Ken, When you ask how members "feel," you open a Pandora's box!
We must still meet some kind of installed bottom line; our equipment must not generate fields above some limit. (We can argue what that should be some other time.) However, when _designing_ an EMC solution, we can estimate field strength based on some arbitrary gain, current and impedance for cables. By assuming all common-mode currents flow in the worst possible directions -- here's our cable arrangement -- we come up with a conservative solution. But cables coming from a (say) two-meter square EUT cannot take all possible configurations. GR-1089 assumes a limited cable arrangement representative of a Central Office installation. And when an EUT gets large enough, it's no longer enough to know what current flows in the cables anyway, because the EUT may be a principal radiator by itself. So I'd not want all radiated tests replaced. We sill need a size limit to tell when we must use antennas, and when current probes. We also need a more flexible definition how and where cable current is to be measured. Not al cables can be run along the floor on a reasonable test site. If we must reach a current maximum with a probe, we may have to get five meters from the EUT. That might require a ten meter diameter ground plane -- which brings to mind the saying: Be careful what you ask for; you might get it! Regards, Cortland (Whose posting here reflect none of his employer's opinions) ====================== Original Message Follows ==================== >> Date: 13-Jan-01 00:50:16 MsgID: 1077-20414 ToID: 72146,373 From: "Ken Javor" >INTERNET:[email protected] Subj: Re: Site Correlation Chrg: $0.00 Imp: Norm Sens: Std Receipt: No Parts: 1 List-Post: [email protected] Date: Sat, 13 Jan 2001 02:43:51 -0600 Subject: Re: Site Correlation From: "Ken Javor" <[email protected]> Reply-To: "Ken Javor" <[email protected]> I must say that this thread has been a refreshing alternative to the EMC-law/regulations questions that typically occupy this service. Not complaining either, because If I suddenly found myself working commercial EMC issues I would likely be flooding this line with those self-same questions. Almost as an aside, Mr. Heald raises an issue of enduring interest to myself and others. "Another important factor... is to manipulate the cables during testing (oh, how much easier our job would be without cables)." The same issue was raised parenthetically in my answer to the question about GTEM polarization. The issue is control of cable-sourced radiated emissions. I am now about to allegorically take a baseball bat to a hornets' nest... Bela Szentkuti pointed out almost twenty years ago that it would be much more efficient and accurate to analytically/experimentally determine the relationship between cable common mode currents and the resultant radiated field based on the maximum possible radiation efficiency of that cable, and use that relationship to derive a common mode current limit for cables from 30 MHz to 1 GHz, using the absorbing clamp as a measuring tool. This would speed up OATS or any other kind of RE testing by deleting the requirement to maximize cable radiation. So this question is a poll. How do the subscribers to this service feel about cable common mode current control in lieu of direct measurement of cable-sourced RE measurement? The idea being that first you would measure and bring cable cm CE into compliance with a cable-type limit and only then would you make the RE measurement. The cables would only be support equipment which did not contribute to the RE profile, hence any measured emissions at or near the limit would be guaranteed EUT enclosure-related. Polite responses only, please!!! Ken Javor ---------- >From: David Heald <[email protected]> >To: "Tudor, Allen" <[email protected]> >Cc: "EMC-PCST (E-mail)" <[email protected]> >Subject: Re: Site Correlation >Date: Fri, Jan 12, 2001, 9:36 AM > > > Greetings again. > I received some questions about this off list and there has been more > discussion in this direction, so I thought I would throw my other two > cents in. > For small fully anechoic chambers with little room for antenna height > adjustment, you should be able to have uncertainty of about 6dB or so > (10dB is much safer realistically) when you apply correction factors for > a 10m site. The reason for this is, as John Barnes pointed out, the > absence of reflected waves being received in addition to the direct > waves. The key importance to a fully lined chamber (including the > floor) is that destructive waves are not present. With a reflective > floor, destructive waves can lower your readings by more than 30dB. Add > this to the 6 dB or so of uncertainty for additive waves and your total > error could be enormous. With an absorber lined floor, the influence of > the destructive waves is eliminated or reduced, so a correlation of 6dB > (again 10dB is safer) should be achievable (this simply accounts for the > absence of constructive interference). > Another important factor to ensure you don't have any surprises when > moving from precompliance to a compliance run is to manipulate the > cables during testing (oh, how much easier our job would be without > cables). Large signal strength changes can be achieved just by moving > cables a few inches. > I also have to agree with Gert's and Ken's comments on far field > measurements. I mentioned this in my original message, but didn't > elaborate at all. These are very important considerations that can > greatly affect any expected correlation to a 10m OATS. > > -- > David Heald > Senior EMC Engineer/ > Product Safety Engineer > > Curtis-Straus LLC NRTL > Laboratory for NEBS, EMC, Safety, and Telecom > Voice:978.486.8880x254 Fax:978.486.8828 > www.curtis-straus.com > > > Tudor, Allen wrote: > >> Greetings: >> >> What's the best way to correlate a pre-compliance chamber (smaller than a 3m >> chamber) to a 10m anechoic chamber? Should I use a signal generator and >> antenna or should I use a comb generator? >> >> Would the answer be different if I were correlating the pre-compliance >> chamber to an OATS? >> >> Thanks in advance. >> >> >> Allen Tudor, Compliance Engineer >> ADC DSL Systems Inc. >> 6531 Meridien Dr. >> Raleigh, NC 27616 >> phone: 919.875.3382 >> email: [email protected] >> >> >> ------------------------------------------- >> This message is from the IEEE EMC Society Product Safety >> Technical Committee emc-pstc discussion list. >> >> To cancel your subscription, send mail to: >> [email protected] >> with the single line: >> unsubscribe emc-pstc >> >> For help, send mail to the list administrators: >> Jim Bacher: [email protected] >> Michael Garretson: [email protected] >> >> For policy questions, send mail to: >> Richard Nute: [email protected] >> >> >> >> > > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Jim Bacher: [email protected] > Michael Garretson: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected] ====================== End of Original Message ===================== ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
