What is Noise Figure (NF)?
Noise figure is defined as the ratio of the signal to noise power at the input
to the signal to noise power at the output of a device, in other words, the
degradation of signal to noise ratio as the signal passes through the device.
Since the input noise level is usually thermal noise from the source the
convention is to adopt a reference temperature of 290°K. The noise figure
becomes the ratio of the total noise power output to that portion of the noise
power output due to noise at input when the source is 290°K.
How is noise figure measured and calculated?
Noise figure is typically determined by using a calibrated noise source which is
traceable to international standards. This noise source is essentially compared
to the unknown noise figure and by measuring this difference noise figure is
computed:
NF = ENR dB – 10 log (Y – 1) + Tcorr
Tcorr is a temperature correction factor that can be applied if the temperature
deviates significantly from 290°K. Y is the Y factor which is the ratio of the
output power with the noise on to the output power with the noise off. By
employing this method of measuring the Y factor, only relative accuracies are
significant which makes the measurement easier than attempting to measure exact
powers which can be quite low and tough to measure.
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