Using an attenuator for NoiseLo/NoiseHi in NFM

A common method of making Noise Figure measurements of a receiver is to use a noise generator of known noise power. The output power of the DUT is measured with the generator off (NoiseLo) and on (NoiseHi), a Y factor calculated, and from that Noise Figure is calculated.

The noise power of a generator could be expressed as an equivalent noise temperature Ts, but is often given as an Excess Noise Ratio (ENR):

ENR=10*log((Thi-Tlo)/290) dB.

An attenuator after a noise source decreases the ENR by the attenuation:

ENR’=ENR-A dB where A is the attenuation in dB.

The property can be used to obtain a different ENR from the noise source, a different Thot.

A known noise source can be used with a known attenuator to provide two different input conditions for measurement of a Y factor and calculation of NF. The variation here is that the noise source is on for both halves of the test, and the applicable source noise temperature needs to be entered into NFM.

Lets explain using an example.

Lets take a noise source of ENR=50dB, and 40dB and 50dB attenuators.


  1. before making measurements, set Tcold in NFM to 290, set ENR to 0dB (50-50) and note calculated 0dB equivalent Thot value, it is 580.0K (this will be used for Tcold later);
  2. set ENR to 10dB (50-40) and a new value of Thot is calculated (3190.0).
  3. now enter the calculated 10dB equivalent Thot value (580.0K) into Tcold;
  4. optional: measure SCZ with the receiver powered OFF;
  5. turn the receiver ON;
  6. with the noise source ON and 10dB attenuator in line, measure NoiseLo;
  7. with the noise source OFF and no attenuator in line, measure NoiseHi;
  8. NFM will try to calculate NF.


Above, the NFM screen showing such a measurement.