________________________________
From: [email protected] [mailto:[email protected]] On Behalf Of Amund
Westin
Sent: Thursday, December 04, 2008 2:56 AM
To: [email protected]
Subject: Radiated emission - time at each frequency
When doing Radiated emission 30-1000 MHz according to EN55022, for hold
long
time do you measure at each frequency (band)?
Best regards Amund
Amund:
I'm not a commercial measurements expert, but I'll toss out my opinion.
The simplistic answer is that you should dwell at a frequency long enough for
the detector to respond to any emission that occurs. Since the charge time
constant of the CISPR detector is 1 millisecond, then 1 millisecond is the
minimum dwell time. However, you should also dwell long enough for any event
that might occur in the EUT. For instance, if the EUT cycles through an
internal function every 35 milliseconds, then you should be dwelling for at
least that long. (Who knows, the EUT may turn on something very noisy only
between 21 and 28 milliseconds into its cycle!)
You are probably using a 120 kHz resolution bandwidth, so each discrete
measurement point is including the spectrum 60 kHz below and above the
indicated frequency. Since the BW edges are defined as the 3 dB down points,
you should hop to your next measurement frequency of something like Fo + 80
kHz to allow for some overlap. Then you can start another dwell period.
There might also be some additional constraint with the decay time of the
detector. IIRC, the decay time constant of the CISPR detector is 550
milliseconds. Unless your measurement system has some kind of rapid drain of
the detector when you initiate a frequency hop, then you may have to wait at
least 550 milliseconds once you hop, but before you read your detector.
Regards,
Ed Price
[email protected] <blocked::mailto:[email protected]> WB6WSN
NARTE Certified EMC Engineer
Electromagnetic Compatibility Lab
Cubic Defense Applications
San Diego, CA USA
858-505-2780
Military & Avionics EMC Is Our Specialty
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