Noise Figure Measurement with Spectrum Analyzer - Page 1
Forum section: electronics / rf-microwave

author: DRBuck
author_rank: Contributor
timestamp: March 15, 2022, 02:34:54 am
content: I've read a few articles on how to measure noise figure using the "Gain Method" with a spectrum analyzer and all they did was confuse me. The procedures and settings appear to be specific to the analyzer being used. I'm looking for help in learning how to do this using my analyzer. I have been playing with a bunch of Chinese broadband "low noise" preamps and would like to find out what the noise figure is for them. The preamps are spec'd 2MHz-2000MHz & 5MHz-6000MHz. I'll like to use them on the VHF/UHF ham bands, 144MHz, 222MHz, 432MHz, and 1296MHz.I have a Siglent SSA-3021X spectrum analyzer and Rigol DSG-815 signal generator. Can someone provide me the preferred analyzer settings and steps to do noise figure measurements?

author: TimFox
author_rank: Super Contributor
timestamp: March 15, 2022, 03:15:38 am
content: The "gain method", simply stated, is to do a careful measurement of the AC gain from input to output of the DUT, then terminate the DUT with the appropriate impedance and measure the total noise at the output with your SA in a well-defined noise bandwidth. Knowing the gain then allows you to determine the noise referred to the input and calculate the noise figure. If the amplifier has a good noise figure, this procedure is hard to do accurately. On some of my best amplifiers, I measured negative noise figures in this manner, which would have earned a Nobel Prize for me if the data had been correct.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: March 15, 2022, 06:25:44 am
content: As luck would have it, I just finished a seminar on noise figure measurements using SA with Y-factor and VNA with "gain" method (which we call "cold-source" method). You can likely view it here:https://connectlp.keysight.com/LP=31495?elqCampaignId=kcomOne key point by the previous poster is that you need two accurate measurements, a measure of gain and a measure of noise. Gain measurements using Signal Analyzers are typically accurate to between 1 and 2 dB; noise power measurements are on the same order so you have maybe 2-3 dB uncertainty on your NF measurement using SA gain method. Y-factor (using a noise source) is much lower error (for reasons you can see in the webinar). The cold-source method using VNA is practical to get 0.1 dB uncertainty because VNA has very low error in gain (0.02 dB) and can get quite low error in noise power measurements (if you have a special noise receiver in the VNA) on the order of 0.08 dB.Bottom line: without a noise source, you can get an idea of NF of an LNA but with really big error bars.Don't know about other manufacturers, but with our SA's you would normally use a noise power band-marker to get the noise power reading, and a fixed CW signal to get the gain.

author: xmo
author_rank: Regular Contributor
timestamp: March 15, 2022, 04:09:23 pm
content: Since you have the spectrum analyzer and signal generator, some experiments could be educational. I came up with the following overnight. Dr. Joel will rap me across the knuckles with a virtual ruler if I was hallucinating.First, place a termination on your spectrum analyzer input. Activate the internal preamp, set your center frequency to one appropriate for the preamp you want to test - say 144.1 MHz, set attenuation to zero, and adjust your reference level for best sensitivity (lowest displayed noise floor)Now, using the marker noise function, get a noise power reading in dBm/Hz. If the displayed reading isn't completely stable, you can add additional trace averaging. If your instrument were an "ideal" device - that is to say 'perfect' with a noise figure of zero, the marker would display the thermal noise floor.Given that the accepted thermal noise of a termination is -174 dBm/Hz, the difference between -174 and your displayed reading is due the the instrument's noise figure. Let's say your reading is -165 dBm/Hz so you conclude that your instrument has a 9 dB noise figure. Record that as Noise figure 2.You also need an accurate measurement of the preamp gain. Using your tracking generator, normalize your test setup and then insert the preamp. Record your gain value. Also try this using your signal generator as a CW source and the analyzer to read amplitude. You should have very close concurrence. Be sure to use source levels appropriate for the device so as to stay in its linear operational range.Next, terminate the preamp input and again take a noise marker reading. Record this as 'cascaded noise figure'Now we will use Friis method: NFC = NF1 + [(NF2 -1) ÷ G1]Normally we use this method to calculate a cascaded noise figure when we know the gain and noise figure of a preamp we are placing ahead of a receiver - but it's just math.We have three of the equation's variables so we can solve for the fourth - in this case - the noise figure of the preamp. Be aware that the equation requires the gain and noise factor terms to be linear.That just takes some more simple math: Gain in dBm = 10(log G), hence G = antilog(Gain in dBm/10)Here's an example. Say you have noise figure 2 = 9 dB, Gain = 15 dB, and cascaded noise figure = 2.56 dB.NF2 @ 9 dB = 7.9433 Noise FactorG1 @ 15 dB = 31.6228 Gain FactorCascaded noise figure 2.56 dB = 1.8045 NFcNF1 = 1.8045 - [(7.9433 - 1) / 31.6228]NF1 = 1.5849Noise Figure of preamp = 2 dBSuppose the manufacturer's specification for the preamp you were testing also happens to be 2 dB. That might give you some confidence in your method. As you test other devices and compare your results to their specs, you may learn more.There will still be uncertainty but the results may be a reasonable way to compare one device against another.Let us know your results.