nduct, producing voltages across their respective load resistors that are equal and opposite. These two voltages cancel one another out at the output SO that no voltage is present. As the carrier moves off to one side of the centre frequency the balance condition is destroyed, and one diode will conduct more than the other. This results in the voltage across one of the resistors being larger than the other, and a resulting voltage at the output corresponding to the modulation on the incoming signal. The choke is required in the circuit to ensure that no RF signals appear at the output. The capacitors C1 and C2 provide a similar filtering function. One of the major problems with both of these circuits is that they require coils and chokes. These are expensive to produce these days, often costing more than an integrated circuit. As a result other methods of demodulating FM are often used. A third form of demodulator is known as a quadrature of coincidence detector. It is widely used in integrated circuits and provides a good level of linearity. It has the advantage over systems such as the ratio and Foster-Seeley detectors in that it requires a simple tuned circuit rather 160 Newnes Guide to Radio and Communications Technology Analogue multiplier (mixer) filter Incoming frequency Audio modulated output signal Figure 5.44 Block diagram of an FM quadrature detector than the more complicated transformers they use. In this way costs can be reduced. It is also very easy to implement in a form that is applicable to integrated circuits. The basic format of the quadrature detector is shown in Figure 5.44. It can be seen that the signal is split into two components. One of these passes through a network that provides a basic 90 degree phase shift, plus an element of phase shift dependent upon the deviation. The original signal and the phase shifted signal are then passed into a multiplier or mixer. It is found that the mixer output is dependent upon the phase difference between the two signals, i.e. it acts as a phase detec