Matt Shaver wrote: > The most basic test for RFI is to tune around the AM radio band on a > receiver with the antenna close to the equipment under consideration, > listening for changes in the level of "static". I'm just hoping to > improve on this; to take it to the next step. Maybe build some sort of > "hobby standard" wide band RFI detector useful for comparison purposes, > rather than a calibrated measurement instrument. > > For example, if I put a ferrite bead on a cable, did it help reduce the > RFI, or was there no change at all? Right now, I'm operating in the > dark, installing filters, chokes and beads in an anticipatory, > prophylactic way ;). What I need is a relative indicator to detect > electrical interference, locate the source, and compare the effect of > different mitigation methods. >
I design large industrial VFDs (great noise generators), and I have been involved in EMC/RFI testing. I don't have the standards - they tend to be written in beaurospeak and you need an expert to translate them and figure out what applies to any specific case. Fortunately others in our company are burdened with that task. Regarding the ability to measure the effect of a ferrite bead or other mitigation attempt - good luck. I have been involved in efforts where even with the fancy spectrum analyzers, it is very difficult to figure out exactly what the noise source is, and how it is escaping from the equipment enclosure. Keep in mind that there are many paths, and that the instruments and standards use dB, a log scale. So the readings are dominated by the worst emitter. Suppose the limit is 70dB, and you have a problem signal that is at 80dB. You put a ferrite bead on a likely suspect and the reading drops by 2dB, from 80 to 78. Maybe the bead reduced the noise transfer through that wire by 2dB, which isn't very encouraging. But maybe the bead actually dropped _that path_ by 20 or 30 dB, but that reduction was masked because another path is emitting at 78dB. If you take the bead off and later put it on that second path, you'll again see only a couple dB of reduction. But if you put beads on both paths (and there isn't a third one lurking somewhere) you'll see a 20-30 dB reduction. It can be very very frustrating. >> Most machine tool controls have had totally uncontrolled >> emissions, because they were mostly exempt as heavy industrial >> equipment. Note the lack of output filters on most PWM servo amps. > > Your own being an exception to this! > >> Devices to inject disturbances into the line and equipment >> chassis are not as bad as the spectum analyzer type gear, and >> you can even make much of this yourself, like with auto ignition >> coils and pulser circuits. > > I did make a vibrating relay type noise generator which helped recreate > an intermittent problem with a spindle speed control board. We call that a "clacker". (Based on the noise the relay makes.) If you attach an antenna wire to the point between the coil and the contact, you can move it around to find the sensitive spots. We have one clacker that we built on a ping-pong paddle - that makee it very easy to move it around along cabinet seams, door openings, etc. > I want to come up with some reference designs for these types of noise > generators so that susceptibility comparisons can be made. Repeatability of results is VERY difficult. One of the issues is that much electrical noise is impulses with relatively random timing. Many vulnerabilities also have timing windows - for example, circuits that usually reject noise, but can be double clocked IF the noise pulse arrives just before or just after the signal has crossed a threshold. When you have failures that depend on a noise spike lining up with a vulnerability window, repeatability goes out the window and reliability becomes a statistical thing. As you close the big windows, the smaller ones get harder and harder to find. Regards, John Kasunich ------------------------------------------------------------------------- Check out the new SourceForge.net Marketplace. It's the best place to buy or sell services for just about anything Open Source. http://sourceforge.net/services/buy/index.php _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
