How it works : FM Modulation


Above: the principles of FM radio. The modulator produces a carrier wave, and alters its frequency in proportion to the changes in amplitude of the signal waveform, to produce the modulated wave. The demodulator reverses the process, producing an output signal whose amplitude changes in proportion to the frequency variations of the FM wave, thus reproducing the original signal.

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
If modulation techniques were not used, it would not be possible to have more than one radio or TV station operating in the same area. This is because there would be no way of distinguishing between them at the receiver. With modulation, however, each station can be alloted a specific carrier frequency with a surrounding frequency band or channel, and by tuning the receiver to a particular carrier frequency that broadcast can be chosen to the exclusion of nearly all other broadcasts and interference in general.

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
In FM, the carrier wave is made to change in frequency around a central frequency. The change in frequency is made proportional to the amplitude of the signal to be encoded. For example, if the frequency changes by 1 kHz for every one volt of input signal amplitude, then an amplitude of five volts produces a change in frequency of kHz. If the input signal to be encoded is a 2kHz sinusoidal wave-form with a peak amplitude of ±10 volts then the frequency fluctuations are ±10 kHz about the carrier frequency. With a carrier frequency of 1 MHz (1 MHz =1000 kHz) this would mean fluctuations from 1.01 MHz to 0.99 MHz and back again 2000 times a second.

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.

Demodulation, and the recovery of the original signal, are achieved by the reverse operations to modulation. Here, a change in the FM signal frequency is made to produce a change in output voltage. One technique is to use a tuned LC circuit which is tuned to the central carrier frequency. When this circuit is fed with an unmodulated FM signal (that is, constant carrier frequency), the circuit oscillates 'in harmony' with the signal-the oscillations are of maximum amplitude. A modulated FM signal will, however, have a frequency slightly different from the tuned circuit and the circuit will oscillate with a smaller amplitude. By detecting these variations in amplitude using a RECTIFIER and low pass FILTER (low frequency with respect to the carrier frequency) the original signal can be recovered.

FM versus AM
FM systems offer a far greater immunity to noise and general interference from other broadcasting channels than AM. This is, however, only gained at the expense of a larger channel 'bandwidth', which is required if the signal is to be reconstructed faithfully at the receiver. Typically, an FM channel for sound reproduction has a bandwidth of 200 kHz, compared with 20 kHz for AM. For television the bandwidth is much higher.

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