ther than LC tuned variable frequency oscillators are Transmitters 201 Supply Audio input Output Figure 6.17 Applying frequency modulation to an oscillator normally needed to provide sufficient stability for these basic trans- mitters. The usually way in which they work is for the oscillator to run at a low frequency, typically a few megahertz, and this signal is multiplied in frequency by a series of multipliers. These are normally amplifiers running in class C to produce high levels of harmonics. Tuned circuits in the output circuit select the correct harmonic. Normally these multipliers are restricted to relatively low multiplication factors, two and three being the most common. It is found that as the frequency is multiplied SO the level of deviation is also increased. This means that if a signal is multiplied 18 times before being transmitted, the level of deviation applied to the oscillator only needs to be relatively small. For a final deviation of 3 kHz, the deviation needs to be 3/18 = 167 Hz for a multiplication factor of 18. This can be achieved relatively easily using the circuit shown. Applying audio directly to the oscillator is not the optimum method of generating FM. The level of FM can vary from one crystal to the next if the specifications of all the crystals are not exactly the same. Also the linearity of the conversion is not always good. This can be improved by applying a fixed bias to the varactor diode to overcome its non-linearities. However, a superior method is to use a phase modulator placed after the oscillator as shown in Figure 6.18. The actual frequency deviation given 202 Newnes Guide to Radio and Communications Technology Supply RF output RF input Audio input Figure 6.18 A phase modulator circuit by the phase modulator increases with the increasing audio frequency at a rate of 6 dB per octave. The phase modulator can be made to give an FM compatible signal altering the audio response of the audio amplifiers to give the inverse response. The use of a phase modulator of this nature 