evel meter is reading the level of the output signal plus any background noise, i.e. (S + N)/N. Note that during the measurements for this test when the signal generator output is turned off the impedance seen by the radio receiver must be 50 ohms. This is normally true because the output attenuators and other matching circuits on the signal generator ensure the output is maintained at this value. Occasionally the signal generator specification will mention that the signal voltage is given as a potential difference (PD) or the electromotive force (EMF). This is very important because there is a factor of two difference between them. The EMF is the open circuit voltage whereas the PD is the voltage under load as shown in Figure 5.55. If the load impedance is the same as the source impedance the EMF will be twice the PD at the load. In theory the EMF is a more correct way of specifying the voltage because the way in which the signal generator works assumes that it has been loaded with 50 ohms for the value to be correct. However, as the values with the PD look more impressive, it is this value which is normally stated. Even when EMF or PD is not specifically mentioned it should be assumed that PD is implied. Signal Matched generator EMF measured when load is not connected, PD measured with load in circuit Figure 5.55 EMF and PD in a signal generator SINAD Sensitivity may also be specified in terms of SINAD measurements. This is very similar to a signal plus noise to noise measurement. The test Receivers 173 method involves applying a signal modulated by a single tone. The audio from the receiver and the audio tone are notched out as required. In this way a measurement of signal plus noise plus distortion to noise plus distortion is obtained. The SINAD value is expressed in decibels. Using this system the sensitivity of a set is normally quoted as a given number of microvolts to give a certain value of SINAD. A figure of 12 dB SINAD is normally used as this represents a 25 per cent distortion and noise factor. SINAD