Electronics > RF, Microwave, Ham Radio

Generate FM using IQ inputs?

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Carlson rule shows how to select required channel bandwidth from a known peak frequency deviation \$\Delta f\$.

But I want to know how to select proper \$\Delta f\$ from a known message signal bandwidth, dynamic range, noise or other properties...

As a rule of thumb, FM with modulation index \$\beta\$ will give you an SNR that is better than analog baseband transmission by \$ 20 \log \beta + 1.76 \textrm{dB}\$. So you can trade off bandwidth for SNR, however there are limits: you need to maintain an RF SNR at the receiver of about 10dB, below that performance degrades rapidly (threshold effect). De-emphasis would give you an additional improvement.

A proper analysis can be quite complicated because of the non-linearity involved. If you want to study this in more detail, "Communication Systems" by Carlson and Crilly may be a good starting point.

Thanks interesting book. I didn't read it before.


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