ese circuits SO that the set can provide new degrees of flexibility and offer a greater number of facilities. Push-button tuning together with frequency memories, scan- ning and a whole host of other facilities are now available in many sets as a result of the combination of microprocessors and frequency synthesizers. A frequency synthesizer is based around a phase locked loop which uses the idea of phase comparison for its operation. The phase of a signal is the position within a cycle. Often this is likened to the point on the 136 Newnes Guide to Radio and Communications Technology Phase difference Figure 5.19 Phase difference between two signals signal travelling around a circle with a complete cycle being equal to 360 degrees. Each point on the sine wave has an equivalent point on the circle as shown in Figure 5.19. If there are two signals they may not be at the same point in the cycle and there is a phase difference between them. The phase angle between them is equal to the angular difference on the circle as shown. From the block diagram of a basic loop shown in Figure 5.20 it can be seen that there are three basic circuit blocks, a phase comparator, voltage controlled oscillator (VCO), and loop filter. A reference oscillator is sometimes included in the block diagram, although this is not strictly part of the basic loop even though a reference signal is required for its operation. The loop operates by comparing the phase of two signals. The signals from the voltage controlled oscillator and reference enter the phase Receivers 137 Phase Voltage Reference controlled parator oscillator filter Figure 5.20 Block diagram of a basic phase locked loop comparator and in this circuit a third signal equal to the phase difference between the two is produced. This is passed through the loop filter which performs a number of functions within the loop. In essence it removes any unwanted products from the error voltage before the tune voltage is applied to the control terminal of the voltage controlled oscillator. The er