How it works : FM Modulation
Frequency modulation (FM) is a technique for coding a signal on a carrier wave and is a rival technique to AMPLITUDE MODULATION (AM). Both systems are used extensively in RADIO broadcasting, but in TELEVISION FM is used exclusively.
In AM, the signal modifies the amplitude of a constant frequency carrier wave (in a sound wave, amplitude is loudness and frequency is pitch). The signal is transmitted in this modulated form and to recover the original signal at the receiver it must be amplitude demodulated (demodulation is the reverse operation to modulation). In FM, the carrier wave is of constant amplitude and the signal is coded in the frequency fluctuations about a central (carrier) frequency. Again, to recover the original signal it must be demodulated-this time by frequency demodulation.
The need for modulation
The complete radio frequency spectrum from low frequencies, LF, to ultra high frequencies, UHF (that is, from 10,000 Hz to 1,000,000,000 Hz) can accomodate over 50,000 AM channels or 5000 FM channels even without duplication. Because the broadcasts can be limited to a specific region or country, those same channels can be used elsewhere without too much interference (depending on the power of the transmitter). Tight control is, however, exercised in the allocation of radio frequencies throughout the world (see BROADCASTING).
Principles of FM
To achieve this in practice some means is required to change the frequency of an oscillator according to the amplitude of the Signal. In high fidelity (hi-fi) FM broadcasting, the circuit can be extremely complicated to provide the required quality of reproduction. But there are several ways of achieving an approximate and cheap equivalent. A simple OSCILLATOR, for example, can be constructed using a tuned inductor-capacitor (LC) circuit in the feedback path of an AMPLIFIER with positive FEEDBACK. The oscillator frequency is determined by the values of INDUCTANCE and CAPACITANCE. By varying either the inductance or the capacitance the oscillator frequency can be changed. One way to achieve this is to use a VALVE [vacuum tube] where the effective capacitance between anode and cathode is dependent on grid voltage. This uses the particular characteristics of a valve but similar arrangements are possible using TRANSISTORS.
FM versus AM
FM was first applied by E H Armstrong during the 1930s. Its development was delayed by the necessary high frequency carrier wave required (as FM requires a channel bandwidth of 200 kHz, it can only be used extensively with carrier frequencies above 1MHZ). FM is mainly used in the VHF and UHF frequency bands, that is, above 10 MHz. By the 1930s AM had become so well entrenched in Britain that it took another 20 years before FM gained commercial acceptance.
Reproduced from HOW IT WORKS p1067