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FM Demodulator Troubles

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akilpatrick

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Hi,

I'm using a 74HC4046A PLL/VCO to demodulate mono audio from an FM carrier. The carrier comes from a signal on CAT5e twisted-pair cable. (it rides on the common-mode on one pair which carries video on the differential mode) The carrier frequency is about 250kHz and the amplitude seen by the receiver is about 250mVp-p. Currently I'm modulating with a function generator using a square wave.

The signal sounds ok, and is fairly low noise. (good enough for my application) But one of the adjacent pairs is carrying 9600bps RS485 data. When data transmits I hear a clicking on the audio. It's already a low-slew rate transceiver on both ends, and I tried slowing it down more with a cap across the data line, but it doesn't have any effect on the noise.

I'm wondering why the demodulator is hearing this noise, and if there is an easy way to filter it out. I've attached a schematic of my circuit. As you can see there is no filter on the input to the PLL.
 

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