difference between narrowband and wideband fm

Hi Scientists;
In some tactical radios, the channel spacing is 25kHz. If the modulation was FM, is narrow-band FM used or wide-band FM? Pls give a reason.
Any book and paper and link will be appreciated.

For further explenations go to:
http://www.ycars.org/EFRA/Module%20B/fmapps.htm
supplementary:
http://www.ccarc.net/narrowband_fm_math.html

Regards,
IanP

Usually Wide-FM name is for the FM broadcast channels using BW greater than 150kHz.
You can use the terminology narrow-FM for 25kHz BW, even if the truly narrow-FM has the same BW with AM (~6kHz).

Since the band available for u is less, that is 25khz, u better go for narrow band FM. It occupies bandwidth somewhat similar to AM.

Hi elf,

It depends not only of the channel bandwidth, but also of the spectrum of the modulating signal.
The distinction between NB and WB FM or PM is in the maximum phase deviation of the carrier, called also FM modulation index (gamma). For a sinusoidal modulating signal of frequency fm, it is gamma=delta_f/fm, where delta_f is the peak frequency deviation.
Gamma=1 is equivalent to a peak phase deviation of 1 radian, and is conventionally considered the limit between NB FM (or PM)and WM FM (or PM).
WBFM occupies more bandwidth (see Carson’s rule) but is more robust against noise.
Regards

Z

You can not tell it's NB and WB only with BB signal bandwidth. You need consider the carrier frequency and the base band signal's amplitude, too.

Some of this is historical. The frequency control was poor in the past and so wide signals widely spaced were used. As spectrum became scarce and frequency control became better both were cut down. There is a lower limit on the FM modulation index. The signal can be no narrower than twice the highest modulating frequency. Reducing the modulation index further only makes the sideband amplitude smaller and the demodulated SNR poorer. About 40 years ago 100 kHz spacing was common.

Another aspect of spacing is the frequency reuse factor. If the channels are closely spaced, the adjacent channel has to be used far away. If the spacing is wider than the signal by a larger amount the channel can be used geographically closer.

Another historical remark:
At the beginning of radio, someone guessed that using frequency modulation with small frequency deviation should produce a narrowband (narrower than AM) RF spectrum, because the spectrum was supposed to be restricted to the range of instantaneous frequencies. That would allowed to increase the capacity of frequency multiplexing systems.
Carson analyzed the problem and demonstrated that that assumption was wrong.

That property (the spectrum has a power spectral density similar to the distribution of the instantaneous frequency) is asymptotically right for very wideband FM. This is known as Woodward Theorem.
Regards

Z

Hi Zorro;
In what book I can study the topics you described?

Hi elf,

For these topics, I like "Principles of Communication Systems, 2nd Ed." by Taub and Schilling.
Regards

Z