ve been quoted. 164 Newnes Guide to Radio and Communications Technology Spurious signals Virtually all receivers generate signals themselves, many of which can by picked up be the set and received as if they had entered the set through the antenna socket. In many cases these signals go unnoticed because they are masked out by real signals, but in some instances they act as unwanted interference degrading the performance of the set. Spurious signals can be generated in a number of ways. Most of today's receivers use a variety of oscillators in their circuitry. It is very easy for these signals, or their harmonics to enter the signal path and appear as signals. On wideband receivers it is often impossible to eliminate all the spurious signals, although most of them can be reduced to levels where they do not cause a major problem. Some receivers will quote known spurious signals. For narrow-band receivers like those in mobile phones it is possible to design the set SO that no significant spurious signals are generated within the frequency range of the receiver. Phase noise and reciprocal mixing One of the main problems with frequency synthesizers is the fact that they can generate high levels of phase noise if care is not taken in the design. This noise is caused by small amounts of phase jitter on the signal, and it manifests itself as noise spreading out either side of the signal as shown in Figure 5.49. Any signal source will have some phase noise. Crystal oscillators are very good, and free running variable frequency oscillators normally perform well. Unfortunately synthesizers, especially those based around phase locked loops, do not always fare SO well and this can adversely affect the performance of the radio in terms of reciprocal mixing. Amplitude Wanted Phase noise signal either side of the signal Frequency Figure 5.49 Phase noise on a signal Receivers 165 Amplitude Strong off-channel Local signal Oscillator Phase noise Width of Weak on-channel receiver signal masked by filter reciprocal mixing noise Spaci