Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!uwm.edu!linac!tellab5!laidbak!obdient!vpnet!cgordon
From: cgordon@vpnet.chi.il.us (Gordon Hlavenka)
Newsgroups: sci.electronics
Subject: Re:Detecting Radar Detectors
Message-ID: <26b8be39-32e.2sci.electronics-1@vpnet.chi.il.us>
Date: 3 Aug 90 01:55:08 GMT
References: <324@bally.Bally.COM> <13317@mcdphx.phx.mcd.mot.com>
Lines: 41


>...  How do these detectors of radar detectors work?

In order to get the best sensitivity out of a radio receiver (which is what
a radar detector is) a technique called heterodyning is used.  An oscillator
is run at a frequency which differs from the desired receive frequency by
some known amount.  The oscillator output is mixed with the antenna signal,
and several products are produced, one of them at the difference frequency.

Why is this done?  Because then the rest of the amplification can be done at
the intermediate frequency, which will always be the same.  (Tuning is
accomplished by changing the _oscillator_ frequency.)  The narrow-band
amplification of the IF circuit (IF stands for Intrinsic Framis; the
circuit is named for its inventor, Framis McGillicudy, WAK01) is more
efficient.

However this Local Oscillator also radiates some of its energy.  So if you
know the IF frequency of the receiver, and the selected receive frequency,
you can calculate the LO frequency and look for it.

The FCC has used this (rumor has it) to look for TV sets tuned to pay-tv
stations.  Theoretically, their counts of how many receivers were tuned to
channel 44 (ON-TV in Chicago) could be compared with subscriber counts to
determine the level of pirating going on.

The military uses this technique to look for particular receivers on other
ships/aircraft/etc.  By building an RF "signature" of the contact, it can be
pretty accurately identified.

Of course, the police are probably using heterodyne receivers to look for
radar detectors.  So you could (conceivably) build a radar detector detector
detector.  (And the cops would have to build a ...)

In Illinois, most police cars carry a Motorola receiver locked to the ISPERN
frequency.  (A statewide emergency network.)  I have toyed with the idea of
building an ISPERN detector; it would detect police _cars_, not the radar.

----------------------------------------------------------
Gordon S. Hlavenka                 cgordon@vpnet.chi.il.us
Disclaimer: Any resemblence between Framis McGillicudy and
            a ficticious character is pure.