didac
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There's a million ways to see the things in life
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manresa,spain
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Hi to all, FM wideband modulation was the modulation that I hate most when I studied Signals and Systems. It's the less straightforward modulation for hand calculations, and only you can manage it using a cosinus as a information signal. The FM SNR is very tricky in fact due to the spreading effect, so it's not very surprising that numbers don't match. The expression for SNR at the demodulator input it's: SNRD=0.75*(BT/W)^2*P*γ, where BT is the transmitted bandwidth(150KHz,200KHz it's the channel spacing for security reasons for a modulation index of 5), W=audio bandwidth(15KHz), P=power at the input of the demodulator γ the equivalent SNR for a Baseband transmission. With that datasheet is difficult to provide any number of their internal blocks, but it looks like the signal path for both AM and FM are common so picking their sensitivity for AM and assuming a transmitted bandwidth of 30KHz and a SNR=26dB I calculated a NF=21.814dB. Regarding FM capture this effect relate to a close interference(in band interference),not noise(it's equivalent to say a C/I intrachannel) due to the fact that what matters in FM is maximum frequency deviation,closest interferences are less harmful that further ones(if they are of less amplitude that the desired signal).
If you are interested in a detailed analysis of FM characteristics take a look at the excellent book: Communication Systems, McGraw-Hill, 1968,A.Bruce Carlson which is for my taste the best book on analog signal theory. Hope it helps,
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