eiver parator oscillator Buffer Demodulated filter amplifier output Figure 5.46 A phase locked loop FM demodulator 162 Newnes Guide to Radio and Communications Technology voltage on the tune line it is possible to generate the demodulated signal. It is found that the linearity of this type of detector is governed by the voltage to frequency characteristic of the VCO. As it normally only swings over a small portion of its bandwidth, and the characteristic can be made relatively linear, the distortion levels from phase locked loop demodulators are normally very low. Many tuners have specifications of fractions of a per cent of distortion. Digital signal processing (DSP) Microprocessors are finding uses in an increasing number of applications. It is therefore hardly surprising to find that they are being used in a variety of areas that normally used only analogue electronics. Functions including filtering, mixing, demodulation and the like can all be performed digitally by specialized digital signal processors. As a result many receivers use these techniques, and in the coming years this process will become far more widespread. In fact digital radio (DAB) receivers rely very heavily on these techniques. The process is based upon the fact that it is possible to build up a representation of the signal in a digital form. This is done by sampling the voltage level at regular time intervals and converting the voltage level at that instant into a digital number proportional to the voltage as shown in Figure 5.47. This process is performed by a circuit called an analogue to digital converter, A to D converter or ADC. In order that the ADC is presented with a steady voltage while it is taking its sample, a sample and hold circuit is used to sample the voltage just prior to the conversion. Once complete the sample and hold circuit is ready to update the voltage Voltage Successive sample points Figure 5.47 Sampling a waveform Receivers 163 Analogue Digital Analogue signal signal signal Sample Analogue signal to digital Signal