Dear John Woodgate,
Thank you very much for the attention paid and for putting the data in
a tabular format. FAC measurement is at 3 meters and the OATS measureent is
at 10 meters. Field strength measured in 3 meters distance in FAC is almost
equal to the field strength measured at 10 meters distance in OATS. So
prescan passed but final testing failed.
Please note that the 3.2 db @ 135.6 MHz difference is NOT within
expectations.
Sincerely
K.Balasubramanian
Sr.Engineer, Hardware.
John Woodgate <[email protected]>
03/21/2004 04:01 PM
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Subject: Re: What is wrong in my 3 meters FAC measurement
[email protected] wrote (in <OFEDF565E1.DCC593DF-
[email protected]>) about 'What is
wrong in my 3 meters FAC measurement', on Sun, 21 Mar 2004:
>
> Dear EMC Gurus,
> I completed my pre-scans for an intentional radiator (13.56
> MHz) in a 3 meters fully anechoic chamber and found that the
> emission are well within the Class B limits. The emission @ 108.48
> MHz (8th harmonic) was around 35.8 db, @ 135.6 MHz (10th harmonic)
> was about 24 db, @ 162.72 MHz (12th harmonic) was about 29 db and @
> 244.08 MHz (18th harmonic) was about 37.5 db. 3 samples were
> tested and the results were similar for all the 3.
> But when another sample of the same model was tested by my
> counterpart in the 10 meters OATS (at USA) the results are
> differing. Emission @ 244.08 MHz is reported about 37.10 db, @
> 162.72 MHz is 31.1 db, @ 135.6 MHz is 27.2 db and the device failed
> to meet Class B limits.
> The test setup is very simple and it is just the 12 volts d.c.
> output of a variable d.c power supply output connected to the
> device. The power supply used by me and by my counter part are not
> same but similar. I tried separately with a shielded cable and with
> an unshielded cable and the results were almost similar. My
> counterpart used only a unshielded cable.
> Now it is great puzzle for us. What is wrong and where? Apart
> from the device being tested, the power supply and the cable used
> there are no other variables involved in the test set up. What
> other factors we should compare between both the testing? I request
> you all provide your valuable feedback.
>
You should be puzzled, but for an entirely different reason. It would be
much better to present your results as a table (don't use tab stops, use
spaces):
Freq. Anechoic OATS
108.48 35.8 ?
135.6 24 27.2
162.72 29 31.1
244.08 37.5 37.1
Now we can see that you have surprisingly GOOD correlation between the
two sets of measurements! Even the 3.2 dB difference at 135.6 MHz is
within expectations. Especially as you are not measuring the same
sample, and you are using different power supplies. Are these power
supplies linear or switching type?
To have confidence that your design is satisfactory for production, if
you are not going to test every unit produced, the measured emissions of
your test samples should be at least 6 dB below the limit.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
