rformance. The most obvious method of achieving this is to measure the difference between the wanted signal and the noise level under specified conditions as shown in Figure 5.54. Amplitude Wanted signal Height above noise indicates the signal to noise ratio Noise Frequency Figure 5.54 Signal to noise ratio The difference between the wanted signal and the noise is expressed as a ratio in decibels. This is often termed the signal to noise or S/N ratio. The signal level also needs to be mentioned as this has a direct bearing on the figures obtained. The bandwidth of the receiver also has a bearing on the figures which are obtained because the noise level is proportional to the bandwidth being used. When specifying the performance for AM the level of modulation also needs to be included in the specification. This is because the audio output from the receiver is measured to give the signal level and this will be dependent upon the modulation level. The usual value for modulation is 30 per cent. A typical specification for a good short wave band communications receiver may be in the region of 0.5 microvolts for a 10 dB S/N in a 3 kHz bandwidth for SSB and Morse. For AM reception it may be in the region of 1.5 microvolts for a 10 dB S/N in a 6 kHz bandwidth with a 30 per cent modulation level. Note that a wider bandwidth is required for AM recep- tion and this means that the noise level is higher and sensitivity is less. In some instances a signal plus noise to noise (S + N)/N ratio is specified. The reason for this arises from the way the measurement is 172 Newnes Guide to Radio and Communications Technology made. A signal generator is connected to the input of the receiver, and at the audio output a meter is used to measure the audio level. With the signal generator turned off the level of the noise is noted at the output. Then the signal generator output is turned on and its level is adjusted SO that the output from the receiver is 10 dB higher than the noise level. When the signal generator is turned on the audio l