Re: [PSES] I would like to hear your thoughts please

[James Pawson (U3C)]

I once tried to do a “worst case” analysis and decided that worst would be 
testing the HDMI output on the product we had designed at the highest 
resolution = highest frequency = biggest problem. I ran all our pre-compliance 
work at this frequency and neglected to check any other resolutions.

 

When we came close to lab time, prompted by my then manager, I checked other 
lower frequency, lower risk (surely?) resolutions and found that the second 
highest frequency was exciting an unintentional resonance in the product and 
the emissions were 6dB higher (just over the limit).

 

However, in our experience of testing a lot of different products, 9 times out 
of 10 it is the more complex configuration that is more likely to have the 
problems, mostly because this has a bigger surface area for risk due to the 
variety of circuits.

 

I like that phrase that “EMC is all about what isn’t on the schematics” i.e. 
the unintended performance. You really don’t know until you test.

 

I would agree with the other voices on here to use pre-compliance testing to 
establish the actual worst case if in any doubt.

 

All the best

James

 

James Pawson

Managing Director & EMC Problem Solver

 

Unit 3 Compliance Ltd

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From: Gert Gremmen F4LDP  
Sent: Saturday, April 6, 2024 3:55 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] I would like to hear your thoughts please

 

Dear All,

Within the framework of the EMCD, all configurations shall be conform, so if 
you choose to actually test, 
all configurations shall be part of the test. The subject of worse case is a 
"miroir d'alouette"... how will you ever know which
is worst case without carrying out the test ? A pre-scan is informative but the 
radiated emission test contains already a pre-scan (peak) for the final 
QP-measurement.  We already require a EMC risk analysis which is a kind of 
pre-scan too. How many pre-pre-prescans will we need to be sure ?
"to repeat some (which?) test to make sure nothing was broken" it's another 
discipline, but that is how Boeing lost a door in flight. And that  is not a 
unregulated sector without thorough quality scans (understatement), and still 
it happens. Imagine the costs and effort for Boeing to rebuild their reputation 
? Didn't we all learned the exponential graph of EMC costs versus development 
time ?
If you need proof (for authorities, or for yourself), nothing can replace the 
actual test. 

Gert Gremmen

On 6-4-2024 0:47, Lfresearch wrote:

Hi folks,
 
I would like to advise a client at where to draw the line on what needs 
testing. I would like to solicit opinions besides my own. Otherwise it’s the 
fox urging the chicken coop…
 
So a manufacturer that makes a product of which there will be several variants. 
All use the same board, but have different sections of circuits populated. This 
may require slightly different code to run on the same uP in each case.
 
So.. The burning question is can we perform and analysis that postulates a 
worse case hardware/software combination and test just one configuration? Or, 
do we have to do every combination?
 
Or, are there some guidelines about where we draw the line of what to test and 
what can be claimed as similarity?
 
Off list responses are welcome too.
 
Thanks,
 
Derek Walton
LFResearch/SSCLabs.
 
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Re: [PSES] Radiated Emissions - How many points to QP?

[Bill Owsley]

 It reads like there are two limits, peak and QP.  Go over either one and by 
how many and by how much over, does not matter.  it is a fail and fix it.
Otherwise, the lab should be recording the 6 points of each P and QP for 12 
points, well,  let the slide if all points are below the QP limit and graph 
shows that.
Now to get lab to show a continuous graph for radiated emissions might be a 
problem.
Easier to get a new lab !
Some cheap labs will fake a continuous plot by connecting the highest dots.  
Run from them !
Long details on the experience that I got.



On Friday, April 5, 2024 at 03:33:01 PM EDT, Stultz, Mark 
<0f79f2e10e47-dmarc-requ...@listserv.ieee.org> wrote:  
 
  
Hello PSES brain trust,
 
When doing radiated emissions measurements, how many frequencies should be 
quasi-peaked? 
 
CISPR 16-2-3 clause 6.4.9 states: 
 
“Of those disturbances above (L-20dB), where L is the limit level in 
logarithmic units, the disturbance levels and the frequencies of at least the 
six highest disturbances shall be recorded.”
 
  
 
We have always done QP measurements on all peak measurements above the limit, 
even if that is more than six points.
 
I have a lab arguing that they only need to measure the top six, regardless of 
how many peak measurements are above the limit.
 
Any thoughts are appreciated.
 
  
 
Thanks,
 
Mark
 
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Re: [PSES] Radiated Emissions - How many points to QP?

[Bill Owsley]

 generally, 2 limits, QP and P which is 20 db above QP.  Gotta meet both.  
Unless the P is so infrequent as to call it a Click.  Which I would not to want 
a challenge over.
On Friday, April 5, 2024 at 03:53:43 PM EDT, Stultz, Mark 
<0f79f2e10e47-dmarc-requ...@listserv.ieee.org> wrote:  
 
 
Hi Larry,
 
  
 
I agree with you completely…I think the lab didn’t see that peak going over the 
limit line and therefore didn’t QP.  We only noticed now that they have sent 
the report several weeks later.  They’re arguing that there is no need to 
retest.
 
  
 
Thanks,
 
Mark
 
  
 
  
 
From: Larry K. Stillings 
Sent: Friday, April 5, 2024 3:46 PM
To: Stultz, Mark ; EMC-PSTC@LISTSERV.IEEE.ORG
Subject: RE: Radiated Emissions - How many points to QP?
 
  
 
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Mark,
 
  
 
If the peak is above the limit (which most standards allow up to 20 dB), then 
wouldn’t you also need to know the quasi-peak of all the signals above the 
limit so you could compare them to the limit? The limit is in QP not peak.
 
  
 
I guess failing is failing, but you really wouldn’t know by how much.
 
  
 
Is it really that much more work to take a QP, or maybe the automation software 
doesn’t support that? We’re old school and still take the data manually and I 
have the guys always take both (peak and quasi-peak).
 
  
 
Larry K. Stillings
Compliance Worldwide, Inc. 
Test Locally, Sell Globally and Launch Your Products Around the World!
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From: Stultz, Mark
Sent: Friday, April 05, 2024 3:33 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] Radiated Emissions - How many points to QP?
 
  
 
Hello PSES brain trust,
 
When doing radiated emissions measurements, how many frequencies should be 
quasi-peaked? 
 
CISPR 16-2-3 clause 6.4.9 states: 
 
“Of those disturbances above (L-20dB), where L is the limit level in 
logarithmic units, the disturbance levels and the frequencies of at least the 
six highest disturbances shall be recorded.”
 
  
 
We have always done QP measurements on all peak measurements above the limit, 
even if that is more than six points.
 
I have a lab arguing that they only need to measure the top six, regardless of 
how many peak measurements are above the limit.
 
Any thoughts are appreciated.
 
  
 
Thanks,
 
Mark
 
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