EMC Testing and monitor

Please do not hate me or make remarks about my lack of knowledge about EMC. I am doing some research to learn some basics about EMC testing.

I found the following report on EMC testing of a PC monitor

I have following questions

1. This montior did pass the EMC testing. The test report shows two graphs on pages 15 and 16 of the report 9 (Level vs Frequency) . Can someone give me some pointers on how to read and understand this graph?

I am unable to understand what is the maximum value dB(uv /m) that is not acceptable by FCC? It seems like that the tester put the limit to 30uV/m. But why this number. Is 30uV/m standard for monitors and could be different for lets say key board or mouse?

1. What is the difference between horizontal polarization and vertical polarization?

melissa

The FCC sets standards mostly so the devices will not cause any interferance with other electronic devices, mostly radio and TV receivers. On the graphs the horizontal axis is the frequency . Higher frequencies to the right. The vertical is in microvolts per meter. This is a way to tell how strong the signal is. The graph increments in decibels. Each 10 DB is 10 times as strong. Starting at 0 DB means you have one microvolt (uV) per meter. At the 10 mark it is 10 times as strong, 20 DB is 100 times as strong, 30 db

Many devices radiate radio frequencies that cause problems with other receivers. While you never reach true zero level, the greater the distance the weaker it gets. If you start off at a low level, then hopefully other receiving devices will be far enough away as not to cause problems. Look at it this way, you have an FM radio and want to use it next to the computer. While on the same desk, the radiation from the computer may over ride the FM broadcast station. You move the receiver across the room and the signal from the computer is weak enough youcan receive the FM station.

As to horizontal and vetical polarization, that is sort of easy. That is in relationship to the earth. If an antenna is parallel to th earth like the old outside antennas, that is horizontal and if the antenna is vertical like most AM radio stations, that is vertical.

Hi, Thanks for your reply. Can you also advise me on the following issues?

I am unable to understand what is the maximum value dB(uv /m) that is not a cceptable by FCC? It seems like that the tester put the limit to 30uV/m in the report. But why this number. Is 30uV/m standard for monitors and could be different for lets say key board or mouse?

melissa

The numbers are SET by the Regulatory Agency, in this case FCC.

BUT, to prevent confusion emanations from electronic equipment are measured in a specific way. Procedures, time to measure, bandwidths etc etc are all specified.

But for simplicity, envision an Open Air Test Site, OATS for short, with a metal Ground plane covering a huge area. Then using what is called a Robert's Antenna, [Robert's Antenna is like a set of completely opened rabbit ears] which is a dipole antenna with its length adjusted (tuned) for the frequency of interest, at a distance of 30 meters sometimes 100 meters, you measure what comes OFF equipment with the equipment sitting on a non-conductive bench approx 1 meter above the Ground plane. [if the equipment is BIG, it simply sits isolated from the ground plan, as in giant set of rack mounted equipment]

To speed up the test, FCC allows you to use a set of non-adjustable, BroadBand Antennas, and 'adjust' the receiver's readings with calibration values, BUT Robert's Antenna always takes precednece.

Obviously, a dipole antenna has polarization. When the 'arms' are vertical to the ground plane, called vertical polariztion. When horizontal, called horizontal polarization. The antenna must also be held anywhere from 1 meter above the plane to ?? forgot is that 12 meters or 10 meters above the plane? Note doesn't always work well because at low frequency when the Robert's antenna arms are extended, the lower arm hits the plane, so got to'fudge' around a bit. To add insult to injury, FCC requires the antenna be moved to where you get the largest signal.

From memory, FCC has set values for maximum emanations measured at 30 meter distance [and 100 meters distance] from the Equipment. THEY set the values after prolonged arguments. Two permissable levels exist. Class A which is assumed to go into Industrial locations so is less stringent than Class B which is assumed to be in residential areas. Note the difference in distance means the receiver is operating in relatively the same reception level for Class A an for Class B.

At distance of 30 meters the signals can be very small, so FCC allows you to move the antenna system closer, and then you 'adjust' the reading as though they were taken at 30 meters. The advantage of being closer is that nearby radio stations, FM stations, and TV stations signals appear relatively smaller. Now, to AVOID confusiion ALL reports are written in terms of 'what if' the measurements were taken at 30 meters, adjusted so when anyone looks at a report they can compare apples to apples, AND the limits can be drawn in, and therefore always the same.

There are a lot of nuances in taking EMC measurements. But, basically, the OATS method is still the 'standard'.

Do a search for OATS and anything written circa 80's to get the basics. Perhaps, HP, now Agilent, or Rhode & Scwartz(sp?) have some basic information in their AppNotes at their websites.

Hi,

So, 30uV/m limit is set by FCC for all the EMC testing for computer monitors! A monitor will not be approved if it generate noise more than 30uV/m. Am I right?

melissa

you need to get a copy of the FCC rules to read tables of limits, read the description os how equipment is categorized, see the plots of those limits.

From memory, FCC Part 15, relating to electronics and unintentiaonal radiators.

I have seen some monitors that cost on the order of $40,000 [Silicon Grpahics?] that due to their price were categorized as Class A, very unlikely to end up in people's homes. Thus their limits were higher.

I didn't have to look the FCC numbers up, limit at what distance, they're in your report!

No, you are NOT right. Depends on what frequency you're looking at. See your report. page 13/17 shows the limits for Class B

Plus, that's a well written report, describes what, how, and with what they tested the LCD Monitor, even great pictures showing those two Schaffner Biconical Antennas!

For what it's worth, Those test results show a measurement fairly close to the limit AND then clearly states that the measurement uncertainty is MORE than that margin. Could easily move the signal 'out of compliance' if literlly applied.

That's one of the reasons Sony's consumer division has their internal margin enforced with an 8 dB margin!! In other words the FCC specification says one limit, but everything you measure in the test lab had better be more than 8 dB BELOW that limit! Not easy to do either.

Think of an electromagnetic wave as an eel swimming through the water. Some eels may squirm sideways to swim and some eels may prefer to squirm up and down. So the e-field, the voltage part of the wave, may be horizontal or vertical, and a dipole antenna has to be oriented to couple to the field.

Various parts of the monitor might radiate at various angles, not necessarily horizontal or vertical (imagine drunken eels) but one horizontal antenna and one vertical antenna will catch anything.

The h-field, the magnetic wave, is at right angles to the e-field. The e-field creates the h-field, and the h-field creates the e-field, and they squirm away at the speed of light.