Ed- I have seen stuff like this. My company makes protective relays that control the power system. That means that these relays work with signals at 60 Hz (or 50 Hz for many parts of the world outside of North America). The test standards call for the use of 1 kHz amplitude modulation. The relays have analog and digital filters that will generally reject frequencies this high. So there can be circuits with marginal RF immunity that will pass the standard test. My fix for this is to also test using 60 Hz amplitude modulation. If there are circuits that respond to RF, the resulting 60 Hz signal cannot be rejected by the filters, as this matches the frequency of the desired signal. I have detected many more marginal circuits using this method.
I view it as a deficiency of the relay-specific test standards that 60 Hz (or 50 Hz) AM is not specified for the test. Donald Borowski EMC Compliance Engineer Schweitzer Engineering Labs Pullman, Washington, USA From: "Ed Price" <edpr...@cox.net> To: <EMC-PSTC@LISTSERV.IEEE.ORG> Date: 08/20/2012 08:03 PM Subject: RE: [PSES] 61000-4-3: Rate of application of field Sent by: emc-p...@ieee.org Ken: The biggest difference I see was that I called it ?rare? and you said it was ?very unlikely,? so that?s not much of a fight. J To dip back into one very interesting program, I was running the qual testing on some soldier-worn electronics. We were doing the typical RS103 exposure of 50 V/M using 1 kHz 50% duty cycle modulation. We had some marginal improper responses (stuttering of the expected response & false positive responses) at a few harmonically related frequencies in the low HF region. So, we go into the usual try-everything-we-can-think-of mode, the sorry-but-ferrite-beads-won?t-help 1 MHz mode, and then the let?s-try-anything mode. I?m sitting there watching the false responses mess up the error rate, and I inadvertently move the modulation frequency just a bit downward. Wow, the system crashed like a rock! It turns out that the system had some broadly resonant responses (hence the harmonically related HF points) that let energy in, but the real killer was the modulation rate. The system really hated (IIRC) something around 960 Hz modulation (pulse or AM didn?t matter), although it would tolerate strong CW at any frequency. The system gurus later explained this as my modulation rate syncing with a harmonic of their data sampling rate; if I was just a bit off, then their error correction algorithm could cover the loss of data. However, if I was right on them, then every time they sampled, I had an RF jolt right in their front end. The lesson was that 461 suggested we use 1 kHz as a default, but if we had studied the system with more diligence, we might have seen that some other modulation (960 Hz) would have been more a much more harsh condition. And of course, 461 testing is all about finding that worst-case scenario, not just proving that your gadget works most of the time. Thus, I often used modulations beyond the popular 1 kHz, 100 Hz, 1 Hz, 1 uS, 50% pulses and AM stuff. (Which is all probably pretty boring to the commercial and EN guys on this list, so sorry.) Ed Price El Cajon, CA USA From: Ken Javor [mailto:ken.ja...@emccompliance.com] Sent: Monday, August 20, 2012 7:26 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] 61000-4-3: Rate of application of field Going to pick a small nit with Ed. In our world, military and aerospace EMC, Ed is correct that some devices can respond more to a cw signal than to an amplitude demodulated signal. That is because the way we do AM, the rms power is higher for cw than for AM. This is because we define the test level as the rms equivalent of the peak of the modulation waveform. Since 1993 MIL-STD-461 requires square wave pulse modulation. Clearly, there will be half as much power in that modulation than cw. But across the Pond, they add the amplitude modulation on top of the cw level, and add 5.1 dB more signal doing so. So it is highly unlikely that something would respond to an EN 61000-4-3 unmodulated signal if it wasn?t already susceptible to the modulated waveform. Ken Javor Phone: (256) 650-5261 From: Ed Price <edpr...@cox.net> Organization: ESP Labs Reply-To: <edpr...@cox.net> Date: Mon, 20 Aug 2012 17:03:45 -0700 To: <emc-p...