Determining the noise parameters of twoports requires measurement of spectral noise-power density. Normally such measurements are carried out with highly sensitive receivers by the radiometer principle or with commercial, automatic noise-factor meters. Lately spectrum analyzers too come equipped for this application, although they are mainly intended for observing spectra with discrete frequencies rather than precise measurement of extremely low noise levels. Stability of receiver parameters with time is an important criterion for these measurements so that calibration should not be necessary more than once a day.
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