Geetha,
FCC routinely accepts anechoic chamber measurements so long as the chamber has
been tested and shown to meet theoretical site attenuation within tolerances,
and the site description has been submitted to FCC for review. In my
experience a good chamber and a good OATS site will yield similar results,
everything else being equal.
Besides lab locations, skills, and standard practices, there are other
variables you need to consider:
What support equipment are you using if any with your EUT? If you used
different pc, mouse, monitor, power supply, etc. in Taiwan than were used in
US lab, you will likely have differences.
Were the same cables and power cords supplied to each lab? Are you confident
that the cables were installed correctly?
Also, what kind of product are you testing, e.g., radio product, printer, pc,
etc? Does it have software to activate different modes? Depending on the
product, you may have different emissions characteristics for different
operations.
Other posts have mentioned cable orientation to maximize emissions. Your test
reports should have set-up photos to give you a clue as to whether the EUT
configurations were the same at both labs.
If a lab has a grounded flush-mount turntable, and the ground wipers are
corroded or missing, there can be spikes of energy recorded by the test
equipment that come from the making and breaking of the ground connection.
Unless one is there to view a live spectrum analyzer trace (not Max Hold) it
may not be apparent from the plot or table in the report.
What is the nature of the culprit emissions - CW, pulsing, broadband, impulse?
This information is useful in determining the source of the emissions.
As is often the case, you have one equation (test results) and around 10
variables. Welcome to the wonderful world of EMC where "I said maybe" is
often the final answer ;)
best regards
Tom Cokenias
T.N. Cokenias Consulting www.tncokenias org
P.O. Box 1086
El Granada CA 94018
tel 650 726 1263
fax 650 726 1252
cell 650 302 0887
At 7:51 AM -0800 3/22/05, Geetha Balasubramanian wrote:
Hi Gurus,
As per the US lab report, the frequency at which the device had
minimum pass margin is 68 MHz. At 68 MHz in vertical polarisation the margin
is 4.7 dB & in horizontal polarisation the margin is 1.6 dB. Measurable
emissions were found only in 40 , 43, 52.38, 64 & 68 MHz.
But as per the Taiwan lab report no measurable emission is found
>from the device at the above frequencies. The device failed in 122.61 MHz at
horizontal polarisation.
There are 2 clock sources in the device, one runs at 12 MHz and the
other runs at 4 MHz. Frequencies showing up in the US lab report are atleast
the harmonics of these 2 frequencies. But frequencies mentioned in the Taiwan
lab report are looking funny (111.27, 135.3, 285.69 MHz. etc.)
US lab tested in OATS but the TAIWAN lab tested in a anechoic
chamber. I heard that chambers are not the correct way to measure ( always the
results will be wrong) that is why FCC never accepts chamber measurement. Is
it so?
Thanking you all for the time & efforts spent on this.
Thanking you all in anticipation
Geetha
"Grasso, Charles" <[email protected]> wrote:
Hi Bob,
Don't you ALWAYS maximize cables. I found that
with cable maximization my test repeatabilty
improved dramatically.
HAs cable maximization fallen out of favour?
From: [email protected]
To: [email protected]
Sent: 3/22/2005 6:58 AM
Subject: Re: Radiated Emission Testing
It is not an unusual occurance. This is one good
reason why you want a good margin under the limit.
Most people like to see a margin of at least 6dB.
One variable is the site itself. A good OATS is the
best. Sites can have up to 4dB variation from
theoretical, and one chamber I've been involved with
had a problem in the 50-60MHz range, where it was
3.9dB out -- barely acceptable. You might want to ask
to see the site attenuation data from the labs
involved.
There can ! also be a big difference depending on the
measuring distance. Some standards allow extrapolation
to be done when measuring at some distance other than
the distance spelled out in the standard, but looking
at the difference in theoretical site attenuation
between a 3m and a 10m site will show that linear
extrapolation is not accurate through much of the
frequency range.
It is not unusual to get different results at the SAME
lab, depending on the person conducting the test. Like
it or not, some technicians are better or more
diligent when it comes to maximizing the emission by
playing with cable placement. When I worked at a
compliance test lab, I ticked off a few clients when
their product was passing by 2dB until I moved the
cables and rescanned only to fail them by 1dB.
Bob Richards
Square D.
--- Geetha Balasubramanian
wrote:
> Dear Experts,
>
> One of our products had b! een tested by an
> accredited lab at US on May 2004 and found to be
> compliant with FCC, CE & VCCI. We started mass
> manufacturing the product and it is being sold in
> the US, Europe & Japan markets for the past 6
> months. Then came a requirement for that product in
> Taiwan market. Since the May 2004 testing at the US
> lab was not run in a BSMI approved system it became
> necessary for us to retest the same product. This
> time the product has been tested in an accredited
> lab in Taiwan. The Taiwan lab tests are showing that
> the product is not compliant to the CISPR22 and need
> a ferrite core in the USB cable to make it compliant
> to the standards. Product tested by the US lab &
> tested by he Taiwan lab are absolutely identical.
> As an engineer responsibled for compliancy I
> am a bit confused now. How to resolve this issue? I
> request you experts to g! uide me.
>
> Sincerely
>
> Geetha Balasubramanian
>
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