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Question about bandwidth of FSK

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duguzhang

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Simple question on FSK

Hi, guys:

I have a question about bandwidth of FSK.

In text book about FM, the bandwidth FM can be calculated by Carson's Rule: BW = 2*(df + fm) where df is the maximum frequency deviation and fm is the maximum bandwidth of modulating analog signal.

But for FSK, some engineers use BW = 2*(df + R/2) where R is the data rate of modulating digital signal and others use BW = 2*(df + R). Which one is more accurate?

Thanks.
 

Re: Simple question on FSK

It depends on two things.

1. The frequency shift compared to the symbol rate.

2. How much low pass filtering is done on the data before modulation.
 

Simple question on FSK

1. Could you explain in more details about why choose BW = 2*(df+R/2) or BW = 2*(df+R) depends on "1. The frequency shift compared to the symbol rate."

2. Did you mean that if there is no low pass filtering before modulation, then we should use BW = 2*(df+R)? Normally for unshaped digital signals of data rate R, the maximum fundamental frequency is R/2 and next harmonic frequency is 3*(R/2). So why choose fm=R instead of fm=R/2 or 3*(R/2) where fm is the bandwidth of modulating signal?

Thanks.
 

Re: Simple question on FSK

You have two types of Frequency shift keying:
1.coherent M-ary C-FSK
bandwidth can be caculated as:
Bc-mfsk=M*Vb/2*ld M=MVm/2
2.nocoherent M-ary NC-FSK
bandwidth can be caculated as:
Bnc-mfsk=M*Vb/ld M=MVm

where Vb is bit rate , Vm symbol rate

Bandwidth for coherent C-FSK is twice smaller than for NC-FSK.
 

Re: Simple question on FSK

Please suggest a good book that contain practical information about FM and other RF Digital systems..(mainly FSK.. that's what i need)
Thanks,
Ahmad,
 

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