Crosstalk Due to Finite Limiting of Frequency-Multiplexed Signals

It is a well-known fact that amplitude nonlinearity produces crosstalk between frequency-multiplexed channels, an important example being clipping in a saturating amplifier. Since the amplitude distribution of the composite signal for a large number of channels is essentially Gaussian, the crosstalk can be calculated with reasonable accuracy by assuming that the input signal is random noise, except for a narrow gap in which a sine wave is inserted to represent the signal in a selected channel. This assumption allows use of standard analytical techniques to determine the output signal-to-crosstalk ratio in the selected channel as a function of the clipping level. The resulting value for infinite clipping is about 9 db. The crosstalk decreases rapidly as the clipping level is raised, and a value of 40 db is obtained for clipping one per cent of time. An optimum clipping level, which provides the highest signal-to-total-interference ratio, may be determined when noise is present in the receiver, and allows definition of "peak factor allowance." An allowance of several db is found to be adequate for frequency-multiplexed binary data channels.