Noise figure

Home measuring noise figure

 

 

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.