I guess the crux of your argument lies in the definition of "decent receiver". While true that a CISPR compliant receiver would warn you, it also has preselection. Less expensive "pre-compliance" instruments require careful attention to this issue.
I have certainly seen this occur in real life, it's by no means merely theoretical. Often it occurs in measuring transmitters, where the filter that is attenuatiing the fundamental is not adequately large. It can however occur from a strong ambient and an inexperienced operator not recognizing the significance of that large ambient signal could readily make an erroneous conclusion. Now of course if it's all carried out in a chamber..... From: [email protected] [mailto:[email protected]] On Behalf Of Gert Gremmen Sent: Thursday, December 04, 2008 1:50 PM To: Ralph McDiarmid Cc: [email protected] Subject: RE: EMI Receiver I think this is more a theoretic exercise, and as true as unlikely to fool you. Any decent receiver in overload does not show a measurement value but blinks, reports an error condition (IEEE-bus) or has a bright red blinking error led or beeper. I have never been able to reproduce this phenomenon with a spectrum analyzer without reported error condition. (and certainly not with all signals -spurious included- meeting limit lines, and make me conclude a compliant result.) Gert Van: [email protected] [mailto:[email protected]] Namens Ralph McDiarmid Verzonden: donderdag 4 december 2008 17:57 Aan: [email protected] Onderwerp: RE: EMI Receiver That's my understanding too. Ralph McDiarmid, AScT Compliance Engineering Group Xantrex Technology Inc From: [email protected] [mailto:[email protected]] On Behalf Of Clif Brick Sent: Thursday, December 04, 2008 8:50 AM To: Gert Gremmen; [email protected] Subject: RE: EMI Receiver I could be wrong as it's been a few years, but it seems to me that a spectrum analyzer receiver in overload will show a signal to be a lower amplitude than it really is (in addition to ghosts, spurs and intermod products). The issue is one of compression, and in fact an overlaoded analyzer will show comply when in fact the device is non-compliant just as readily as the opposite. If an overlaod condition exists, you may get a reading of a real (from the EUT) signal that is say 0dBm, you add 10 dB of attenuation and get -3dBm. This means that in fact your signal is at least 7dB larger than your display. If you add 10 dB more and get say -7, you're still in overload. You would need to continue until you get linear response. Important to understand is that it may be well outside of the band of interest that the overlaoding signal is comin in on. This is why preselection helps.... Best regards, Clif From: [email protected] [mailto:[email protected]] On Behalf Of Gert Gremmen Sent: Wednesday, December 03, 2008 2:27 AM To: [email protected] Subject: RE: EMI Receiver >From "EUT cost to compliance" point of view I fully agree with this discussion about preselectors. The only error one makes without preselector is that one may (not will) need much more effort and cost to make your product compliant, as ghost emissions may appear due to overload effects in any receiver amplifier/mixer part, transient limiter or elsewhere. However, if your receiver/spectrumanlyser without preselector indicates compliance the result is valid (other non-compliance aspects neglected). Without preselector: A non compliance result is NOT reliable A compliance result IS reliable. Another remark: Most transient limiters contain an attenuator (20dB) in front of the limiter. As a transient limiter will clip somewhere between 1 and 3 volts pk, the spikes that triggered the transient limiter were generated by that "immature design" were of an amplitude very likely to have destroyed the input of the analyser without the transient device (>20V) !! I have no relations with this company!!! I stick to the (older?) R&S receivers, as they have shown to produce consistent results over time, and have sufficient head room in their amplifiers to never create overload problems, and do have decent pre-selectors. In addition, they have no problems in meeting calibration requirements over their lifetime. Also, from a electronics designer point of view, these instruments have been build without compromises. (and I have looked inside !) >From a company whose core business is professional RF one can expect corresponding performance. No durable experience with the newer receivers yet. Regards, Ing. Gert Gremmen ce-test, qualified testing bv Van: [email protected] [mailto:[email protected]] Namens Bob Richards Verzonden: Wednesday, December 03, 2008 4:10 AM Aan: [email protected] Onderwerp: RE: EMI Receiver --- On Tue, 12/2/08, Jim Eichner <[email protected]> wrote: >Amazing the spectrum you get when a really large 50kHz switching >fundamental slams into the transient limiter. I remember reading a long time ago that when a transient limiter starts to clamp, it becomes a comb generator. I ran into that recently when testing a product that initially appeared to fail by ~20dB. I don't know what made me think to try it, but I removed the transient limiter and installed an attenuator in its place. The emissions at 150khz dropped drastically. A linearity check showed that I still had a problem, though. I grabbed the one analyzer we have with a preselector, and got passing results. FWIW, this was a power supply for VCCI testing. It easily passed (with transient limiter) at 120Vac, but appeared to fail miserably at 100Vac, 50Hz. Bob Richards, NCT. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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