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# Bandwidth transmitted signal and lowpass equivalent signal

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#### zzungboy

##### Newbie level 3
Hi,

I have simple question.
I think it is basic concept for digital communication.
However, I'm a little bit confusion on this.
Maybe, I'm missing some point.

In many books, when the bandwidth of lowpass equivalent signal is B, the BW of the transmitted signal is 2B.
Why? Do you provide the reason or give me some reference to understand this?

Re: Bandwidth transmitted signal and lowpass equivalent sign

zzungboy said:
Hi,

I have simple question.
I think it is basic concept for digital communication.
However, I'm a little bit confusion on this.
Maybe, I'm missing some point.

In many books, when the bandwidth of lowpass equivalent signal is B, the BW of the transmitted signal is 2B.
Why? Do you provide the reason or give me some reference to understand this?

Hi Deares,

The formula of that is
BW=(B+1)F
F is central frequency that is carrier frequency
B is modulation index
B could be between 1 to 5 (I think)

Best regards,
Kevin

Re: Bandwidth transmitted signal and lowpass equivalent sign

At the baseband the real bandwidth of signal is B, when the signal is modulated for passband (IF) the complex envelope of signal is shifted from 0 Hz to Centre frequency and the -ve frequency spectrum goes into +ve axis and doubles the BW.

If a real signal bandwidth is 2B, then its "equivelent" low-pass model would be B becasue the low pass model is complex, it has I and Q components EACH one is B in bandwidth, thus retaining the 2B bandwidth of the "real" signal.

zzungboy

Points: 2