Buy / DIY GTEM Cell ??? - Page 1
Forum section: electronics / rf-microwave

author: dazz1
author_rank: Frequent Contributor
timestamp: July 19, 2019, 10:15:32 am
content: HiI have an electronic device that needs EMC compliance testing. The cost of having a test house do it is eye-watering. It's a straight expense with no residual value or return on investment.I can see that as the product range increases and gets modified, the EMC testing is going to be a major financial burden.The item I want to test is about 0.5m x 0.5m x 0.5m cube. Too big for a TEM cell.Does anyone have experience buying/installing/running that size GTEM? I suspect they are excessively priced but a lot cheaper than an OATS.Is there any published papers (apart from the ABB patent) with enough info to DIY build these things? I suspect building one for commercial use (not sale) might not comply with patent rights.Are these GTEMs an acceptable alternative to OATS?Dazz

author: biastee
author_rank: Regular Contributor
timestamp: July 20, 2019, 12:17:15 pm
content: Quote from: dazz1 on July 19, 2019, 10:15:32 amU didn't say what the frequency range is. If <= 150 MHz, CISPR 20 / EN 55 020 has instructions for constructing a TEM cell that can accommodate a 0.7m high DUT. The instruction doesn't say what are the dimensional limits for L / W, though your specified cube can definitely fit the space inside.GTEMs are acceptable alternative to OATS.

author: Neilm
author_rank: Super Contributor
timestamp: July 20, 2019, 05:58:18 pm
content: Does your product have cables going to if? (test leads, mains etc).If so then you could have problems getting a reasonable reading. I have had units "pass" EMC in a GTEM then fail on an OATS and visa versa. Doing various experiments showed that the lead layout was causing additional reflections in the chamber and threw off the resultsNeil

author: dazz1
author_rank: Frequent Contributor
timestamp: July 21, 2019, 09:08:40 am
content: Quote from: biastee on July 20, 2019, 12:17:15 pmWhich is why I am interested in them. From my reading, it appears that 61000-4-20 has a new section on GTEMs.I have been doing a lot of reading up on EMC testing but I haven't purchased any standards yet. I am trying to figure out the ones I actually need. There are so many to choose from and they are so expensive, and they get amended.Very few are free and useful, like CENELEC Guides 24, 25 and 32.I think I need the following guides/standards:Low Voltage Directive (2014/35/EU)EMC Directive (2014/30/EU)The RED guide 2014The Blue guideRF Immunity Test IEC 61000-4-3,RF Conducted RF Immunity IEC 61000-4-6EN-61000-6-1 Commercial and Light Industrial emissionsEN 61000-4-20 Testing and measurement techniques - Emission and immunity testing in transverse electromagnetic (TEM) waveguidesEN/IEC 61000-3-2 Power Harmonic EmissionsEN 55032 (CISPR32)EN 55024 (CISPR24) ImmunityHave I missed any that I need? Are there any on my list I don't need?I know that building my own OATS would not be cost effective. Same for a shielded room. I don't expect a commercially built, calibrated GTEM will be cheap, but probably a lot more cost effective than tipping $$$k into a EMC test lab. At least the a GTEM, I could sell it on if/when I have finished with it. Maybe I could hire out testing time/access.Dazz

author: schratterulrich
author_rank: Regular Contributor
timestamp: July 22, 2019, 09:00:12 am
content: I find this Master's Thesis very useful:https://www.mikrocontroller.net/attachment/89388/dip_icheln.pdf

author: dazz1
author_rank: Frequent Contributor
timestamp: July 22, 2019, 11:08:29 am
content: Quote from: schratterulrich on July 22, 2019, 09:00:12 amThat is gold. It looks like it was the original research.1995 was a long time ago.I found this patent on converting GTEM to OATS measurements.https://patents.google.com/patent/US5825331I didn't realise a method could be patented.Dazz

author: dazz1
author_rank: Frequent Contributor
timestamp: July 23, 2019, 07:36:42 am
content: HiOK so the Masters thesis shows making a GTEM is not trivial. The devil is in the detail and some really expensive test equipment is needed for fine tuning.It basically looks like a high Q wave guide that just wants to resonate, connected to an imperfect RF absorber.I suspect refining termination of the fat end of the inner conductor could yield performance improvements.Dazz

author: wilhe_jo
author_rank: Regular Contributor
timestamp: February 13, 2020, 02:10:15 pm
content: Sorry for bringing this thread back to life...did someone succeed in this?I'm just thinking of making a GTEM cell for my business.For me, the biggest problem is sourcing reasonably priced RF absorbers.Has anyone tried to just glue some 100pcs of EMI ferrites on a panel???Some decades ago, grid-shaped ferrite tiles were quite common to get reasonable performance beyond 1GHz.For even higher frequencies, maybe 1mm thick graphite sheets or graphite powder could be a start for experiments...73

author: dazz1
author_rank: Frequent Contributor
timestamp: February 13, 2020, 06:02:17 pm
content: HiBack in my time at university, a team experimenting with circular wave guides, made absorbers from carbon copy paper sandwiched between thin layers of expanded polystyrene. The absorbers were wedge shaped. The carbon copy paper was also wedge shaped and orientated so the apex pointed at the incoming field. This worked well enough for their experiments but possibly not good enough for a GTEM cell.I think carbon film used for heating elements could also be used but a lot of experimentation would be required.If you could build the chamber and buy the absorbers, you would still save a ton of money. Resonance is the enemy of GTEM cells. These are mechanical devices with electrical properties defined by very detailed design and construction.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: February 15, 2020, 03:09:11 pm
content: Well, you're certainly right about the mechanical difficulties. However, I intend to go dirt-cheap.250-500mm septum height is fine for now. You can get aluminium foils up to 1.5m wide for very reasonable prices.Another Option would be Aluminium covered MDF... However, joining them would get difficult and covering sheets of plywood (maybe MDF is the better choice) my own gives me the ability to have generous overlaps. So RF tightness should be ok.So the real issue seems to find suitable absorbers to get beyond an open-TEM cell.BTW I intend to have reasonable performance up to some 100MHz because another interesting alternative to TEM cells in the recent CISPR standards is the reverb chamber...73

author: dazz1
author_rank: Frequent Contributor
timestamp: February 15, 2020, 08:33:12 pm
content: HiI guess the choice is paying for tiles where someone else has done R&D, or doing the R&D to figure out what works. No right answer, just a choice.Do you have a free access reference to reverb chamber?Dazz

author: wilhe_jo
author_rank: Regular Contributor
timestamp: February 15, 2020, 11:57:58 pm
content: Well, just buying ferrite tiles isn't really sufficient.The dielectric backing is very important as well... So still a lot of work...I have access to the IEC standards... but these are difficult to share.You'll find some information here:https://interferencetechnology.com/reverberation-chamber-emc-testing/https://en.wikipedia.org/wiki/Electromagnetic_reverberation_chamberBut there several scientific publications. Most, however, deal with susceptibility...73

author: dazz1
author_rank: Frequent Contributor
timestamp: February 16, 2020, 07:51:54 am
content: HiI haven't seen/heard of reverb chambers being used for emissions.Dazz