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FSK synchronous demodulator

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Ata_sa16

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Hi guys,

I have a question about this topology

synchronous_fsk_detector (1).png

I understand the first part, so basically it is demodulating mark and space modulated signals by mixing with their carrier frequeny, so at the end it will have

lets we have received mark:

cosw1t x cosw1t =0.5+ 0.5*cos2w1t

now the question is we need LPF in order to filter 0.5*cos2w1t

but why BPF is used ???


Thank your for your time and attention :)
 

When you mix two signals together, you get two frequencies out: the sum and the difference of the two inputs.
sin(a).sin(b) = 1/2(cos(a-b) - cos(a+b))
(Using cos() instead of sin() really makes no difference)
Therefore the BPF is to select the resulting frequency that you want (the sum or the difference).
The mixer does not take in two identical frequencies (as would be implied by your equation) but rather the local oscillator is chosen to be a different frequency to the one you are actually interested in. It could be that you actually try to mix the frequency down to 'zero' in which case an LPF could be of use but a BPF is a more general solution.Also you can think of an LPF as a BPF with a 'zero' lower cutoff.
Susan
 

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