Re: Phase or frequency modulation?

On 2025-07-08 18:20, Ralph Mowery wrote:

In article <pu1o6kp8b27fbq470dh8lrkjiu15cldu76@4ax.com>, jl@glen--
canyon.com says...

>
I meant measure the actual FM from your crystal oscillator.
>
I wonder if the crystal's Q limits modulation bandwidth somehow.
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 Sure it does.  You can only move the frequency a small amount.  FM and
PM is usually generated at 8 to 12 MHz and multiplied up to 50 to 450
MHz.
This is for the old crystal controled  ham and pulic servce radios.
 

It limits the frequency deviation, but not strictly the modulation bandwidth.  The modulation frequency is the frequency of the modulation, i.e. what goes into the modulator or comes out the demodulator.
You can do PM at a 1 MHz modulation frequency and a 1-Ht  deviation, no problem.  You get weak PM sidebands at 1 MHz frequency offset.
The problems arise when the modulation frequency or the frequency deviation becomes comparable to the center frequency.  At that point you get sideband folding, which loses information unless you have an I/Q system.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com