Receiver circuit
Figure 17.1 shows a typical inter-carrier sound system for a basic TV set. The 6-MHz sound carrier is selected from the demodulated vision carrier by a ceramic filter, resonant at 6 MHz and having a bandwidth of about 200 kHz. The separated sound carrier passes through several stages of amplification followed by a limiter. The limiter clips off any amplitude modulation caused by the vision signal: the AM rejection is about 55 dB. This clipping action produces a square wave output, rich in harmonics of the 6 MHz carrier frequency. They are suppressed by a low-pass filter which restores the carrier wave shape to approximately a sine wave for application to the FM demodulator. The reference carrier is generated by the high-Q tuned circuit L1/C3.
Next comes a voltage-controlled amplifier (VCA) whose gain depends on the d.c. voltage set by R3. The demodulated audio signal is de-emphasised next, by an RC network. The audio signal is now ready for application to the driver and output stages. The output stage generally consists of a push–pull pair of transistors operating in class B (or class D) mode, the d.c. mid-point voltage being isolated from the loudspeaker by coupling capacitor C10. Resistors R4 and R6 form part of a negative feedback loop to control the gain of the output stage. Capacitors C7 and C8 set the amplifier’s upper fre- quency limit. R5 and C9 suppress any tendency for HF oscillation due to the inductive load of the loudspeaker and its wiring.
Frequency demodulation
In frequency modulation, the carrier frequency deviates above and below its centre frequency in accordance with the amplitude of the modulating signal. The FM detector or demodulator thus has to convert the frequency deviations back into the original signal.
There are two main types of FM demodulator that are used in TV receivers: the ratio detector and the quadrature (coincidence) detector. The ratio detector uses discrete components and has the advantage of provid- ing its own rejection of amplitude modulation. Although more complex, the quadrature detector lends itself more easily to IC packaging and hence its extensive used in modern TV receivers.
The operation of the FM detector is based on the fact that the impedance of a tuned circuit is resistive at the resonant frequency but becomes induc- tive at lower frequencies and capacitive at higher frequencies. Consider the LC circuit in Figure 17.2. At the tuned frequency f0, the circuit is purely resistive with voltage V0 in phase with the current i. If the frequency falls below the resonant frequency, voltage V0 leads the current and if the fre- quency rises above the resonant frequency, the voltage lags the current. The amount of phase shift is determined by the deviation away from the tuned frequency of the circuit. If the input to the circuit is the FM carrier, then pro- vided the circuit is tuned to 6 MHz, the phase shift represents the original modulating sound signal. In the FM detector, this phase shift is translated into a voltage variation to reproduce the audio signal.
Figure 17.3 shows an IC package (TBA 120) incorporating a quadrature detector together with an amplifier. R302 is the volume control and S300/C304 forms a 6-MHz tuned circuit external to the chip.