ormance of the amplifier in this region is not practical. P(A+B)H2+6+P(A)+P(B) (20.15) As the output level is reduced and moves out of the overload region, the shape of the harmonic curve passes the knee and enters a linear portion of the graph where the second harmonic level P_(3A)=H1++PP(A) (20.16) reduces by 2dB and the third harmonic by 3db for every 1dB drop in fundamental signal. The harmonic performance is characterised m(2AB)H3a+9.6+2P(A) = P(B) (20.17) in this region. If the amplifier transfer characteristics in the 'linear' region are assumed to be a square law, Equation 20.10 is obtained, where is the input voltage, given by Equation 20.11, V is the output voltage, +P(A)+P(B)+P(C) (20.18) and a , az a a re constants of the amplifier. 20/12 Frequency division multiplexing Thus, once H2a and H 3a have been determined by measurement, lation products for a particular type (i.e. A + B, A + B, etc.) falling all the second and third order intermodulation products can be in a particular channel of interest. deduced for a specific transmission level. The factor U(x) is derived from Bennett's formulae but for sim- plicity are usually shown in graphical form as in Figure 20.12. These graphs are valid for systems of greater than 500 channels 20.7.4.4 Determination of unlinearity noise from a multi- (Bell, 1971). channel load The technique used (Bennett, 1940) is to show that, for the purpose 20.7.4.5 Approximate value for the weighted intermodulation of calculating unlinearity or intermodulation noise, a channel noise contribution loaded with a single sinusoid can be considered equivalent to a channel loaded with gaussian (or speech ) noise by the application For wide band systems there are three major intermodulation con- of a suitable factor k(x) to the noise contribution A band of n speech tributors A + B, A - B and Assuming the CCITT accepted channels thus becomes a band of n sinusoids and the problem is values for channel loading (see Table 20.6) a rule of thumb calcula- reduced to one of counting intermodula