Ambient noise calculator


This article describes a simple method of assessing the ambient external noise arriving at an SSB receiver system.

The method measures the change in audio output power due to input noise with a change in input attenuation, and given the equivalent noise power of the system components, calculates the external noise power.

The technique depends on the fact that the audio output power of an SSB receiver is linearly related to the RF input power (including the equivalent internal noise power) up to the onset of AGC action, which is typically more than 20dB above the equivalent  input noise power. By using a known external attenuator to keep measured signals in this linear range, valid relative measurements can be made of the receiver audio output power and absolute results calculated by factoring in the attenuator, receiver equivalent internal noise power, and other configuration variables.


Fig 1: Block diagram of the measurement configuration.

The terms used in the calculator are as per Fig 1. The reference plane would usually be the antenna connector.

The audio voltmeter should ideally be a true RMS voltmeter with response that is flat across the full audio frequency range in the receiver output. It should not be assumed that all true RMS digital voltmeters are flat across the audio band. In the absence of a suitable true RMS meter, a conventional rectifier type AC voltmeter with adequate frequency response will introduce only a small error.

The Noise Figure of the receiver must be known. If the noise figure is not known, but some other expression of sensitivity and bandwidth is known, the   Receiver sensitivity metric converter may be of help to obtain the Noise Figure.

The attenuator must have calibrated steps or settings.


The measurement process depends on linear operation of the receiver. Care must be taken that the receiver does not suffer overload, audio clipping or any AGC gain compression when measurements are taken.

The measurement process is as follows:

  1. setup the equipment as shown in Figure 1
  2. enter the Noise Figure and Loss Rx to ref plane figures on the calculator form
  3. set the attenuator to zero
  4. select the appropriate Noise measurement units on the form
  5. set the receiver to conditions for which the Noise Figure is known
  6. tune the receiver to a quiet channel
  7. adjust the attenuator for the minimum attenuation consistent with no AGC compression / S-meter deflection
  8. enter the attenuator setting on the calculator form in the Observation #1 / Attenuator box
  9. adjust the receiver audio gain and measure the audio output
  10. enter the measurement in the Observation #1 / Measurement box
  11. increase the attenuation until the audio output changes noticeably
  12. enter the attenuator setting on the calculator form in the Observation #2 / Attenuator box
  13. measure the audio output (do NOT adjust the receiver audio gain, if you need to adjust the receiver audio gain, you must go back to step 9
  14. enter the measurement in the Observation #2 / Measurement box
  15. hit the form calculate button for results.

The calculator uses the entered data to calculate the ambient noise factor (Fa) and the ambient noise temperature (Ta).

Data entry

Noise measurements may be entered depending on your selection of "Noise measurement  units" on the input form, (consistently) in:

Temperature for each of the elements can be entered. In the absence of other knowledge, use room temperature of 290 K.

Noiseless Gain Adjustment (NGA) is noiseless gain or loss, use it to factor in noiseless gain adjustment at the antenna (such as mismatch loss).

Enter numeric data only as a number in decimal format, no units.

The calculator will stop processing if it detects conditions that would not occur in practice (eg negative Ta).

Input form

Noise measurement units NGA (dB)
Temperature (K)
Loss Rx to ref plane Temperature (K)
Measurements: Observation #1 Observation #2
Attenuator (dB)
Temperature (K)


If you're clever, you will have worked out you can download this page to your hard disk. If you're smart, you won't, you will run it from the web site and automatically take advantage of any updates.

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