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Relation between FSK modulation and OBW (Occupied Bandwidth)

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Marialena07

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My question is: In FSK digital modulation why is OBW (Occupied Bandwidth) reduced while the data rate is increased? (the exact opposite happens with other digital modulations such as QPSK, 16-QAM etc which is justifiable to happen).
 

kaz1

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My question is: In FSK digital modulation why is OBW (Occupied Bandwidth) reduced while the data rate is increased? (the exact opposite happens with other digital modulations such as QPSK, 16-QAM etc which is justifiable to happen).
You need to tell us how did you arrive at the assumption in the first place? It will be great if so...
 

FvM

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The assumption is basically wrong. Minimal OBW according to Carson's rule is (fH-fL) + data rate. The actual OBW is higher if you don't suppress the side lobes.
 

Marialena07

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You need to tell us how did you arrive at the assumption in the first place? It will be great if so...
I conducted some measurements on the spectral analyzer with the FSK digital modulation and I was surprised to see that. I am trying to finish my thesis 😅 I am not very experienced with communications.
 

JohnBG

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FSK is modulation that does not really contrains BW, contrary to the other modulations you have mentioned in the question, that have a fixed BW.

The faster the transport stream the faster the constellation spins thus sizing more BW.

Some military systems still use FSK with really broad frequencies shifts, out of range, keying too fast for conventional civilian equipment.

To prevent FSK signals overlapping and interferring, usually but not always, there's a limiter that prevents data rate above certain limit.

FM systems have same safety, that consists of 1. xonsumption limiter reducing or even switching off input to prevent, potentially, loud input overheating the transmitter, and 2.- an output filter.

It would be interesting to know what system are you using, because the limitting effect may be even caused by simply the output stage demanding too much current and some safety in the power supply reducing consumption.
--- Updated ---

how is it writing a digital modulations thesis and writing in same online post ' .. not very experienced in communications ..' !?

potential recruiters, now more than ever, scan media for this kind of 'shot-own-foot' comments to thin big stacks of job applications.
 
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