In message
<64D32EE8B9CBDD44963ACB076A5F6ABB0268DF76@Mailbox-Tech.lecotech.local>,
dated Thu, 10 Apr 2014, "Kunde, Brian" <[email protected]> writes:
There is a phenomena that is sometimes seen during the Radiated
Immunity Test (IEC61000-4-3) where the EUT will fail as the frequency
changes to the next step but if you dwell on that frequency and turn
the RF on and off it doesn't fail.
From previous discussions on this email group some experts believe that
phenomena occurs because the Signal Generator glitches during a
frequency or power level change causing the failure which would be a
false failure but inherent to most such test equipment.
It can't help but 'glitch', because a step change in frequency or level
generates a transient spectrum containing many frequencies. This applies
even if the change occurred at a zero crossing, and unless it was done
quite slowly (when the transient spectrum would be dense but weak).
Others believe that some electronics can be more sensitive to a
frequency change than a power level change and that the failure is a
real failure.
I think that might be difficult to explain in physical terms.
I'm also learning that many test labs are coming up with their own test
methods to improve the inherent uncertainties. Here are a few I'm aware
of:
1. Level at each frequency using the forward power value with
modulation turned off. Once leveled, turn modulation on for the dwell
time.
2. Ramp down the power, change to the next frequency, then ramp it up
to the value determined during calibration.
3. Reduce power by 5db, change to the next frequency, then increase
power to cal value.
4. Reduce power by 5db, change frequency, read forward power value,
calculate next power level and apply.
I'm sure there are other methods being used. One downside to any method
other than the straight forward "change frequency and power method", it
adds time to the test. Customers don't like paying for extra lab time.
Indeed: if people thinks such modifications are needed, they should make
a case to CISPR/A for review.
Another issue is if a Customer fails at one lab and then goes to
another lab, they may get different results if a different test method
is being used.
Yes. This can be very troublesome.
Finally, here are my questions. What test methods do you use in your
lab or used by 3rd party labs that you go to? Is it common for labs to
offer more than one test method depending on the pros and cons
presented for each? Is there a preferred method being used?
It should be 'horses for courses' - where there are alternative methods
given in the standard, the method most appropriate for that *product
type* should be chosen, not the method that happens to pass a specific
product.
I apologize for bringing this subject up again, but I was hoping to get
some updated perspective and latest opinions regarding this subject. I
know every lab wants to do the best job possible and make improvements
by varying the test method, but then you may introduce differences
where you no longer correlate your results with other labs.
The temptation to 'improve' should be restricted to minor changes.
Anything more should be proposed as an improvement of the relevant Basic
standard.
--
OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk
Nondum ex silvis sumus
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
-
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