Ambient noise calculator

Overview

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.

Measurement

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:

setup the equipment as shown in Figure 1
enter the Noise Figure and Loss Rx to ref plane figures on the calculator form
set the attenuator to zero
select the appropriate Noise measurement units on the form
set the receiver to conditions for which the Noise Figure is known
tune the receiver to a quiet channel
adjust the attenuator for the minimum attenuation consistent with no AGC compression / S-meter deflection
enter the attenuator setting on the calculator form in the Observation #1 / Attenuator box
adjust the receiver audio gain and measure the audio output
enter the measurement in the Observation #1 / Measurement box
increase the attenuation until the audio output changes noticeably
enter the attenuator setting on the calculator form in the Observation #2 / Attenuator box
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
enter the measurement in the Observation #2 / Measurement box
hit the form calculate button for results.

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

Data entry

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

dBs;
Watts (or proportional to Watts by a constant, eg all input in mW); or
Volts (or proportional to Volts by a constant, eg all input in mV)

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 T_a).

Input form

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

Notes:

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.

Other VK1OD tools.