o main signals, higher order mix products, i.e. 3f1 + 2f2 and 3f2 + 2f1, will appear a further 1 kHz away, and SO forth as shown in Figure 6.9. In the case of a signal modulated with speech or music, there will be a whole variety of different audio frequencies making up the waveform. All these various frequencies intermodulate with one another to generate noise or splatter which spreads out from the main signal. Normally the worst intermodulation products will be those which are nearest to the wanted signal, and their levels reduce as the offset increases. The specifications for intermodulation products are usually given in terms of the difference between the wanted or main signal and the various intermodulation products. This figure is expressed in decibels. Often a transmitter specification will say that all intermodulation Transmitters 193 products are below a given level. In this case the worst ones are bound to be the third order products. Sometimes the levels of specific products will be stated. Typically the third order products will be around -25 to -30 dB for the third order products and 5 or 6 decibels lower in the case of the fifth order products. Spurious outputs Intermodulation products are one type of spurious signal. Other types can be generated in transmitters typically as unwanted mixer products or as harmonics. These may appear on frequencies that are far away from the wanted signal. It is very important that a transmitter does not radiate these unwanted signals otherwise they could cause serious interference to other users of the frequency spectrum. Accordingly it is necessary to ensure that they are kept to acceptable levels and this is done by ensuring that filters are used within the designs themselves as well as at the output. While it is never possible to completely eliminate spurious signals their levels are kept to limits where they are deemed not to cause a problem and their levels are often specified in the data sheet for the equipment. When specifying the levels they are given as leve