Preamplifier to Improve SA Noise Floor? - Page 1
Forum section: electronics / projects

author: TimNJ
author_rank: Super Contributor
timestamp: October 01, 2020, 03:30:53 am
content: Hi all,I'm back to bother you all again with another RF question. I'm having some issues trying to perform a pre-compliance conducted EMI measurement. Instead of direct connection to an LISN RF out port, a current monitoring probe is placed around one of the AC input wires (line, for instance). The spectrum analyzer is R&S/Hameg HMS-X. From what I can tell, it's not the world's best SA in terms of noise performance.The noise floor is only about 10db below the limit line, even when using the lowest measurement bandwidth (1KHz), and with the HMS-X's premplifier turned on. The preamplifier improves SNR by about 10dB.In a perfect world, I think a noise floor 10dB below the limit line would be okay. (Well, I guess in a perfect world there would be no noise floor.) But, in this case, the actual measured peaks seem to change in amplitude, depending on the noise floor level. (Does that make sense?) In a perfect world, the measured peaks are not really correlated with the noise floor, so as long as the peaks can "poke their heads" through the noise floor, then you can get an accurate measurement. In this case, the relative level of the noise floor seems to impact the measured level of the peaks too. (Why...I'm not sure.)So, I'm considering adding a low-noise external preamplifier. But, my eyes start to cross when I start to read the spec sheets of some commercial offerings. (Once again out of my league.)How might an amplifier like Mini-Circuits ZFL-500LN-BNC+ fare? (I don't have a big budget, so this seemed reasonably priced.)https://www.minicircuits.com/pdfs/ZFL-500LN+.pdf1. How do you make sense of the stated noise figures? (Is it good enough?)2. It says 5dBm max @ 1dB compression point, but then has powers in the range of 8dBm in the table on Page 2. What's the difference?Thank you.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 01, 2020, 04:51:23 am
content: The noise figure is about 3 dB. That's equivalent to a 300 K noise temperature, or 3dB above the thermal noise floor. This is more than good enough for your application.I don't understand whats up with the 1 dB compression points, I'd trust the plot (~8 dBm), but verify if you care.That said, I would look again at your SA. I'm not familiar with that model, but frequently spectrum analyzers have programmable attenuators built into the input stage. If that exists, turning it off may fix your problem for free.

author: TimNJ
author_rank: Super Contributor
timestamp: October 01, 2020, 11:18:30 pm
content: Quote from: ejeffrey on October 01, 2020, 04:51:23 amThanks for the explanation and for the idea. During this measurement, the programmable attenuator was set to 0dB. But, that's a good point, as it automatically chooses an attenuator based on the reference level selected. In this case, reference level is at minimum. Fiddled around a little more and looks like the noise floor dropped a little more...but still getting a ton of "noise" between 100KHz and 1MHz. It's quite narrow band, which makes me a bit suspicious of it being "real" or not. We've also done normal CISPR/EN55011 conducted measurements on this DUT and there's nothing crazy narrow band like that.Is it possible these are some sort of inter-modulation artifact? In the attached image, you can vaguely see the envelope of what *I think* is the real EMI, but so much noise on top of it. The noise seems to improve with frequency. Is this evidence of the spikes being 1/f noise? I suppose that would make sense but why are the spikes not there when the DUT is unpowered?Thanks!