[SI-LIST] Re: Question on radiation limits
- From: John Barnes <jrbarnes@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx, AAnbazhagan@xxxxxxxxxxxxxx
- Date: Mon, 08 Jul 2002 15:08:11 -0400
Anbazhagan,
I would trust the second lab's measurements. Their limits match CISPR
22 section 6, which calls for class B devices to meet:
* 30dB(uV/m) quasi-peak from 30MHz to 230MHz
* 37dB(uV/m) quasi-peak from 230MHz to 1000MHz
at a 10m measuring distance, over a conductive ground plane.
The 10.35dB higher limits used by the first lab suggest that they
assumed a 1/r^2 dropoff, and nothing more. This would be valid in free
space, for the electromagnetic field from an isotropic radiator, under
far-field conditions. Three major error sources that they have
neglected are:
* No reflected wave off the ground plane- measurements up to 3dB
low from this factor alone.
* Coupling between the noise source's antenna and the receiving
antenna (not enough information to quantify).
* Coupling between the receiving antenna and the ground plane (not
enough information to quantify).
Measured according to the CISPR 22, signals from the equipment under
test (EUT) not only go directly to the receiving antenna, but also
bounce off the ground plane. The receiving antenna thus sees the vector
sum of two or more signals, with:
* Additional path loss for the reflected signals.
* Phase differences due to the difference in the path lengths.
Thus the signals may do anything from almost cancelling to almost
doubling the direct signal, depending on:
* Signal frequency.
* Antenna orientation.
* Height of the noise source.
* Height of the receiving antenna.
* Horizontal distance between noise source and the receiving antenna.
The result is that in the azimuth (going up and down) you will see lobes
and nulls. CISPR 22 requires you to scan from 1m to 4m to find the
maximum emissions (worst lobe) before making quasi-peak measurements.
90+% of the time vertically-polarized signals have lobes on the horizon
(1m antenna height), while horizontally-polarized signals have
high-angle lobes (> 4m antenna height) for typical table-top equipment.
But even using an Open Air Test Site (OATS) or semi-anechoic chamber,
you may have trouble extrapolating 3m measurements to 10m measurements.
As you back away from the antenna the lobes will move vertically. So
you may have a particularly-nasty lobe that was above 4m or below 1m at
3m distance, but slides into the 1m-4m height range at 10m... Using a
3m Radiated Chamber, with 1m to 1.74m antenna height, we had to design
in up to 9dB additional margin to make sure that we would pass on the
OATS at 10m:
* Negligible correction for horizontal polarization.
* 1.5dB Vertical Correction Factor (VCF) at 30MHz, increasing to
* 9dB VCF at 240 MHz, decreasing to
* 0dB VCF at 450MHz and higher.
Before the 10m semi-anechoic chamber was built, we sometimes would
re-verify a product in just the 3m semi-anechoic chamber after an
engineering change. But this was based on:
* Using the same EUT, support equipment, and setup as the original
verification measurements, with both 3m and 10m plots for comparison.
* No new significant noise peaks.
* Radiated emissions no worse than the original measurements.
* Usually only for class A devices.
John Barnes
dBi Corporation
http://www.dbicorporation.com/
AAnbazhagan@xxxxxxxxxxxxxx wrote:
>
> Dear All,
>
> Though it is not strictly related to signal integrity, I have a ques=
> tion
> on the radiation limits set by the CISPR (Eurpoean emission standard,
> EN55022 for emission and EN55024 for immunity).
>
> We have a got a lab in which we did the radation test in a fully anecho=
> ic
> chamber at 3 m distance (Antenna to EUT horizontal distance) to get the=
> CE
> certification. The limit was set as 40.35 dB=B5V/m from 30MHz to 230MHz=
> and
> 47.35 dB=B5V/m from 230 to 1GHz.
>
> The same test was conducted in another lab located at a different count=
> ry.
> There they had set the limit as 30 dB=B5V/m and 37dB=B5V/m and the boar=
> d failed
> with that limits.
>
> Which one of the limit is correct? It was argued that the 40dB=B5V/m li=
> mit
> for 3m distance is valid only for OATS and not for anechoic chambers.
>
> Can someone clarify it.
>
> Thanks
> Anbazhagan.
>
> =
>
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