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Noise Figure, the Y Factor Method and Uncertainties

April 30th, 2011. Posted in Test and Measurement, Tutorials

Any electronic device or component produces Noise. Its a fact of life. Even a passive resistor when at room temperature has movement of its electrons which results in noise. This is termed as Thermal Noise and the minimum this can be with a system at room temperature is -174dBm/Hz (-174dBm measured in a 1Hz bandwidth).

The noise power in a resistor is given by:

Noise Equation

where, k = Boltzmann’s constant in Joules per Kelvin, T = absolute room temp = 293 kelvin, B = Bandwidth, 1 Hz in this example. Then converted to dBm’s.

What is Noise Figure?

Noise Figure is a value that defines the amount of noise that a device adds to a signal that passes through it. When many components are added together in a circuit each component contributes to the overall noise generated. This is important for many types of circuit including transmitters and receivers. In a transmit stage, the noise contributes to degradation of the signal quality. In the receiver, noise limits the sensitivity of the circuit. This makes Noise Figure an important parameter to be measured for many electronic circuits.

Noise Figure itself is defined as:

Noise Figure Equation

Where F is the Noise Factor and is given by:

Noise Factor

Signal to Noise Input to Output

Noise Figure can be measured using various different methods. Indeed it is now possible to measure Noise Figure using a Vector Network Analyser and without the need for any kind of Noise Source as this document from Rohde and Schwarz describes.

The most common method of Noise Figure measurement involves using a Spectrum Analyser with a Noise Source and is called the Y Factor Method.

This technique makes use of a calibrated broadband noise source that contains two temperature states: A high temperature state, T(ON source) with a higher output of noise power, and a low temperature state, T(OFF source) with reduced noise output. The noise source is applied to the input of the device under test and the noise power at the output of the DUT is measured for each of the two input noise states. The noise figure and gain of the DUT are calculated from these measurements.

A typical setup for this is below:

Typical Noise Figure Setup

A new application note titled “The Y Factor Technique for Noise Figure Measurements” by two colleagues Mike Leffel and Rick Daniel has been released which aims to give a detailed account of Noise Figure measurements. The equations that define noise figure, how Y Factor works along with calculation of uncertainties when using various pieces of test equipment. This is highly recommended for a anyone who needs to make noise figure measurements.

Pn = k.T.Bw (k = Boltzmann constant, T = absolute room temp = 293k, B = BW, 1 Hz in this example). Then converted to dB….10xLog(Pn.1000) due to power
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