|Halligan's Hallicrafters International|
maintained by Scott Neader, KA9FOX
Date: Sun, 9 Dec 2001 21:11:06 EST
Subject: Re: [AMRadio] Narrow Band FM
http://www.amfone.net - Home of the AM Radio List.
To post or see items for sale or wanted, go to the new AM'ers Trading post below.
It's refreshing to see some really technically sound tutorial info on the net
regarding a topic as important to our hobby as modulation theory.
Congratulations and thanks for taking the time to create this.
One issue that I think bears emphasis is the relative spectral bandwidth
required for the various modulation modes. I suspect that many believe,
especially those who operate primarily in the VHF domain, that narrowband FM
takes less bandwidth that AM. Correct me if I'm wrong, but I recall that NBFM
requires about 16khz of spectrum to get enough sideband energy to deliver
reasonable audio power, where AM requires at least 6 to 8 kHz, and sideband
about half of that (Let's encourage more use of traditional AM in the VHF
bands. There's a reason the commercial air traffic control system still uses
it!). Angle modulation (FM or PM), in fact, always generates an infinite
number of sidebands, although, depending on modulation index, most are highly
attenuated and insignificant for small mod indices. As you point out the
Bessel function determines the relative amplitudes of carrier and the
multiple sidebands as a function of mod index. I think there is a widespread
misconception that peak deviation equals bandwidth which is not the case. The
bandwidth of the significant sidebands of an angle modulated signal is
determined both by the rate of frequency deviation, i.e., the modulating
frequency, and by the peak deviation, the latter being determined by the
amplitude of the modulating signal in a given phase or frequency deviation
circuit. Commercial FM broadcast allows a much higher modulating frequency
and much higher modulation index which results in a much wider transmission
bandwidth (200 kHz as I recall) in order to achieve higher received audio
frequency fidelity with a very good signal-to-noise ratio.
It's curious to me that the single most important parameter determining
effectiveness of communication whether eyeball-to-eyeball or via radio or
light, i.e., Signal-to-Noise Ratio, is hardly ever mentioned in our hobby.
The nonlinear relationship of receiver output S/N Vs received signal S/N for
angle modulation, which gives us "capture effect," compared to a linear
relationship for the same in AM modes would be a topic of interest for future
Thanks again and please feel free to comment, or correct.
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This page last updated 18 Jul 2001.