@ieee.org> Subject: RE: [PSES] 61000-4-3: Rate of application of field Brian: ??Other labs level the forward power at each frequency with modulation off, then turns on the modulation only for the dwell time, then turns off modulation ?? This technique increases the test time, but more importantly, it includes a double exposure, first to the CW RF field while the leveling is taking place, and then for the dwell time with the modulation turned on. I have seen EUT?s which were more sensitive to a CW field than to the modulated field. Granted, that was rare, but it can happen. You would need to consider the possibility that you may be ?over-testing? using this technique. ?This method would be more accurate and gives you a better verification method?? I wouldn?t say that. You may have any number of non-linearity?s in your RF signal path. Logically, you might say that if you were using a pulse modulation, then leveling on CW would accurately establish the level, and then all you would be doing is turning it off and on at the pulse rate. But in the real world, maybe your power amplifier changes output power with average loading. Oops, you could be applying a higher peak level during the modulated portion of your exposure. Essentially, leveling on CW and then applying modulation is sorting counting on a blind assumption that modulation doesn?t change the signal amplitude. Applying the modulation, leveling the field and then sweeping (or stepping) is an older technique, but equally accurate. It makes the assumption that you know what the modulation envelope does to the detector that you use to monitor the field. For instance, if you wanted to apply a 50% duty cycle pulsed RF field of 100 V/M peak, then the detector might read 100 V/M with a CW field, but would drop to the RMS value of a 50% duty cycle (70.7%, or 70.7 V/M) when the modulation was turned on. Of course, using RMS detectors with this technique has an inherent limit; when the duty cycle becomes much shorter than say 10%, it becomes very difficult to remain accurate. As I see it, both techniques have shortcomings. The ?tune / level in CW / apply modulation / dwell / turn off modulation & RF / tune again? method leaves you wondering about what happens during the modulation time, and the ?apply RF / apply modulation / sweep (step)? method requires you understand the modulation effect on your measurements. I preferred the first method for low duty cycle and complex pulse modulations, but preferred the latter more direct method for CW or 80% AM modulated sine wave modulation. All this discussion would be moot if we had true peak reading broadband field strength sensors, and if the immunity standards were all referenced in terms of peak applied field strength. Until then, we have to wrestle with techniques that are not perfect. Now why am I thinking of Lucha Libre? Ed Price El Cajon, CA USA From: Kunde, Brian [mailto:brian_ku...@lecotc.com] Sent: Monday, August 20, 2012 7:14 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] 61000-4-3: Rate of application of field James, What version of the standard are you looking at? The ?family? standard we use calls out the 2002 version but the latest version we have on-hand is the 2006 version. I have noticed that different labs perform the test differently regarding how the power is established, verified, and how the modulation is applied. I do not see where the standard calls out the details of how this is performed during the test. I assume most labs are using whatever method they use which gives the most accurate results within the limitations of the test equipment and software they are using. For instance, some labs just set the signal generator output level that was established by the calibration and leave the modulation always turned on throughout the test. Other labs level the forward power at each frequency with modulation off, then turns on the modulation only for the dwell time, then turns off modulation and level at the next frequency. This method would be more accurate and gives you a better verification method, but as far as I can see this is not specifically called out in the standard. As far as gradually applying the carrier or modulation strength I really don?t know. The standard does not reference it as far as I can see. One issue your email points out is the fact that EMC labs are performing the test differently which may or may not cause a larger variation in the results when comparing one lab to another. Our customers often ask about our test method and request variations to be made for one reason or another; often to try and match a test method from another lab that they use. The Other Brian From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On Behalf Of Pawson, James Sent: Monday, August 20, 2012 5:54 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: 61000-4-3: Rate of application of field Hello, I can't find any clauses in 61000-4-3 (radiated RF immunity) that deal with the rate of application of the RF field. My understanding is that the test is generally performed by setting the unmodulated carrier to the level contained within the calibration file and then suddenly applying the modulation. Is there any precedent for, or problem with, gradually increasing the modulated carrier field strength up to the required level instead of a more sudden application? I imagine a system like a mobile radio would involve a suddenly applied burst of RF when the transmitter is "keyed". Many thanks James - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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