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From: Luke Turnbull [mailto:[email protected]]
Sent: Wednesday, March 21, 2007 7:48 AM
To: [email protected]
Subject: Radiated Emissions pre-scans for Aerospace / Military / Automotive
Standards.
Hello emc-pstc'ers,
I have an EMC question about military / aerospace / automotive emissions
standards. I hope I can get a wide / global answer to how standards 1. should
be interpreted and 2. are interpreted when performing a test.
It has been suggested that we as a test lab should perform "pre-scans" when
making emissions measurements of a product. The purpose is to ensure that any
intermittent emissions will be captured and that it should involve the use of
max hold on a spectrum analyser with fast sweeps to ensure all frequencies are
revisited many times a second for at least 20 seconds in each span.
For each of: DO-160, MIL STD 461, CISPR 25
1. Do people believe the standard requires such a pre-scan?
2. Are test labs worldwide actually performing such a pre-scan?
Thanks for your help,
Luke Turnbull
Let me talk about MIL-STD-461E. (I would apply the same logic to DO-160, since
military mission success and safe commercial operation are fairly similar.)
Paragraph 4.3.9 of MIL-STD-461E says:
OPERATION OF EUT: During emission measurements, the EUT shall be placed in an
operating mode which produces maximum emissions. During susceptibility
testing, the EUT shall be placed in its most susceptible operating mode. For
EUTs with several available modes (including software controlled operational
modes), a sufficient number of modes shall be tested for emissions and
susceptibility such that all circuitry is evaluated. The rationale for modes
selected shall be included in the EMITP.
There is no mention of a "pre-scan", so I don't believe that it would be
required. But it does say, in effect, that you should intimately know how the
EUT works. Every mode of operation needs to be considered; sometimes an
experienced analysis can eliminate duplicative modes of operation. Sometimes,
you just don't know what will be the worst case, so you need to investigate
the EUT to find out. You might call that a pre-scan.
Knowledge of the EUT should extend to short duration events, or to unusual
modulation characteristics. For instance, one of our products is an aircrew
personal locator transponder beacon; just the kind of thing you need if you're
hiding under a cow in Bosnia while waiting for the good guys to find you. A
device like this might compress data and then send it as a burst of a few
milliseconds, but only once every few seconds. And it might only do that when
specifically interrogated. The "on" time is short; less than 0.1% duty cycle
(and even that is a digital frame of words of bits).
If you do only one measurement scan, at the Paragraph 4.3.10.3.1 Table II
speed limits, you will probably miss everything. Some things you can do are
make multiple scans using a digital peak-hold, scan slowly, and possibly speed
up the operating rate of the EUT. Whatever you do, it's still going to be
time-intensive. I can't tell you how many seconds are needed for your product;
determining a decent "probability of intercept" is central to your test
planning. I can say that I have run RE102 data acquisitions that lasted 15
hours or so (and that was just one of the frequency ranges).
You can get you knowledge of your EUT from a combination "pre-scans", previous
experience with similar products, and thorough engineering analysis.
I can't say if all labs work this way, but here's what I put in my formal EMC
Test Procedures:
The measurement receiver bandwidths and the minimum measurement times listed
in Table II of Paragraph 4.3.10.3.1 of MIL-STD-461E will be used for emission
testing. Specialty emissions (frequency regions identified during automated
testing which require manual investigation) may be investigated at slower
rates as desired.
The underlying rationale for all swept frequency emission testing is to allow
the measurement detector to charge to the full peak of the observed signal. To
ensure this condition, consideration must be given to the inter-related
factors of detector time constant, detector bandwidth, tuning and settling
time, post detector filtering and probability of signal intercept. Typical
measurement times used by CDA are much slower than these allowable maximums.
I am biased toward finding out all I can about the EUT. Compliance with 461 is
like buying insurance, and nobody likes spending money on that. OTOH, there's
a large body of experience (supported by Murphy) that says what you miss at
the 461 level will eventually bite you at the system integration or
operational level.
Regards,
Ed Price
<mailto:[email protected]> [email protected]
NARTE Certified EMC Engineer & Technician
Electromagnetic Compatibility Lab
Cubic Defense Applications
San Diego, CA USA
858-505-2780 (Voice)
858-505-1583 (FAX)
Military & Avionics EMC Is Our Specialty
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