Image Courtesy: Microwave Journal

    The figure above depicts a test setup for NPR (noise power ratio) measurement. In this method, an AWGN generator produces white noise. The noise is bandlimited by a filter before being fed to the DUT (Amplifier). The filter used is a notch filter with a notch greater than 50 dB below the passband amplitude and a width of approximately 1% or less than the filtered noise BW.

    The amplitude of the applied noise signal is increased to the point of nonlinearity. As discussed earlier, IMD products result from the mixing of multiple tones when the amplifier is in the nonlinear region. As a noise signal consists of many tones, measuring individual IMD products is difficult. Therefore, the aggregate power of IMD products is measured at the notch frequency using a spectrum analyzer or VNA. As the products increase, the depth of the notch decreases.

    This method can be expensive as it requires a high-quality filter with sufficient notch depth to observe the desired IMD products.