Using the assumption below that both signals are BB and not synchronized, could you get away with something as simple as converting the noise amplitude and noise plus EUT amplitude to linear terms and subtract the noise? I believe there is precedence in CISPR regs for this approach.
Brent DeWitt, AB1LF Milford, MA From: Ken Javor <ken.ja...@emccompliance.com> Sent: Thursday, April 11, 2019 7:23 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] [EXTERNAL] Re: [PSES] EMI testing with ambient If both signal and noise are broadband and not synchronized, then they would add rss. It would be interesting if you could trigger off the signal - if there are two BB low duty cycle signals at the same frequency, you might be able to time window out the noise. You didn't say radiated or conducted (sounds like RE), but clearly easier to do conducted because higher level signal - maybe can use o'scope. If radiated you could likely use a current probe around the culprit radiating cable(s) to increase the signal level to where maybe an o'scope would work. Ken Javor Phone: (256) 650-5261 _____ From: "Grasso, Charles" <charles.gra...@dish.com <mailto:charles.gra...@dish.com> > Reply-To: "Grasso, Charles" <charles.gra...@dish.com <mailto:charles.gra...@dish.com> > Date: Thu, 11 Apr 2019 23:10:51 +0000 To: <EMC-PSTC@LISTSERV.IEEE.ORG <mailto:EMC-PSTC@LISTSERV.IEEE.ORG> > Conversation: [EXTERNAL] Re: [PSES] EMI testing with ambient Subject: Re: [PSES] [EXTERNAL] Re: [PSES] EMI testing with ambient Thanks Bob - Yes I have had to use this technique occasionally. What I have is BB + BB - hence the question. Thanks! Charles Grasso W: 303-706-5467 From: Sykes, Bob [mailto:bob.sy...@gilbarco.com] Sent: Thursday, April 11, 2019 1:00 PM To: EMC-PSTC@LISTSERV.IEEE.ORG <mailto:EMC-PSTC@LISTSERV.IEEE.ORG> Subject: Re: [PSES] [EXTERNAL] Re: [PSES] EMI testing with ambient This message originated outside of DISH and was sent by: owner-emc-p...@listserv.ieee.org <mailto:owner-emc-p...@listserv.ieee.org> Also not rocket science or breaking news (to many)... If you are measuring a CW NB signal in the presence of BB ambient, decreasing the Resolution Bandwidth of the EMI receiver will generally reduce ambient signal level while having negligible effect on the CW signal. Of course there are limits to this.. And yes, you are no longer using the specified CISPR RBW for the measurement so YMMV :) From: Ken Javor [mailto:ken.ja...@emccompliance.com] Sent: Thursday, April 11, 2019 12:01 PM To: EMC-PSTC@LISTSERV.IEEE.ORG <mailto:EMC-PSTC@LISTSERV.IEEE.ORG> Subject: [EXTERNAL] Re: [PSES] EMI testing with ambient CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe. Seeing as no one has yet taken a crack at this, I will weigh in with a non-specification opinion. Meaning this is just statistics, or math, not wording from some officially sanctioned publication. No rocket science here or revealed truth. Probably not telling Chas anything he doesn't already know... You can calculate the effect of an ambient on an existing signal at the same frequency, but you have to decide if the signal and noise add in phase or not. If the ambient is just thermal noise, that is an rss type calculation, like this: (Measured amplitude)^2 = (Actual amplitude)^2 + (noise amplitude)^2 If the signal and noise are in phase, then it's just a linear addition. I only mention this latter because Chas' post says the noise is BB. He doesn't elaborate, but that could mean impulsive noise. I'm not sure if impulsive noise adds to a cw signal the same as thermal noise. Never tried to measure that. Worst case you assume it adds rss. Using Chas's values: If the signals do add linearly, then if the measured amplitude is 6 dB over the limit and the noise is at the limit, that means the signal is just at the limit. If the signals combine rss, then if the measured amplitude is 6 dB over the limit and the noise is at the limit, that means the signal by itself is about 5 dB over the limit. Ken Javor Phone: (256) 650-5261 _____ From: "Grasso, Charles" <charles.gra...@dish.com <mailto:charles.gra...@dish.com> > Reply-To: "Grasso, Charles" <charles.gra...@dish.com <mailto:charles.gra...@dish.com> > Date: Wed, 10 Apr 2019 18:02:50 +0000 To: <EMC-PSTC@LISTSERV.IEEE.ORG <mailto:EMC-PSTC@LISTSERV.IEEE.ORG> > Conversation: EMI testing with ambient Subject: [PSES] EMI testing with ambient Hello all - My question is regarding the validity of data taken with a high ambient condition present. Due to test configuration constraints we have high broadband noise entering a test chamber via the cables entering the enclosure. The bb noise is about 20MHz wide and is hitting the FCC Class B spec. Of course I have failing data (from the EUT) reported by the lab. The failing data exceeds the FCC spec by about 6dB. Question: What are the conditions under which a data point is considered invalid in the presence of high ambient bb noise?? Thanks! Charles Grasso W: 303-706-5467 - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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