Noise Figure Measurement Using Spectrum Analyzer

- details and method for using a spectrum analyser to measure noise figure..

For any radio frequency, RF amplifier or system, the noise figure is a key parameter.

Measuring noise figure may not always be easy and while the easiest way is to use a specialised noise figure meter, one of these may not always be available.

Therefore using a spectrum analyser to measure the noise figure can be a very useful option, as these test instruments are often available within RF laboratories.

Noise figure measurements with a spectrum analyser

Using a spectrum analyser to measure noise figure has a number of advantages:

  • Uses available equipment:   Using a spectrum analyser to measure noise figure is often convenient because it utilises test equipment that will be found in many RF development or test laboratories. A dedicated noise figure meter may not be available.
  • Wide frequency range:   Spectrum analyser noise figure measurements can be made at any frequency within the range of the spectrum analyser. A variety of frequencies can be used for different devices without changes to the test configuration.
  • Frequency selective noise figure measurements:   Measurements can be made to be frequency selective, independent of device bandwidth and spurious responses.

Using a spectrum analyser may not as accurate as those obtained when using a noise figure meter, but this is very dependent upon the spectrum analyser and the measurement method used. The "Y" factor method is often accepted as being equally as accurate as that of a specialised noise figure meter, but requires the use of a noise source. Some spectrum analyzers have software built in to run these tests.

Noise figure measurement basics

Noise figure measurements are important because the limit of sensitivity of a radio or wireless receiver is limited by noise. In an ideal system this would be limited by noise picked up at the antenna, but in reality all systems generate some noise themselves.

Note on Noise Figure:

The noise figure for an RF system or an element within an RF system is a figure of merit expressed in decibels indicating the level of noise introduced. The ideal noise figure is 1, and anything above this indicates that noise is introduced.

Click on the link for a Noise Figure

In order to measure noise figure using spectrum analyser it is necessary to manipulate the equations a little.

    N = noise power output
    k = Boltmann's constant - 1.374 x 10^-23 joule/°C
    T = temperature in ° Kelvin, i.e. 290° is room temperature
    B = bandwidth in Hz
    G = device gain
    F = Noise factor (Noise figure is the noise factor expressed in decibels)

It is possible to set the equation based on the Noise Figure in dB and split it into the different elements of the equation.

This noise figure equation can be further reorganised. In this equation the noise power bandwidth is set by the resolution bandwidth of the spectrum analyser. Therefore the equation be reorganised as follows:

In some instances it may be necessary to add some factors to accommodate for the real responses, etc of the analyser against the theoretical requirements for making noise figure measurements with a spectrum analyzer:

  • Filter:   As the filter response cannot be a complete rectangular shape and the noise power bandwidth and the resolution bandwidth are not the same. A typical figure quoted for some analyzers was that the filter bandwidth was around 1.2 times the resolution bandwidth which equates to an adjustment of 0.8 dB.
  • Noise level:   The averaging effect of the video filters, etc on analogue spectrum analyzers tended to give a reading that was around 2.5 dB below the RMS noise level.

The overall adjustment is around +1.7 dB (i.e. 2.5 - 0.8). However most modern spectrum analyzers will have corrections for these discrepancies and the makers literature or help material should be consulted regarding them.

To make the noise figure measurements required, the spectrum analyzer should have a noise floor that is 6dB lower than the noise emanating from the device under test. As this will typically be an amplifier, its noise level is likely to be greater. If not a further low noise amplifier can be added after the device under test to bring the noise level up - note that the noise will tend to be at the input to the device under test if it is an amplifier.

The noise figure can then be calculated as:

    N = noise power output
    Gd = Gain of device under test in dB
    Gamp = Gain of additional amplifier in dB
    B = Bandwidth in Hz

In this way the noise figure can be measured with a knowledge of the device gain, measurement bandwidth and noise power output.

Noise figure measurement procedure

The test for measuring noise figure using this method is quite straightforward. It consists of simple stages:

  1. Measure gain of system:   One of the key elements in the noise figure equation is the system gain. This needs to be measured by the system. Typically this is achieved by using a signal generator fed into the device under test. The gain can be measured simply by measuring the signal level directly from the output of the signal generator and then with the amplifier in circuit.

  2. Measure noise power:   The next step is to disconnect the signal generator and terminate the input to the device under test with a resistance equal to its characteristic impedance.

    With the lower signal level, i.e. just the noise power from the device, the input attenuator level of the spectrum analyser may need to be adjusted, e.g. to 0dB, and sufficient video averaging applied to obtain a good reading for the noise level.

  3. Calculate noise figure:   Using readings for the average noise power and the bandwidth for the analyzer, these can be substituted in the equation above to give the noise figure for the device under test.

By Ian Poole

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