led systems and bench evaluation of individual circuits. In Figure 20.14 the system is loaded with Gaussian noise from a If V, is assumed Gaussian then Equation 20.26 may be obtained, generating source that has a flat frequency spectrum of noise, ignoring d.c. terms and where * represents the convolution integral. bandwidth limited by the filter Flim A single defined channel at frequency f(c) is removed from the spectrum of noise by a filter Fsc before the noise band is transmitted (20.26) into the system. A measurement on the same channel at frequency f(c) selected by The second order noise power NP2 in watts is therefore given by filter Fpc at the receiver will therefore measure noise that has been generated only within the system itself. Equation 20.27, where Pch is the traffic load per channel in watts. The system is loaded with noise power P that represents the expected multichannel load, as in Equation 20.30, where Ps The (20.27) total power of the signal applied to the system, P is the multichan- ,,(f) nel load for N channels, and TL is the transmission level point in Likewise the third order noise power NP3 3 in watts is given by Ps ==Pn+TL (20.30) Equation 20.28, where the mean square voltage is given by Equation 20.29 and Pch is the traffic load per channel in watts. For instance, the figures for a 4MHz system at a -30dBm trans- mission level point will be P. = +14.8dBmO (assuming the normal channel loading of -15dBmO). P will evaluate to -15.2dBm at TL= -30dBm. Two measurements are taken. The first is with the channel stop (20.28) filter at the transmitter by-passed with the switch S in the closed position. This gives the power due to simulated traffic in a single (20.29) channel referred to the meter calibration. Measured figure = =No. The second measurement is with the channel stop filter at the transmitter in circuit with the switch S open. This gives the noise The convolution is best performed numerically on a per system power generated from the system referred to the same meter cali- basis. bratio