The combined output is screened by a lowpass filter to significantly reduce the second and higher-order harmonics of the input signal. The DUT output is connected to a spectrum analyzer for the IM measurement. In order to prevent interaction that creates IM signals, generators must be sufficiently isolated from each other.
Additional isolation is provided by the power combiner and matching pads.
To ensure a proper measurement, the harmonics and IM products of the input signal applied to the DUT must be low enough to avoid errors. Thus, the input signal to the DUT must have sufficiently low second-harmonic levels at 2f 1 and 2f 2 . The second harmonics of the input signals should also be at least 80 dB down. This must be done for each DUT at every measurement frequency. Another important aspect of IM measurement is ensuring that the input-signal strength to the analyzer is not too high, since the analyzer itself can generate IM and produce erroneous readings.