NFM implements the method of Agilent application notes AN 57-1 and AN 57-2 for noise figure measurement, with the addition of an adjustable attenuator after the noise source and between the DUT and instrument. This web page is not a tutorial on noise figure measurement, the above two applications notes do such a good job that I wouldn't try to improve on them. The terminology on this page is largely consistent with that in the applications notes.

NFM also incorporates a facility for true relative audio power measurement using the PC sound card, with a selectable integration time.

Fig 1: Noise Figure measurement using a two stage system.

The terms used in the calculator are as per Fig 1. 

Data entry

Audio output noise measurements may be performed using the PC sound card, or entered directly.

Direct entry can be in your selection of "Noise HI/LO" units on the input form, (consistently) in:

Enter only a number in decimal format, no units.

The measurements in the second stage must be wrt the same reference as used in the first stage for correct results. The calculator will stop processing if it detects conditions that would not occur in practice (eg negative Te).

Measurements using the sound card will be displayed in dBfs.

Fig 2: Screen shot of NFM

Saved measurement results

Measurement results can be saved to a text file or copied to the clipboard to paste into another application.

Additionally, a condensed snapshot of measurement results can be saved as a row of a Tab Separated Values (or tab delimited text) file for import to other applications (eg Excel).

Internal audio power measurement with high resolution

Internal audio true power measurement with selectable integration times from 0.15s to 100s allows reduction of measurement uncertainty due to sample set size. Unregistered copies are restricted to a maximum integration time of 0.5s.

For example, an integration time of 10s with post detection bandwidth of 2kHz has an uncertainty of 0.07dB to 90% confidence.

Imports noise source calibration file

Calibration file import facility, ENR at operating frequency calculated by cubic spline interpolation of the calibration file data points.

Windows Scripting

Important controls and menu functions in NFM have shortcut keys to allow convenient automation of test processes, especially repetitive measurements, using Windows Scripting.




Term Meaning
dB decibel - power ratio
dBm decibels - power wrt 1 mW
dBfs decibels wrt full scale


Version Date Description
1.01 28/05/2007 Initial.

Use at your own risk, not warranted for any purpose. Do not depend on any results without independent verification.

© Copyright: Owen Duffy 1995, 2021. All rights reserved. Disclaimer.