Noise Figure Measurement with the National Instruments RF Signal Generator and RF Vector Signal Analyzer

All devices have inherent noise. When noise is quantified, it is usually referred to the device input. In other words, all noise power a UUT inherits is assumed to come from its input. The noise figure of a UUT is the ratio in dB of its noise power to the noise power that a matched resistive load would deliver at room temperature.

Measuring Noise Figure

If you terminate a UUT input with a matched resistive load (typically 50 Ω) and measure the noise power density at its output (No), the noise figure (NF) is given by the following equation:

where G is the power gain of the UUT, k ≈ 1.38 ×10-23 is Boltzmann’s constant, and T0 ≈ 290° K is the room temperature.

When you use an analyzer such as the NI RF Vector Signal Analyzer (RF VSA) to measure the output noise of a UUT, the result of the measurement contains not only UUT noise but also noise intrinsic to the RF VSA. If the UUT gain (G) is known, compute the noise figure of the UUT with the following equation:

where Nrfsa is the noise measured by the RF VSA when its input is terminated with a matched resistive load and Nm is the measured noise with UUT attached. Both Nrfsa and Nm are given in Watts; G is a linear power gain.

Measurement Setup

A typical noise figure measurement setup is shown in the figure below:To measure the noise figure, perform the following steps:

1. Turn on the RF VSA and let it warm up for 20 minutes.
2. Turn on the UUT if it is active.
3. Set the RF VSA to the frequency of interest, and decrease the resolution bandwidth to about 1 kHz.
4. Terminate the RF VSA input with a broadband resistive load.
5. Obtain an average reading of the noise level. Make sure to take enough readings to obtain a good average.
6. Convert a reading taken in dBm to watts and normalize it to 1 Hz by dividing by the resolution bandwidth. This value is the noise floor of the RF VSA at that frequency, which is Nrfsa in this document.
7. Remove the load termination from the RF VSA input.
8. Attach the output of the UUT to the RF Signal Analyzer input.
9. Generate a known small signal from the PXI-565x RF Signal Generator into the UUT input. This signal level should be less then 10 dB below the 1 dB compression point of the UUT.
10. Measure the output signal level on the RF VSA to determine the gain (G) of the UUT.
11. Disconnect the PXI-565x and terminate the UUT input with a broadband resistive load.
12. Make another averaged reading of the noise with the UUT attached by repeating steps 5 and 6. This average is the noise value for the UUT and the RF VSA (Nm).
13. Substitute your values into the equationas follows to determine the UUT noise figure:

Where:
o The value from step 6 is N_rfsa
o The value from step 10 is G
o The value from step 12 is N_m

Resources

Common RF and Microwave Measurements
This is the main page of a series of tutorials focused on common RF measurements involving signal generators and analyzers.

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Reader Comments | Submit a comment »

I am amazed at how simple NI seems to make out Noise Figure measurements to be. 'Terminated Load' is a nice term especially when you do not define anything about it. This is not some DAQ application which works by assembling different blocks. Noise Figure has an inherent dependence on temperature, this setup pays no respect to that. What will be the accuracy of the readings taken from this setup? Will you be able to get any sort of repeatibility at all? The title is grand and eye-catching but have you even tried this before writing this article.
- Santosh Reddy, Agni Instruments. santosh.reddyn@gmail.com - Sep 4, 2007