# How does a comparator look inside/ can an opposing voltage source act as a resistor

#### Msaf1123

Joined Mar 25, 2016
5
Right now I'm trying to use an inverted ac voltage source to cancel out a fm carrier wave an leave me with the modulated signal. Would that work or am I just wasting my time?

#### kubeek

Joined Sep 20, 2005
5,757
Please post a schematic of your circuit, and a much bettter descirption of what your inputs are, voltage, frequency, etc.

#### MrAl

Joined Jun 17, 2014
8,475
Right now I'm trying to use an inverted ac voltage source to cancel out a fm carrier wave an leave me with the modulated signal. Would that work or am I just wasting my time?
Hi,

First, the inverted wave would have to be in perfect phase relation to the carrier.
Second, if the FM varied from 100kHz to 120kHz, you'd be left with a wave that varies from 0 to 20kHz, or -10kHz to +10kHz. We could look into this more though, it may not even be that good It probably has to be 'mixed'.
When i do the subtraction sin(w1*t)-sin(w2*t) with f1=12 and f2=10 i get f2 modulated as 'bursts" which look like the beat frequency times 2. Full wave rectified they look like modulated half sines, so they would have to be filtered and every other one inverted or something like that to recover the beat frequency.

You might want to look into known methods of demodulating FM signals. It's much harder to do than demodulating AM signal.
Sometimes a constant frequency is mixed with the input to get a lower mid range frequency to work with too.

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