Some more speculation on the FAC measurement:
It appears that you have common mode signals on the power cable. At the OATS,
when the EUT is connected to GND though the AC powered 12VDC power supply you
created a monopole antenna consisting of the power cable (12V cable and the AC
power cord of the 12 VDC supply). When the 12V cable is shielded some of the
CM current is returned directly to the EUT chassis. You then might have had
less CM current on the power supply power cord and less radiation. When using
a battery to power things it was all floating, correct? Your "antenna" is now
a dipole with the ends being the battery and the EUT. This "antenna" will
behave differently than the monopole and could be better or worse, depending
on many factors.
In the FAC the monopole antenna performance may have dropped due to the lossy
"GND".
The bottom line is the common mode RF on the power leads. This needs to be
fixed. Bypassing the + and - directly to the chassis at the chassis wall may
help.
Dave
From: [email protected] [mailto:[email protected]]
Sent: Wednesday, March 24, 2004 7:19 AM
To: drcuthbert
Cc: [email protected]; [email protected];
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Subject: RE: What is wrong in my 3 meters FAC measurement
Dear All,
Sorry I think I have not explained properly.
FAC Measurements (with shielded or unshielded cable almost the same result)
244.08 MHz 37.5 dB @ 3 meters
162.72 MHz 29.0 dB @ 3 meters
135.60 MHz 24.0 dB @ 3 meters
OATS Measurement (unshielded cable used to connect the EUT to the 12 volts
d.c. power supply)
244.08 MHz 37.1 dB @ 10 meters
162.72 MHz 31.1 dB @ 10 meters
135.60 MHz 27.2 dB @ 10 meters
Some more trials we did in the OATS. By conneting the EUT to the 12
volts d.c power supply using a shielded cable we passed comfortably. While we
tested by using a 12 volts battery even without the shielded cable EUT
comfortably passed Class B limits. But in the FAC there was no difference
found between a shielded cable and an unshielded cable.
Now the question is what was the reason for the failure at OATS. The
power supply? The cable? The combination of power supply and cable? Anything
else?
Sincerely
K.Balasubramanian
Sr.Engineer, Hardware.
[email protected]
03/23/2004 01:37 AM
To: <[email protected]>, <[email protected]>
cc: <[email protected]>, <[email protected]>,
<[email protected]>, <[email protected]>,
<[email protected]>, <[email protected]>, <[email protected]>,
<[email protected]>, <[email protected]>, <[email protected]>,
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Subject: RE: What is wrong in my 3 meters FAC measurement
Are the measurements in dBuV/m at 3 meters and at 10 meters?
Dave Cuthbert
Micron Technology
From: John Woodgate [mailto:[email protected]]
Sent: Sunday, March 21, 2004 5:27 AM
To: [email protected]
Cc: [email protected]; [email protected];
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drcuthbert; [email protected]; [email protected];
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Subject: Re: What is wrong in my 3 meters FAC measurement
[email protected] wrote (in <OF570A07AC.7B1B4A15-ON65256E5
[email protected]>) about 'What is wrong in my
3 meters FAC measurement', on Sun, 21 Mar 2004:
> 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.
I assumed that you had already corrected the figures for the different
measurement distance. If you give insufficient data, you WILL get
misleading responses.
> Please note that the 3.2 db @ 135.6 MHz difference is NOT
within
>expectations.
It is within MY expectations, given the differences between the two sets
of tests.
--
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
