What is FM ?

FM broadcasting is capable of better sound quality than AM broadcasting, the chief competing radio broadcasting technology, so it is used for most music broadcasts.

Are FM programs being broadcast at the present time? Yes, from a number of transmitters operating under experimental licenses, most of them in the East and the Mid-West. You may not be able to get FM programs in your locality at the present time, but every month new FM stations are springing up in all parts of the country.

Usually 200 kHz is allowed for each wide-band FM transmission. For communications purposes less bandwidth is used. Narrow band FM (NBFM) often uses deviation figures of around ±3 kHz.

It is narrow band FM that is typically used for two-way radio communication applications.

Very high frequency is the ITU designation for the range of radio frequency electromagnetic waves from 30 MHz to 300 MHz, with corresponding wavelengths of ten to one meters.

Can I hear FM programs on my present set?

FM radio stations use the VHF frequencies.

Having a narrower band it is not able to provide the high quality of the wideband transmissions, but this is not needed for applications such as mobile radio communication.

What wave lengths does FM use? FM has been assigned a band in the ultra-high-frequency range, extending from 42,000 to 50,000 kilocycles. That wave length is far below the regular broadcasting stations you hear at the bottom of your radio dial.

As with any form of modulation, it is necessary to be able to successfully demodulate it and recover the original signal.

The term "FM band" describes the frequency band in a given country which is dedicated to FM broadcasting.

Will the introduction of FM make my present radio obsolete? No. Your favorite programs will continue to be broadcast by the existing AM method for some time to come. Your present set won’t be able to pick up the programs broadcast solely by FM transmitters, but FM’s proponents expect that the major radio shows will soon be broadcast by both FM and AM simultaneously. Possibly both types of broadcasting will continue to exist side by side. But if FM ever entirely replaces AM, a good many years will have passed and you will be in the market for a new set anyway, as your present receiver will be obsolete.

The FM demodulator may be called a variety of names including FM demodulator, FM detector or an FM discriminator.

There are a number of different types of FM demodulator, but all of them enable the frequency variations of the incoming signal to be converted into amplitude variations on the output.

These are typically fed into an audio amplifier, or possibly a digital interface if data is being passed over the system.

Varactor diode oscillator: This method simply requires the use of a varactor diode placed within the tuned circuit of an oscillator circuit.

It is even possible to use a varactor diode within a crystal oscillator circuit.

Typically when crystal oscillators a re used the signal needs to be multiplied in frequency, and only narrow band FM is attainable.

Phase locked loop: Phase locked loops provide an excellent method of generating frequency modulation.

It is often necessary to manage the constraints within the loop carefully but once done it provides and excellent solution.

Resilience to noise:   One particular advantage of frequency modulation is its resilience to signal level variations. The modulation is carried only as variations in frequency.

This means that any signal level variations will not affect the audio output, provided that the signal does not fall to a level where the receiver cannot cope.

As a result this makes FM ideal for mobile radio communication applications including more general two-way radio communication or portable applications where signal levels are likely to vary considerably.

The other advantage of FM is its resilience to noise and interference. It is for this reason that FM is used for high quality broadcast transmissions.

Early developments in radio were called ‘wireless telegraphy’, which is why the radio used to be called the wireless.