using this chip I threw together an Fm Rx that works well, untill I followed part of a schem from researchgate that depicts a Afc circuit.
The demodulated audio out pin is low pass filtered down to 1 hz or so the the Dc is then fed back to a varicap which forms part of the tank circuit, to lock the freq to the received signal.
In test it only works when the freq is one side of the required freq instead of both above and below.
Scoping the signal and very slightly tuning the Rx each side of said signal I do get a Dc voltage, but it goes positive above and below the received signal, whether a few hz or khz, shifting the local osc back to the received signal one way, but pushing it further away the other.
Can I implement Afc like this with the Mc3361, or is this something it was never meant to do.
I worked it out by looking at the datasheet, inside the chip the audio out pin has a transistor to the + rail and a current mirror to ground, putting a 4k7 resistor from the audio output to ground now enables the chip to drive the pin from 1v to 4v with the tuning centred on signal at 2.5v, acceptable with a 0 - 6v supply and enables aboit +-100khz of control, I can probably even reduce that some.

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