rcuit to detect changes in the frequency of the incoming signal. It has three windings. The primary and secondary act in the normal way to produce a signal at the output. The third winding is untuned and the coupling between the primary and the third winding is very tight, and this means that the phasing between signals in these two windings is the same. The primary and secondary windings are tuned and lightly coupled. This means that there is a phase difference of 90 degrees between the signals in these windings at the centre frequency. If the signal moves away from the centre frequency the phase difference will change. In turn the phase difference between the secondary and third windings also varies. When this occurs the voltage will subtract from one side of the secondary and add to the other causing an imbalance across the resistors R1 and R2. As a result this causes a current to flow in the third winding and the modulation to appear at the output. The capacitors C1 and C2 filter any remaining RF signal which may appear across the resistors. The capacitor C4 and R3 also act as filters ensuring no RF reaches the audio section of the receiver. The Foster-Seeley detector is very similar to the ratio detector in many ways, but there are a few fundamental differences. Although the circuit Primary Secondary Third winding on the transformer Audio output Figure 5.42 Circuit of the ratio detector Receivers Audio output RF choke Figure 5.43 Circuit of the Foster-Seeley detector topology looks very similar, having a transformer and a pair of diodes, there is no third winding. Instead this is replaced with a choke. While there are a number of differences the circuit again operates on the phase difference between signals. To obtain the different phased signals a connection is made to the primary side of the transformer using a capacitor, and this is taken to the centre tap of the transformer. This gives a signal that is 90 degrees out of phase. When an unmodulated carrier is present at the centre frequency, both diodes co