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Non-Zero Noise Figure After Calibration Application Note 1484 The increasing demand for wireless products combined with the availability of modern easy-to-use measurement equipment mean that noise figure measurements are more popular than ever. This is especially true in the microwave region of the spectrum where more and more wireless standards are being developed. Noise figure measurements rely on a calibrated noise source as a reference. The general measurement process is shown in the figures below. In Figure 1, the noise source is connected directly to the input of the measuring instrument and a user calibration is performed. This measures and stores the instrument's own noise figure at its various attenuator settings. These results are used to remove the effect of "second-stage" noise contribution during a corrected measurement. The measurement arrangement, with the device-under-test (DUT) inserted between the noise source and the instrument is shown in Figure 2. Calibration setup Noice source Figure 1. Calibration Noice source F1, G1 DUT F2 Figure 2. Measurement Immediately after calibration, the noise source is still connected directly to the instrument, which automatically switches to the corrected measurement mode. In this configuration the instrument would be expected to display a noise figure and gain of 0 dB, because there is no DUT present. In practice however, the instrument may show a noise figure of plus or minus a few tenths of a dB as well as an even lower level of gain, but not zero. While this is quite normal, some users assume that this zero-error will be added to their DUT's noise figure, producing an inaccurate result. This application note attempts to explain the reasons behind the zero-error and shows with examples that it does not have a compromising effect on instrument accuracy in a measurement situation. Theory Figure 2 along with the basic noise figure equation will be used as the basis of the explanation. The general equation for the noise factor of two cascaded stages as defined by Friis (Proceedings of the IRE, July 1944, pp.419-422) as: (F2