Some years ago, I got tired of having to deal with "unable to reproduce"
measurements, and acting on a hunch, I put two 8447D preamps in series,
and hooked them up to a new acquausition, a 6 GHz oscilloscope.
Connecting this to an antenna inside the SAC, I was overjoyed to see a
waveform that I could ascribe to a particular circuit.
Yes, the old HP 8000-series spectrum analyzer stacks (et al) were
wonderful instruments; compared to the 141T with plug-ins that I bought
for a few hundred dollars some time ago, they were (with the right
software) miraculous time savers in their own way -- but scores of
otherwise knowledgeable EMC engineers have still written up test reports
"with unable to reproduce".
During a recent EMC chapter meeting out here in West Michigan, I
suggested that one could obtain an approximation of that kind of
information by connecting a broadband oscilloscope to a high-frequency
IF port of a conventional spectrum analyzer. This might still be a good
idea for those of us whose employers or customers are unable or
unwilling to buy a more modern FFT based instrument.
Or those of us who, for reason of the reduced income available in
retirement, must make do with older analyzers on our own test benches.
Cortland Richmond
On 8/14/2017 10:29 AM, Ken Javor wrote:
Another benefit of Time Domain EMI receivers Not a question to the
group, a fable – a story with a lesson learned.
Was at the IEEE EMC Show south of DC this past week, and stuck around
to attend a Friday afternoon session (!) One of the presentations
concerned a test in which I had played a part, and it reminded me of
something interesting that happened there. A lot of you will not have
attended, and of those of you who did, perhaps not all had the
fortitude to stick it out until the final hours. And, I had made a
mental note to disseminate this information post-test, and then
promptly forgot, as is my wont these days.
There was a spacecraft integration test where they decided for
budget/schedule/risk reduction reasons not to move it into a SAC for
the EMC portion, but leave it in a high bay, clean room facility where
the balance of the testing occurred. The entire facility is just a
few miles from a major airport, and it being an industrial plant,
there are many mobile radios in use, and those sorts of intermittent
transmissions being the most difficult to pin down as ambients, we
decided to do an rf survey of the clean room facility and determined
that we needed an rf tent to meet our ambient requirements.
We were using one of these newer EMI receivers which had the
capability to look at large portions of the spectrum at once, as
opposed to tuning and dwelling frequency-by-frequency. Using that
capability, we could look at the entire launch vehicle
command-destruct band (uhf, a 60 MHz wide band just above 400 MHz) and
also, separately, over the entire required spectrum at S-band. S-band
had to be monitored to ensure payload transmitter compatibility with
some launch site communication links operating at close to the same
frequency. The command-destruct band was monitored to ensure that
unintentional emissions from the spacecraft as a payload did not
interfere with reception of that emergency command, in the event the
launch had to be terminated after lift-off.
The first rf test was ensuring that the spacecraft didn’t emit
excessively in the command-destruct band. When the tent was up, we
noticed that from time to time the entire noise floor jumped up above
our limit, and then settled down. Some of the less experienced
engineers took this to be an intentional radio transmission, but as we
were looking at a 60 MHz + wide spectrum, this clearly wasn't the
answer. It had to be a broadband event, and it turned out to be
either people brushing against the tent and depositing charge which
then flowed all over, and/or the ventilation in the facility blowing
over the tent and causing the material to bow and ripple like a sail,
with the same undesirable ESD end result. We dealt with these
problems by tying down that which could be tied down to avoid flapping
in the breeze, and cordoning off the tent and placing “rf test in
progress” signs around the periphery. People being people, even
trained engineers and technicians, they completely ignored the roped
off area and signs, so that in addition to the restricted zoning,
after several violations and required retests, I assumed the
responsibility for guarding the perimeter, doing so with much the same
fervor as a junkyard dog, and just about as mean by the time it became
obvious that all other enforcement had failed.
But all that aside, the point of the story is that with a traditional
frequency sweep, these accidental discharges would have occurred at
random frequencies to which the EMI receiver just happened to be tuned
when the ESD event occurred, and it would have been very difficult to
discern that these were in fact broadband events as opposed to random
keying of transmitters around the plant or at the airport, i.e.,
attempting to discriminate between pulsed cw and impulsive signals.
This is illustrated graphically in the MIL-STD-461G Table II
supporting appendix material dealing with the proper use of such EMI
receivers. The ability to observe a very wide portion of spectrum
jumping up and then receding was a clear signal to the experienced
observer of what was happening, and time domain capability, in
addition to greatly speeding the test along, also allowed immediate
interpretation of the clue, and then eventually, after solving the
people problem by making use of the old (junkyard) dog EMC engineer,
the entire problem was put to rest.
Ken Javor
Phone: (256) 650-5261
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