Here is the problem with the ‘2 dB rule’. If there are only two ports it simply makes more sense to cable the ports. If the device passes, go no further. If you only fill one port, then you have to still fill the remaining port to meet the 2dB rule, even if it passes. So, why waste the time?
The 2dB rule is basically meant for those systems having many identical ports. The same reasoning can be applied to adding identical modules. If a host only has the capacity of two identical modules, you will need to put both modules in and test once. If you use the 2dB rule for identical modules, you test one host with one module, then you must still add the second identical module to meet the 2dB rule. Again, why waste the time – test both, get done. The 2dB rule in both scenarios is still for those instances where many of the same identical modules or ports are used in a single host’; but then it appears no one defines what “many” means. In that case, put one module in, then put the second. If no more than 2dB is seen, test done; if more than 2dB put the third in. If still more than 2dB keep going till all modules are in place or no more than 2dB increase seen. 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From: Bill Stumpf [mailto:bstu...@dlsemc.com] Sent: Thursday, March 9, 2017 11:26 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] EMC co-location distance question Right - ANSI C63.4 still makes reference to the "2dB rule" Bill From: Adam Dixon [mailto:lanterna.viri...@gmail.com] Sent: Thursday, March 09, 2017 1:03 PM To: EMC-PSTC@LISTSERV.IEEE.ORG <mailto:EMC-PSTC@LISTSERV.IEEE.ORG> Subject: Re: [PSES] EMC co-location distance question It's been awhile, but at the time when wanting to understand modular test requirements, the 2dB asymptote/leveling off was a reference in FCC Part 15 Section 15.31 (a) (3) pointing to ANSI C63.4-2003. Regards, Adam in Atlanta On Thu, Mar 9, 2017 at 1:16 PM, Sundstrom, Mike <mike.sundst...@garmin.com <mailto:mike.sundst...@garmin.com> > wrote: Regan, In regards to the daisy chaining: Hook up equipment (daisy chaining) until such a time as you don’t get a 2 dB increase of any unwanted signals. I think this is in ANSI or CISPR? Everyone, What am I remembering partly here? Thanks, Michael Sundstrom Garmin Compliance Engineer 2-2606 (913) 440-1540 <tel:(913)%20440-1540> KB5UKT "We call it theory when we know much about something but nothing works, and practice when everything works but nobody knows why." -- Albert Einstein From: Regan Arndt [mailto:re...@empowermicro.com <mailto:re...@empowermicro.com> ] Sent: Thursday, March 09, 2017 11:50 AM To: EMC-PSTC@LISTSERV.IEEE.ORG <mailto:EMC-PSTC@LISTSERV.IEEE.ORG> Subject: Re: [PSES] EMC co-location distance question Hi Bill. Thanks for this great info! Much appreciated. The second part of my original email was merely philosophical discussion points. Let me clarify. I would love to hear what people think: 1. How many different types of ancillary equipment shall you test with your main EUT? i.e. PCs or laptops with HDMI & USB ports……the sky’s the limit as to the myriad of devices that now can be connected. Where does one ‘stop’? a. Also, not all devices (that claim to be Class B) are noise free. I’ve personally experienced extremely noisy devices using a reputable brand name (yes, FCC logo on the device), but had to exchange it for their competitor to ensure no unintentional emissions, so my main EUT passes. (Moral of the story….don’t believe everything you read on the label – lol) 2. Another related question: what about identical devices that can be daisy chained? i.e. some devices allow a daisy chain of up to 12 or more. (i.e. telecom trunk card). Does one use the chamber table as the determination for the quantity of daisy chained samples to test? Or does one compile all the max # of daisy chained devices on the table despite the congestion of I/O & power cables (not recommended)? Or is there a rule of thumb that one shall prove that there will not be an increase in emissions past a certain number of devices? (The latter is my preference). Thoughts? From: Bill Stumpf [mailto:bstu...@dlsemc.com] Sent: Thursday, March 09, 2017 6:54 AM To: Regan Arndt <re...@empowermicro.com <mailto:re...@empowermicro.com> >; EMC-PSTC@LISTSERV.IEEE.ORG <mailto:EMC-PSTC@LISTSERV.IEEE.ORG> Subject: RE: EMC co-location distance question Regan, I'm not sure I understand your question correctly, but I will try to provide an answer. ANSI C63.4 is used when testing for FCC part 15 subpart B compliance for unintentional radiators. When testing a table-top system, the collocated equipment should be spaced at 10cm, unless this is not typical of normal operation. If so, then the EUT and its accessories/peripheral devices should be placed as they would be in typical applications. It is important to read the text of the ANSI standard, as it goes into great detail on how to set up and select accessories for testing. The drawings are for general reference only and the text always takes precedence. As for EN/CISPR standards, they mostly accept similar setup conditions to ANSI C63.4, but you will have to verify the specific requirements in each standard. The <20cm you refer to was at one time unofficially (FCC) considered the distance for collocation of transmitter antennas, but this is no longer the case. The FCC considers that two transmitters/antennas are collocated if they are in the same product / enclosure. I hope this helps. Bill From: Regan Arndt [mailto:re...@empowermicro.com] Sent: Wednesday, March 08, 2017 1:50 PM To: EMC-PSTC@LISTSERV.IEEE.ORG <mailto:EMC-PSTC@LISTSERV.IEEE.ORG> Subject: [PSES] EMC co-location distance question Greetings members, Can anyone shed any light on what ANSI defines as the max distance/limit of a ‘co-located’ piece of ancillary equipment or other support equipment (on the table) to the main EUT being tested for FCC Part 15 class B for ‘unintentional radiators’. I seem to recall 20 cm but I think this was referring to ‘intentional’ radiators. Is there a similar requirement in the EMC directive and/or EN/CISPR standards? Or is this really dictated on your recommended set-up? Shall that device be removed from the test bed if typical installation indicates that it will be greater than 20cm from the main EUT? I believe that the set-up in ANSI only showed the PC, printer, mouse, keyboard & monitor but that’s it. There are so many other/different electronic devices that connect to a laptop/PC/other, etc. (i.e. smart phone) these days, it is not viable for anyone to test all devices/configurations. I understand that one must use good judgment and exercise due diligence but there must be a limit. Agree? Some of these electronic devices state compliance to Class B, it may still have interference with another closely co-located device because they did not test it fully to the myriad of other electronic devices out there that it could be connected to. There is no guarantee per se. Can anyone comment? Thanks in advance. 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