A block diagram of a typical frequency stabilizing system used is shown in Figure. It uses a basic frequency, Standard, a crystal oscillator, and the carrier frequency of the FM signal is compared with it. We know that reactance modulator works across the tank circuit of a LC oscillator, whose output is isolated by a buffer stage. The output of buffer is fed to an amplitude limiter and subsequently to the class C power amplifiers (not shown).
A small portion of the signal is taken from the limiter output and fed to mixer in which this signal is mixed with a signal from the crystal oscillator. The difference signal which is usually about one- tenth- to one-twentieth of the master oscillator frequency is amplified and applied to a phase discriminator. The output of the phase discriminator is a DC signal which is applied to the reactance modulator as a correcting voltage to counteract any drift in the average frequency of the master oscillator.
The discriminator is so designed that it reacts to slow changes in the incoming frequency, but not to the normal frequency variation due to frequency modulation. Also the discriminator provides positive 0/p voltage if the input frequency exceeds the discriminator tuned frequency and 0 negative 0/p voltage, if it is lower.
Stereo Broadcast Transmission
A stereo signal consists of two channels which can be labeled L and R (left and right), once channel for each speaker. The ordinary mono signal consists of the summation of the two channels, i.e. L + R, and this can be transmitted in the normal way. If a signal containing the difference between the left and right channels, L - -R, is transmitted then it is possible to reconstitute the left and right only signals. By adding the stun and difference signals i.e. (L + R) + (L – R), gives 2L, i.e. the left signal.
and subtracting the two signals, i.e.(L + R) – (L – R) gives 2R, the right signal. This can he achieved relatively simply by adding and subtracting signals electronically. It only remains to find a method of transmitting the stereo difference signal in a way that does not affect any mono receivers.
This is achieved by transmitting the difference signal above the audio range. It is amplitude modulated onto a 38 KHz sub-carrier. Both the upper and lower side-bands are retained, but the 38 KHz sub-carrier itself is suppressed to give a double side-band signal above the normal audio bandwidth as shown in Figure This whole base-band is used to frequency modulate the final radio frequency carrier.
To regenerate the 38 KHz sub-carrier, a 19 KHz pilot tone is transmitted. The frequency of this is doubled in the receiver to give the required 38 KHz signal to demodulate the double side-band stereo difference signal. The frequency of pilot tone is also used to detect whether a stereo signal is being transmitted. If it is not
present, the stereo reconstituting circuitry is turned off. Forever, when it is present, the stereo signal can be constituted.
To generate the stereo signal, a system similar to that shown in Figure is used. The left and right signals enter the encoder where they are passed through a circuit to add the required pre-emphasis. After this they are passed into a matrix circuit. This adds and subtracts the two signals to provide the L + R and L — R signals. The L + R signal is passed straight into the final summation circuit to be transmitted as the ordinary mono audio, The difference L — R signal is passed into a balanced modulator to give the double side-band suppressed carrier signal centered on 38 KHz. This is passed into the final summation circuit as the stereo difference signal. The other signal entering the balanced modulator is a 38 KHz signal which has been obtained by doubling the frequency of the 19 KHz pilot
Tone. The pilot tone itself is also passed into the final summation circuit. The final modulating signal consisting of the L + R mono signal, 19 KHz pilot tone, and the L – R difference signal based around 38 KHz is then used to frequency modulate the radio frequency carrier before being transmitted.