Noise Figure Measurement [Gain Method]
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- Опубліковано 23 сер 2024
- This video shows how to measure the Noise Figure of an amplifier using nothing but a spectrum analyzer using the 'Gain method.' The D.U.T. used in this video is a Mini Circuits ZKL-2+ amplifier. A Tektronix MDO4104B-6 serves as spectrum analyzer.
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![Noise Figure Measurement [Y Factor Method]](http://i.ytimg.com/vi/4SB4YHdqRSQ/mqdefault.jpg)
![Noise Figure Measurement [Y Factor Method]](/img/tr.png)
Nice video. You really need to terminate the input of that amp with 50 Ohms, as it will make a noticeable difference in your measurement; notice it is -141 dBm/Hz in this video but -138 dBm/Hz in your other video with the noise source off.
Also, if the MDO uses digital down conversion to compute the spectrum then you are OK since it is taking the log of the average; i.e. it's all DSP following linear down conversion. Although in a typical spectrum analyzer it is taking the average of the log (a logarithmic amplifier), which is why they use a special noise marker mode, since white noise is statistically different than a sine wave, and there is a 1.4 dB difference between the two after logarithmic amplification.
Anyway, nice videos.
Hi Sebastian, nice video showing the basic "Gain Method". Many modern spectrum analyzers offer very low displayed average noise levels and very small resolution bandwidths which are very helpful when using the "Gain Mehod". My spectrum analyzer provides a resolution bandwidth of 1 Hz when the frequency span is small which really makes this method a breeze, and the displayed average noise level is very low when running a 1 Hz resolution bandwidth.
The best NF video on UA-cam. Thank you!
Thanks, glad you enjoyed it!
@@BalticLab Can you share that PDF in the video with the Noise Power table
Just noticed you were doing trace averaging though, which is interesting, since that is equivalent to averaging the log instead of log of the average. This would give the 1.4 dB error even with a signal analyzer such as the MDO. Need to reduce the VBW instead, which would average prior to the log.
very helpful information. the MDO also should have signal path compensation preformed before a measurement like this.
How can you compare gain at 9V with NF at 12V. Both of them should be calculated at same voltage!
I thought this was great! Thank you very much for a clear explanation of this procedure.
If you use kTB as input noise, the input has to be terminated with 50 ohm
Hmmm, is it necessary to account for the noise figure of the spectrum analyzer itself?
It only plays a role if the NF of the amp is very low. And generally, this method is only suitable for high gain / high NF type D.U.T.s. I'll show a different, more accurate method shortly.
@@BalticLab Actually it plays a very significant role even if NF of the amp is relatively high, 15+ dB. This is because typical spectrum analyzer has a high NF, 11+ dB. There are two ways to check this. 1) Compare noise floor of a spectrum analyzer versus calculated termal noise 2) Measure NF of the given amplifier using 'Gain method' and Y-factor method, calculate NF of the spectrum analyzer using Friis formula for a NF of a cascaded amplifiers.
how to calculate noise when my output from amplifier is not a straight line as in video but have crest and troughs over the spectrum. where should I measure the noise. you have chosen the marker at 10MHz since my response is not a straight line where should the marker be? why is the span arbitrary, shouldn't it be equal to bandwidth of the amplifier? what does resolution bandwidth do and how do you choose a value?
In my day, spectrum analysers typically had an inherent noise figure of about 25 dB. Unity gain, but NF 25 dB. So, with the input of the SA input terminated and a RBW of 10kHz, the display of the SA would typically be -134 plus NF of 25 = -119dBm . The gain of your DUT is lost in this. I don’t think this methodology is appropriate.
On Amp datasheet we look at 9V and 10MHz, on thermal worksheet we use the 10kHz number and then after all the math we compare it to the 12V noise figure. I like the concept here but it seems like we didn't use all the right info to analyze actual noise figure
We have taken 10 kHz which is the RBW and used that in the equation. 1 MHz is the frequency point at which the Noise power density is measured.
Is the measured NF lower because the MDO's NF was not accounted ?
Hey Gregory, there's various reasons. The MDO's NF was not accounted for, the input wasn't properly terminated and the trace averaging was adding some error as well.
Million likes for that video straight to the karma!
thank you very much
The real gain of DUT should be reduced by cable loss, about 0.5dB
Would I be able to see -174dBm on spectrum analyzer when nothing to connected at the input? What is the setting? Setting RBW to 1 Hz, what about Video BW?
video about stereo multiplex from scratches
still waiting aI
But leaving the input port open the noise figure is not minimized. According to my university lectures reminds there is an optimum source impedance which minimize the noise source. It is correct? Thanks!
+federico filosomi You are correct, it's supposed to be terminated properly. I ignored it in this video to make it as basic as possible
it's somewhat strange, i am curious what about the sa that cannot have ktb measurement results.
for 10kHz, it should be -134 dBm, but sa shows -89dBm without any DUT connection.
That -89dBm is the noise floor of your spectrum analyzer. Basically the noise from within the SA is much higher than the input noise.
Did you measure the noise spectral density of your spectrum analyzer before connecting the amp?
+hasanRubaie Negative. This was supposed to be a very basic video for people who have no clue how to make noise figure measurements at all. There's a lot of technical errors and oversights in this video. But they're all fine for the purpose of this vid.
I think there is something called Noise correction in Spec An,that resets the effect of noise introduced by the spec An itself to zero for measurement purpose.
is it possible to meassure Scattering-Parameters with spectrum analyzer ?
You need phase information for accurate measurements. So not really.
Nice explained #discreteoptical
Measuring NF is not this easy....without feeding actual signal you are just measuring the noise floor with amplifier on and off. But there is lot of stuff happens when you actually feed in signal as biasing will effect noise performance, your non linearity will and most of all this is not actual platform which noise will follow when the amplifier is in operation. Feeding through 50 ohm makes a lot sense coz you need antenna while feeding in the signal and how its input matching could deteriorate the noise performance you can see in simulations. That is the reason we measure noise figure with SNRinput/SNRoutput ratios.
This technique is indeed one agreed upon technique for determining Noise Figure (its called the Gain Method which KB5OFS mentioned). Many modern Spectrum Analyzers provide displayed average noise levels of -161 dBm/Hz or better and resolution bandwidths of 10 Hz or better (mine provides 1 Hz resolution bandwidth if the frequency span is very small), and this makes the Gain technique a viable bench top test for the average person that's not complicated. Granted you raise some interesting issues, but for basic Noise Figure comparisons between various preamps, the Gain Method is a reasonable technique.
good video