karlist July 8, 2019 at 6:41 am
- This topic was modified 11 months ago by karlist.
Hi I am making this topic as a continuation to another topic from a while ago (http://signalhound.com/support/forums/topic/bb60c-emc-measurementshow-to-lower-the-noise-floor-of-my-setup/).
I made a new topic because I wanted to extensively describe our setup so professionals from Signal Hound might see where is(or isn’t?) the problem.
So, we we upgraded our chamber and now the setup is like this:
Antenna – 1m cable RG316 – 20 db amplifier – 3m Rg316 cable – RF / Coaxial Adapter – 5m cable – BB60C
Factor diagram parameters I entered in Spike software
20 dB amplifier TBWA2/20dB https://www.tekbox.com/product/tbwa2-wideband-rf-amplifiers/
3m Rg316 cable:
parametri no signal hound
• Damping 1GHz: 1,2dB (0,240dB/m)
• Damping 2GHz: 1,9dB (0,380dB/m)
• Damping 3GHz: 2,2dB (0,440dB/m)
• Damping 6GHz: 3,5dB (0,700dB/m)
EMC table parameters:
Radiation if there is no DUT in our chamber:
Radiation of DUT in horizontal polarization:
Radiation of DUT in vertical polarization:
Same DUT in certified lab :
Pictures of our chamber.
So our goal is to get measurements in our lab as close to measurements of certified lab.
As you can imagine, we have already invested a small fortune in our equipment so we are expecting a little bit better results than this.
Main issue is that I see 48 dBuV/m 180 Mhz peak in certified lab results but in our lab it is 70…
Regarding the low frequency noise – big reason for it is our noisy poe supply and our ethernet filter for chamber probably is not as good as for certified lab. Our filter: https://adauratech.com/anechoic-chamber-filters/specifications/
1. Is this radiation that BB60C is measuring in chamber without DUT “legit”? I have no previous experience in EMC measurement field, so it seems kinda wierd that we are not seeing
lower numbers…Can you please recheck the Path Loss table information shown above? Maybe it is just an error in these. I have checked them multiple times…
2. How can we increase the quality of our measurements? I guess we should add some more ferrite absorbers. Any tips where to put them? We do not have enough to cover all walls.
3. How far in to plywood box should we put the antenna?
4. How far from wall should the DUT be?
ANY advices would be really appriciated! This issue is becoming more and more exhausting..karlist July 8, 2019 at 10:36 pm
I failed with the image upload, so I will upload them as attachments in replies.
Kārlis TučsParticipantKārlis Tučs July 9, 2019 at 12:53 am
more images in attachment..I couldn’t add more image with original account..karlist July 9, 2019 at 3:12 am
4 last images
Justin CrooksModeratorJustin Crooks July 9, 2019 at 11:40 am
This is a difficult question to answer. My first question is, what antenna distance are you using? Is it the same distance in both cases (certified test and pre-compliance test)? If not, there is a 20 log (d2/d1) dB correction to apply.
Obviously your chamber is not anechoic, which will change results, and VSWR on your antenna and amplifier can change things. I can see these stacking to produce maybe a 6-8 dB variation, but 20 dB seems a bit excessive.
Do you have two of the antennas? If so, you could calibrate your system in position using a signal generator instead of your DUT…karlist August 9, 2019 at 1:18 am
Yes, antenna distance is same in both cases – 3 meters.
I just added some more ferrite tiles to chamber which decreased the spike at 179 Mhz by few dBs.
Do I understand correctly – because of my antenna which is very lossy up until 250 MHz, I wont be able to measure anything less than about 40 dBuV/m right at low frequencies?
As you can see in one of the images, when no device is in the chamber the radiation levels are shown as pretty high, and that happens because of Antenna Factor path loss table. At 30 Mhz it is 39 dB..
I have concluded I will not be able to somewhat precise measurements in frequencies below 250 MHz with this antenna..too bad..karlist August 20, 2019 at 3:46 am
Can someone please confirm my theory about the reason of bad measurements?
Is antenna the issue here?
thanks in advance
kaiserParticipantkaiser August 20, 2019 at 8:04 am
So, have you tried a free-field measurement yet (aka not in the chamber)?
Antenna test is intricate. It’s hard to truly say what is or is not the problem here because there can be dozens of things that cause something like this, or it could really just be that.
Just looking at it, given the frequencies you’re talking about, if an engineer came to me asking about this and what to do next. I’d suggest a free-field measurement. In general, my comments would be that you’re testing at a low frequency (250MHz) has a ~4′ wavelength. In order for the fields to enter into the antenna, you’re going to want other things (walls, absorber, etc) at least that far away from it unless you’ve specifically verified the test setup, that you have absorber behind the antenna to prevent reflections, but far enough away to not artificially couple and affect the antenna. There are a lot of things much closer than that at the frequencies your talking about. Also, the chamber could also end up acting more like a resonant cavity than a chamber depending upon size and exact geometry. Again, maybe not a problem, but could be. Also, have you ran S11 on the antennas post install? Are they being detuned, or a reflection coefficient issue created during install? And so on.
The bottom line is that from the pictures, given the issue that you’re seeing, you could spend days/weeks/months tinkering with the test setup and never hit a definitive conclusion on the issue, as there are potentially many, and they could interact in complex ways. So, I’d tell my engineer to check S11 an as-installed items, and check it in various chamber configurations. S11 open-air vs chamber should be the same; if substantially different, something is up.
Then separate out the test chamber from the measurement, and then add pieces back in. Go find some open space somewhere, and just run a real quick simple test. Collect that data, and then start introducing your test setup piece by piece, verifying that you’re good every step as you do so. You’ve already half did that with the test from a certified house, but it’s just hard to truly say you’re replicating or there’s an antenna problem, etc when all we really have with these exact items is a somewhat problematic test chamber setup.
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