Interaction between test site and amplifier nonlinearities in susceptibility tests

Susceptibility tests performed with nonlinear power amplifiers may give nonrepeatable test results or even show susceptibility levels at the spurious frequencies which may be wrongly related to the frequency characteristics of the specified emitted signal. The control of the emission system can be performed in three different ways: local-field AGC, net-power AGC, and incident-power AGC. The first two modes may interact with the test site forcing the amplifier to work in saturation and therefore to generate a large amount of spurious signals which do not represent the wanted test signal. The test site interacts with the instrumentation because of reflections from the environment (mainly the ground plane) and-the high values of VSWR of the transmitting antenna. The incident-power AGC which is adopted in most recent EMC specifications may overcome these problems but only if the test site is carefully standardized-at least in the same manner as done for emission tests