cy (MHz) Figure 6.8 Diagram of a typical spectrum analyser screen plot words a spurious signal will be said to be a certain number of decibels below the carrier. Sometimes this will be referred to as a certain number dBc. In the previous example the figure could have been given as -30 dBc. The more negative the number is the smaller the spurious signal level. In other words a spurious signal level of -40 dBc is better or lower in level than one of -30 dBc. This method is convenient to use because it is easy to see the difference between the main signal and the spurious signal on a spectrum analyser. Intermodulation In just the same way that non-linearity of a receiver amplifier can cause intermodulation distortion, the same can be true of an amplifier in a transmitter. In many cases non-linearity itself is not a problem, for example where the signal is frequency modulated, but where the signal is 192 Newnes Guide to Radio and Communications Technology Two wanted signals being transmitted Unwanted signals that may fall outside the channel bandwidth and cruise interface to other users Amplitude 2f1-f2 2f2 f1 3f1 2f2 3f2 - 2f1 4f1 3f2 4f2 3f1 Frequency Figure 6.9 Intermodulation products produced in an amplifier amplitude modulated, either as AM or SSB, etc., then it will give rise to unwanted products. The problem is encountered where a linear amplifier is used on the output of an AM or SSB transmitter or when other types of modulation such as quadrature amplitude modulation (QAM) are used. An AM or SSB signal consists of a variety of different signals dependent upon the audio being applied. However, to illustrate the problem the example of two signals being transmitted will be taken as an example. This might happen when using an SSB transmitter where just two audio tones, one at 1 kHz and another at 2 kHz, are being transmitted. Here the third order products will produce signals at 2f1 + f2 and 2f2 + f1. In the case of the example the two third order intermodulation products will appear 1 kHz either side of the tw