TABLE OF CONTENTS Many of the circuits that are used in electronic systems are considered to be linear. Th means that for a sinusoidal input, the output will also be sinusoidal with perhaps a different amplitude and phase. [1] In the time domain, the user expects to see an output waveform that has the exact same shape as the input waveform. In the frequency domain, we expect to see at the output the same frequency that was at the input (and only that frequency). Any other frequencies that are generated due to the input signal are considered distortion. [2]
Most of the distortion mechanisms measured with spectrum analyzers are low level. That is, the devices producing the distortion are mostly linear and have only a slight nonlinear behavior. Such a weakly nonlinear system can be modeled with a power series.
| (7-1) |  |
The first coefficient, k 0, represents the DC offset in the system. The second coefficient, k 1, is the gain of the circuit associated with linear circuit theory. The remaining coefficients, k 2 and above, represent the nonlinear behavior of the circuit. If the circuit were completely linear, all of the coefficients except k 1 would be zero.
The model can be simplified by ignoring the terms that come after the k 3 term. For gradual nonlinearities, the size of k n decreases rapidly as n gets larger. For many applications the reduced model is sufficient, since the second-order and...
