is far more satisfactory than modulating the oscillator. Sometimes this method of modulation is known as indirect FM as shifting the phase of a signal gives a corresponding change in frequency. Once the FM has been generated it is amplified (and frequency multiplied) to the correct level (and frequency). Class C amplifiers can be used because all the information for the modulation is contained within the frequency changes. There should not be any amplitude variations. The filtering stages are particularly important for this type of transmitter where a large number of harmonics are generated. Each stage should contain sufficient filtering to ensure that the levels of spurious signals are Transmitters 203 Reference Phase Programmable Voltage oscillator controlled detector divider Output oscillator filter Summing amplifier Audio input Figure 6.19 Applying frequency modulation to a frequency synthesizer kept to an acceptable level. Then at the output the filtering and matching circuit must ensure a good match to the antenna system as well as keeping any unwanted signals from being radiated. Instead of applying the modulation to a comparatively low frequency signal that has its frequency multiplied, a frequency synthesizer may be used to generate the signal. Modulation may then be applied to the control input of the VCO. Provided the loop bandwidth is less than the lowest audio frequency required then the modulation will appear on the signal. When a synthesizer is used the signal is normally not multiplied. Instead sufficient modulation can be applied to provide the required level of modulation. 7 Broadcasting Most people first come into contact with radio by listening to radio broadcasts. Today many households have a variety of broadcast receivers from portable sets for use around the home, and car radios to very expensive hi-fi systems. These are used to pick up broadcasts on the long, medium and short wave bands as well as the higher quality transmis- sions on the VHF FM band. In addition to this new digital broadc