________________________________
From: [email protected] [mailto:[email protected]] On Behalf Of John
McAuley
Sent: Monday, September 29, 2008 2:18 PM
To: [email protected]
Subject: Immunity of commercial equipment to radar at frequencies over
5 GHz
Dear Group
I am seeking information on the immunity of commercial equipment to
radar at
frequencies over 5 GHz. The field strength will be in excess of 300 V/m.
There is little information available on the immunity of commercial
equipment
to radar pulses at these high frequencies and levels. My guess is that
equipment will be relatively immune given the bandwidth of IC technology. The
rule of thumb for the bandwidth of IC technology is 10 or 20 times the
bandwidth. There may, of course, be non linear out of bandwidth responses.
From a search on the web it appears that PCs will not exceed 4 GHz for
some
time based on the current state of the art 45 µm and 65 µm technology. What
is the upper bandwidth of a 3 GHz PC? I would also assume that the high
frequency clock is contained within the processor and would be relatively well
protected?
I would be interested to hear from military test labs that have
experience of
testing immunity of equipment to pulsed fields up to 18 GHz. How frequently do
conventional circuits fail high level EMI testing?
Many thanks for any information provided.
Best regards
John McAuley
Compliance Engineering Ireland Ltd
John:
Don't count on the semiconductors in your commercial equipment being
non-responsive to GHz range energy. A susceptible response might be elicited
just by the application of the RF, but more often, the RF couples into your
device through apertures or other paths, and then non-linear circuit elements
"demodulate" the RF, allowing the modulation rate to create the susceptible
response.
Paragraph A.4.3.10.4.2 of MIL-STD-461F says it more thoughtfully:
Modulation is usually the effect that degrades EUT performance. The
wavelengths of the RF signal cause efficient coupling to electrical cables and
through apertures (at higher frequencies). Non-linearities in the circuit
elements detect the modulation on the carrier. The circuits may then respond
to the modulation depending upon detected levels, circuit bandpass
characteristics, and processing features.
Pulse modulation at a 1 kHz rate, 50% duty cycle, (alternately termed 1
kHz
square wave modulation) is specified for several reasons. One kHz is within
the bandpass of most analog circuits such as audio or video. The fast rise and
fall times of the pulse causes the signal to have significant harmonic content
high in frequency and can be detrimental to digital circuits. Response of
electronics has been associated with energy present and a square wave results
in high average power. The modulation encompasses many signal modulations
encountered in actual use. The square wave is a severe form of amplitude
modulation used in communications and broadcasting. It also is a high duty
cycle form of pulse modulation representative of radars.
There's a lot more after that, but to paraphrase it a bit, the RF is the
vehicle and the modulation is the payload. I have tested a number of systems
where the EUT is rock solid in the presence of CW, but goes nuts when you turn
on the modulation.
Let's add another fun effect; FM demodulation. I have seen a couple of EUT's
where the sweep rate of the illuminating signal was the culprit. Sweep the RF
frequency too fast or too slow, and the EUT is OK. Sweep at a moderate rate,
and the EUT responds!
Clearly, you can't test for every possible modulation and sweep rate, so
following the guidelines of MIL-STD-461 by using a 1 kHz & 1 Hz square wave
modulation and sweep speeds compliant with Table III yields a conservative and
prudent test regime.
Getting back to your original question, I would be pessimistic about the
chances of typical commercial equipment operating in the presence of 300 V/M,
1 kHz 50% modulation 5 GHz energy. I would expect susceptible responses, and I
would not be surprised to encounter destructive effects too.
Ed Price
[email protected] <blocked::mailto:[email protected]> WB6WSN
NARTE Certified Lab Rat
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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