One of the first ways in which noise was specified was related to radio receivers. Here noise specifications relate to the receiver sensitivity and include specifications including signal to noise ratio, noise figure and the like.

Note on Receiver Sensitivity:

The main limiting factor on receiver sensitivity is noise. Accordingly receiver sensitivity specifications revolve around noise specifications. The chief receiver specifications are signal to noise ratio, SINAD and noise figure. Each looks at the receiver performance with respect to noise.

Click for more information on Receiver sensitivity.

For many other applications, the noise spectral density is used. This is a measure of the noise power within a certain bandwidth. Noise power density has the dimension of power / frequency.

Noise in 1 Hz bandwidth

Typically the specification is specified in dBm and within a 1 Hz bandwidth, i.e. dBm / Hz.

Interestingly the thermal noise in a 50 Ω system at room temperature is -174 dBm / Hz.

It is then easy to relate this to other bandwidths:

BANDWIDTH (ΔF) HZ	THERMAL NOISE POWER DBM
1	-174
10	-164
100	-154
1k	-144
10k	-134

Noise measurement techniques

There are many ways of measuring noise in an electronic of RF system. There are specialised meters for measuring noise figure, SINAD, and other figures. Apart from these, standard test equipment may also be used.

• Meter method: It is possible to utilise a simple meter and other elements to measure noise levels. The concept of noise measurement system can be explained by looking at what is required.

Simple noise measurement system

The noise measurement system shows a typical measurement circuit. Noise generated by the unit under test is amplified to a suitable level - the gain of the amplifier must be known.

The meter used should have an averaging capability, because the noise level is random and will vary. While most digital meters have an averaging capability, analogue meters inherently average any changes.

• Spectrum analyser: Most modern spectrum analysers have built in capabilities for measuring noise levels. The analyser settings should be selected to ensure that the optimum conditions are set. These will depend to a large degree on the analyser used.

Plot of phase noise using spectrum analyser

Noise measurement precautions

One of the easiest ways of measuring noise levels is to use a spectrum analyser. It is able to determine the noise power in a given bandwidth. This can then be related to another bandwidth by scaling the power level measured to the required bandwidth.

However there are other factors to take into account.

• Filter shape: As the spectrum analyser filter shape is not completely square and needs a finite band to change from its pass-band to its stop-band, this shape needs to be accommodated when calculating the noise in a given bandwidth.

Filter responses for noise measurement

Fortunately this is a simple calculation for modern analysers that can give readings of noise power in a given bandwidth - this is often normalised to dBm / Hz.

• Spectrum analyser noise performance: The noise performance of the spectrum analyser must be better than the noise to be measured. If not, the reading will be that of the spectrum analyser.

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