Hi,
I once had equipment that did processing of analogue measurements.
Nobody explained what is was doing and so I seemed to get random failures.
The measurement technique was to set the levels, trigger the equipment and
check the result, then move to the next frequency.
A failure would occur but when retested, the failure would not be there.
I eventually modified the frequency step size and also took the measurement 20
times at each frequency. The response was that there were (at first) no
failures and then one in twenty measurements - then four in twenty until I hit
the jackpot with twenty failures in twenty measurements - then the response
tailed off again.
I turned out that the processing was taking a rolling average of 10 readings.
The equipment had some narrow - band susceptibilities but with only taking one
measurement the transmit frequency had to match the susceptibility exactly
otherwise the induced error would be averaged out to become a pass!
I never discovered if the averaging had been introduced deliberately to get a
pass, or if it was part of the normal operation. However the project guys were
not amused when the discovered I had found many susceptible frequencies....
Other possible causes -
* rate of change of frequency during a step
* switching power up/down in steps at a critical point in the UUT process
* Duration of exposure - while sweeping the RF is on for say 10 minutes -
the
UUT fails - you remove power, the UUT recovers - the repeat tests last 1
minute and you don't see the failure... (capacitor charging up, component
heating up)
I am sure that there will be more explanations. If you find out - will you
post the answer?
Tim Haynes
Electromagnetic Engineering Specialist
SELEX Sensors and Airborne Systems
Tel: +44 (0) 1582 886239
Fax: +44 (0) 1582 795871
Mobile:
e-mail: tim.hay...@selex-sas.com
Homepage www.selex-sas.com
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