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From: karn@epic.bellcore.com (Phil R. Karn)
Newsgroups: sci.electronics
Subject: Re: Radar detector detectors
Message-ID: <1991Feb20.135310@epic.bellcore.com>
Date: 20 Feb 91 18:53:10 GMT
References: <3078@mindlink.UUCP> <11690@crdgw1.crd.ge.com> <6277@tekred.CNA.TEK.COM>
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Organization: Packet Communications Research Group (Bellcore)
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It seems to me that there are some simple counter-counter-measures for
the radar detector detector problem.

Radar detector detectors work by sensing the stray radiation from the
first local oscillator in the radar detector. The stray radiation
leaks through the first mixer and comes out the antenna.

1. Use an RF amplifier. Amplifiers tend to be one-way devices; not
only would a good one suppress LO energy, but it would improve
receiver sensitivity.

2. Use a balanced mixer. A balanced mixer, by design, suppresses LO
energy from going out the input port. Cheap unbalanced mixers simply
combine the RF and LO signals and feed them to a nonlinear amplifier,
and there's nothing to keep the LO energy from going right out the
antenna.

3. Spread the LO before feeding it to the first mixer.  Then despread
the IF signal. Any stray LO radiation will also be spread, thus making
it much more difficult (if not impossible) to detect.

Spreading should be easy to do - use a common PN sequence generator
for both the spreading and despreading operations. This will also
require two additional balanced mixers, one each for spreading and
despreading.

Phil


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