Dear All,
A cautionary tale. Some twenty years ago, a young engineer was doing some “immunity testing” of a circuit to high power uWave. The engineer “cranked” up the power into the TWT until the output was as high as he could get. The TWT was an experimental wide-band amplifier (many octave wide). The test set-up had a horn antenna radiating towards the circuit with a block of RAM behind – all in a screened room. He had done the safety calculations for the maximum power output of the TWT, the gain of the horn at the operating frequency and the distance between horn, and the RAM was within its ratings. Then the fire started – the RAM was smouldering, choking fumes everywhere. After the “fire” was out – I was called in to investigate. I am not going to describe how this was all investigated – but the findings were that the TWT was so overdriven that the harmonics were within fractions of a dB of the fundamental. The measuring equipment could only measure to the seventh harmonic and all the harmonics and the fundamental were present. The antenna was fed with waveguide which, although overmoded, presented little in the way of attenuation at the harmonics. The antenna itself also worked quite efficiently at the harmonics and the effective gain increased with the square of the harmonic number (approximately). The power at each harmonic was approximately 1/7th of the rated TWT output Instead of there being one unit of power at the RAM he had created 1/7+4/7+9/7+16/7+25/7+36/7+49/7 = 140/7=20 times the original incident power! It is the best example of harmonic induced problems that I have ever seen. Regards Tim ************************ Tim Haynes A1N10 Electromagnetic Engineering Specialist SELEX Sensors and Airborne Systems 300 Capability Green Luton LU1 3PG * Tel : +44 (0)1582 886239 * Fax : +44 (0)1582 795863 * Mob : +44 (0)7703 559 310 * E-mail : [email protected] P Please consider the environment before printing this email. There are 10 types of people in the world-those who understand binary and those who don't. J. Paxman ________________________________ From: [email protected] [mailto:[email protected]] On Behalf Of Oglesbee, Robert A Sent: 02 November 2009 15:23 To: [email protected] Subject: RE: [PSES] 61000-4-3 and 61000-4-20 Field Uniformity Requirements *** WARNING *** This message has originated outside your organisation, either from an external partner or the Global Internet. Keep this in mind if you answer this message. James, It’s a combination of the cabling to your antenna and the antenna factor as well. You have to take it all into account at least once. After you have an idea of your cable/antenna losses vs frequency you can just do outside-the-chamber measurements. Regards, Rob From: Pawson, James [mailto:[email protected]] Sent: Monday, November 02, 2009 10:05 AM To: [email protected] Subject: Re: [PSES] 61000-4-3 and 61000-4-20 Field Uniformity Requirements Hi Ken, thanks for the reply. So if one was to measure the output from the power amplifier into the test chamber (with a directional coupler and spectrum analyser say) and observed the harmonics, then that would that tell you if you had excessive harmonics or would that be a feature of the transmitting antenna and would only be measurable in the chamber itself? Also, would that apply to a GTEM? I would have thought a GTEM would be fairly efficient at low frequencies. Thanks, James ________________________________ From: Ken Javor [mailto:[email protected]] Sent: 02 November 2009 13:52 To: [email protected] Subject: Re: [PSES] 61000-4-3 and 61000-4-20 Field Uniformity Requirements Your point number 2 is necessary because the field-sensing devices are broadband and if a signal harmonic is higher than the fundamental, the field sensor can’t tell the difference and the field will be leveled on the harmonic amplitude, not that of the fundamental. Thus you have not the case of an over-test, but an under test at the fundamental. This problem is the reason (my assumption) why 61000-4-3 starts at 80 MHz; the biconical antenna below 80 MHz becomes progressively less efficient and it is very easy to have second and third harmonics of signals below 80 MHz radiate at higher levels than does the fundamental. The actual 6 dB number is something of a traditional limit; with a broadband device performing an rss of everything in its bandwidth, a signal 6 dB under the fundamental will increase the overall measured level by 1 dB, so that you are under-testing at the fundamental by 1 dB if a spurious signal 6 dB below the fundamental is included in the mix. Ken Javor Phone: (256) 650-5261 ________________________________ From: "Pawson, James" <[email protected]> List-Post: [email protected] List-Post: [email protected] List-Post: [email protected] Date: Mon, 2 Nov 2009 11:03:12 -0000 To: <[email protected]> Conversation: 61000-4-3 and 61000-4-20 Field Uniformity Requirements Subject: 61000-4-3 and 61000-4-20 Field Uniformity Requirements Hi, As far as I can tell, the requirements for field uniformity in 61000-4-3 and 61000-4-20 are the same. Primary field components within 6dB (after discarding 25% with the biggest deviation) and of these remaining points no secondary field components greater than 6dB of the primary. 1) Is there an issue if the secondary components were higher than 6dB of the primary? Even if the EUT was tested in multiple orientations? As far as I can see, it would result in an over test of the EUT and isn't compliant with the standards, but more importantly would testing like this mask an issue that would only occur if the field was primarily polarised in one direction? e.g. a slot orientation forming an antenna? Provided the EUT was rotated and the field component was high enough so this situation occurred, do we care that much about the secondary components? 2) Another scenario: what if due to some kind of resonance in the test facility the primary field component couldn't be achieved without overloading the amplifier, but the isotropic field (square root of the sum of the sqaures of the X, Y and Z components) was of the right level. That doesn't feel right because of the questions I outlined above. 3) Also the standard calls for the front face of the EUT to be "initially placed with one face coincident with the calibration plane" (61000-4-3, clause 8.2). Why the front face? Why not the central plane of the EUT? Do we not care what happens 'after' the calibration plane? Does anyone have any thoughts / experience that they could share on these points? Thanks in advance James James Pawson Leading Hardware Engineer EchoStar Europe T: +44 (0)1535 659000 e: [email protected] - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]>
