apter 5, Figure 5.31. The output from this circuit must be buffered using a further amplifier. This will reduce the effects of any changes in load that may occur and cause frequency shifts. Buffer amplifiers generally have a high input impedance to ensure that the oscillator circuit is not loaded to any degree. In some instances more sophisticated oscillator systems may use a crystal mixer system. Here a VFO running at a relatively low frequency is Buffer Crystal Mixer amplifier oscillator with tuned circuits Variable frequency oscillator Figure 6.1 Block diagram of a crystal mixer VFO Transmitters 181 mixed with a crystal oscillator running at a higher frequency. If wideband operation is required the crystals in the crystal oscillator can be switched to increase the frequency coverage. The inherent stability of the crystal oscillator enables the whole oscillator system to remain stable despite the fact that part of the circuit is switched. Also by keeping the variable oscillator running at a relatively low frequency the stability is maintained. However, the output of a VFO which uses a mixer must be tuned to prevent unwanted or spurious signals from entering the later stages of the transmitter, and ultimately being radiated. This approach is seldom used these days. Today most transmitters that need variable frequency capability use a frequency synthesizer. This approach gives much greater stability while still being able to operate over a wide range of frequencies. Even where a number of preset frequencies may be required, the technology is SO cheap these days that synthesizers offer a very effective way of generating the required signal in a transmitter. Synthesizers for use in transmitters are exactly the same as those employed in receivers. In transceivers the same one is used for both transmit and receive functions. Synthesizer phase noise is also important in transmitters. In the same way that receivers suffered from reciprocal mixing as a result of phase noise generated by a synthesizer, a poorly designed 