E-field probes for EMC immunity - cheap vs. expensive ? - Page 1
Forum section: electronics / testgear

author: Jolle
author_rank: Contributor
timestamp: September 15, 2022, 12:05:26 pm
content: I have gained access to a TEM cell for pre-compliance emission and immunity measurements. When I am going to perform an immunity test I understand that I can roughly estimate the field strength by dividing the signal strength with the distance between the septum and bottomshield. But in case I want to verify that my setup gives me approximately what i expect, it would be nice to have an E-field probe I can put into the TEM cell.Looking at ebay I can find stuff like "Amplifier Research FP5000 Isotropic E-Field Probe, 10kHz -1GHz", which looks like a "proper" EMC field measurement probe, costing well above $2000:https://www.ebay.com/itm/325089673884?hash=item4bb0dac29c:g:5MgAAOSwd~ZiJqSr&amdata=enc%3AAQAHAAAAoGLQKQhrHhjQg5OSNv%2F%2FIjSl2vg4yFnWuP191KuB%2ByzpVgcdskFKLDN53%2F3MWAQAwyLYux%2Fn%2BqvFeLpfuRb70%2B30EK20A5kM%2Fqtxz%2BPEqzB3spPd6Kk3qME4bmbWYiW5O68a7Zx0IYDPmITGpsqs3%2FOqReuauQ7BotZ3GNAAelhA9K7U37w9GL8lKxUuDH0qSaJFgEJTG8ryS7THHuLkN1s%3D%7Ctkp%3ABk9SR7yfjI_oYABut looking elsewhere I can also find equipment like Extech 480836 or Tenmars TM-195, which costs much less:https://www.newark.com/extech-instruments/480836/rf-emf-strength-meter-50mhz-to/dp/61M1195?ost=480836http://www.tenmars.com/m/2001-1682-689905,c19132-1.php?Lang=enI expect that the AR FP5000 would be more accurate, but would the Extech/Tenmar be sufficient in my case?Can anyone elaborate what the difference is between the cheap E-field meters and the much more expensive field probes like the AR?

author: Berni
author_rank: Super Contributor
timestamp: September 15, 2022, 01:20:15 pm
content: For most cases you don't really need to be very accurate with these things.It is mostly about just seeing if you are in the right ballpark. You can always intentionally go over/under the spec to make sure you are inside. If you crank up the signal generator to 500% the specification yet it still works, then you can be pretty sure it will handle 100% the specification at the EMC lab even if your setup was crap and was only reaching 1/4 the power you thought it made.Similar for EMC emission tests, you don't really need to count individual dBms. As long as you know the rough ballpark you want to be in, then you can just make sure you take care of anything that is reaching close to it. This way you make sure there is no catastrophic fail once you send it off to the real EMC lab. If you are unlucky with the test setup and do fail then you know exactly by how many dB you have to lower that offending peak.

author: nctnico
author_rank: Super Contributor
timestamp: September 15, 2022, 01:50:08 pm
content: Quote from: Berni on September 15, 2022, 01:20:15 pmYes and no. Depending on the DUT and TEM cell design you may already have a 3dB uncertainty in field strength. And there is also a practical side to it; you still need quite a bit of power to get to high field strengths in a reasonably sized TEM cell. So having some assurance that a 3dB to 6dB margin is enough to be in the the ball park can save you from buying a very expensive amplifier (that may not even be needed after all).

author: rhb
author_rank: Super Contributor
timestamp: September 15, 2022, 05:20:44 pm
content: Personally, I make them from semi-rigid SMA jumpers by soldering the center lead to the shield with as small a gap in the shield as possible. I made them in several diameters.Dave did a video on the subject, though not using RG 402. Remember, the amplitude of the signal from an H field probe is dependent upon the orientation of the loop relative to the magnetic field. So “calibration” is not really a valid concept unless you’re measuring the orientation very accurately.Oops. Didn’t register that you want the E field. The pickup of those is simply a function of the length. It’s a small antenna. I doubt that the absolute magnitude means anything. I select a size based on how much spatial resolution I need.

author: nctnico
author_rank: Super Contributor
timestamp: September 15, 2022, 05:23:59 pm
content: This is not about H (magnetic) field probes, but E (electric) field probes. Basically a calibrated antenna + receiver.BTW: I've found this report on E-field probe basics:https://www.researchgate.net/publication/325342186_Standard_Electric_and_Magnetic_Field_for_Calibration/fulltext/5b06c106aca2725783d9604e/Standard-Electric-and-Magnetic-Field-for-Calibration.pdf?origin=publication_detail

author: Jolle
author_rank: Contributor
timestamp: September 16, 2022, 07:17:34 am
content: Thank you for your responses.I have been looking a bit more into the datasheets of the FP5000 and the TM-195 and there is no doubt that the 480836/TM-195 is less accurate. It also seems that the 480836/TM-195 are not properly calibrated since it is possible to enter a calibration factor (which it then uses throughout the entire frequency range...).From these information I guess the price comes down to the accuracy of the instruments, like buying a 0.1% voltage reference vs a 0.001%. For my application I do not need ±1dB accuracy and a FP5000 or similar therefore seems like overkill to me.But the cheap alternatives I have been looking at might also be a bit too cheap. Does anybody have any experience with any RF E-field strength meter they would recommend ?

author: Feynman
author_rank: Regular Contributor
timestamp: September 17, 2022, 09:48:03 am
content: Many TEM cell fabricators offer spread sheets where they show field strength dependent on the RF power (dbm) fed to the cell. Measuring RF Power might be cheaper than measuring V/m (in the case your RF source doesn't tell you already the RF power). And often times the fabricators give you hints in which areas of the cell to expect a reasonably uniform field.

author: Xandinator
author_rank: Contributor
timestamp: October 03, 2022, 10:42:47 am
content: Maybe the cheapskate solution we employ is something that works for you as well:Use some (inexpensive) EMC field probes to characterize homogeneity and use the theoretical value as reference for the center (if you supervise students that also constitutes a nice task for them).Better than ±5 %.

author: ExaLab
author_rank: Regular Contributor
timestamp: October 07, 2022, 11:11:12 pm
content: I use the Tenmars TM-195. Using the typical CAL factor of the unit, its accuracy above 1.9GHz is +/-1dB (plus the isotropic deviation)For those interested, I have the graph/table of the calibration factor vs frequency (not easily available ...)

author: ExaLab
author_rank: Regular Contributor
timestamp: January 24, 2023, 06:44:49 pm
content: Below, the Calibration table and related graph for the TM-195.Please take into account its limited accuracy at low frequencies of the operating range.