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