(CCITT, 1985; Bell, 1971.) Fundamental level at amplifier output Table 20.6 Values for Lo Pvo g and t Assumed Transmission Level -15dBm (=0dBmO) Pvo(dBmO) Figure 20.11 Amplifer characteristics (dBmO) CCITT -15.0 -12.9 -14.5 -13.9 (20.10) International Vi = V cos w t (20.11) 20.7.4.2 Multichannel load From this transfer characteristic Equations 20.12 and 20.13 may be obtained. The average traffic power Pn in a multichannel load of N channels is given by Equation 20.8, where N 240, assuming that a single (20.12) channel load figure is given by the -15dBmO value in Table 20.6. Pn = dBmO (20.8) (20.13) For N < 240 the statistics of the activity factor dictate that a slightly higher value for the average load should be used. A best fit curve is given by Equation 20.9 (CCITT, 1985). Thus the second harmonic (cos 2 W is proportional to the square Pn = - 1 + 4log N dBmO (20.9) of input signal amplitude V. Likewise the third harmonic (cos 3 w 1) is proportional to the cube of the input signal amplitude. In dB terms it can be shown (Bell, 1971) that at a transmission level of 0dBmO the power in a sinusoidal harmonic or intermodu- 20.7.4.3 Unlinearity characterisation lation tone, P ( x) in dBmO, at the amplifier output is related to the output level of the fundamental tones A and B by Equations 20.14 For a sinusoid fundamental signal at the output of a typical ampli- fier, the output power of the second and third harmonics would to 20.18, where m(x) is the harmonic or intermodulation power in appear as in Figure 20.11. In order to calculate the effect of the dBmO, P(x) is the fundamental power in dBmO, H2a and H3a are the second and third harmonic powers in dBmO, characterised for the unlinearity on the transmission it is necessary to be able to quantify amplifier with fundamentals at 0dBmO. (Figure 20.11.) the harmonics and associated intermodulation products. At the overload point, or clipping region, the harmonics increase very rapidly with increasing fundamental level and the charac- (20.14) terisation of harmonic perf