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From: alvitar@madhat.UUCP
Newsgroups: net.auto
Subject: Re: radar detectors
Message-ID: <190@madhat.UUCP>
Date: Mon, 25-Nov-85 03:22:11 EST
Article-I.D.: madhat.190
Posted: Mon Nov 25 03:22:11 1985
Date-Received: Wed, 11-Dec-85 09:29:23 EST
References: <633@mtung.UUCP>
Organization: DataVision, Huntsville AL
Lines: 31

Note:  I  have never  designed a  speed detector,  radar detector, or
radar jammer;  therefore, this  information is  obviously second hand
and may not be correct.

[ In <633@mtung.UUCP> Jonathan Clark writes: ]
> I'll ask it again: Is the transmission from a radar gun modulated
> in any way so that the gun can distinguish the signal it sent out 
> from the signal coming back in IN THE SAME BAND?
> [...examples of extraneous noise...]

Some police radars use a heterodyne technique to  compute speed based
on the doppler  shift in  the received  signal.   The transmit signal
(Tx)  is  mixed  (beat)  with the  received signal  (Rx) resulting in
signal components which are the sum (Tx+Rx) and difference (Tx-Rx) of
the original frequencies.   Filters  are used  to reject  all but the
Tx-Rx component, which  is fed  into a  frequency counter.   Speed is
supposedly proportional to the resulting count.  

This  technique  improves  the  accuracy  of  the  gun,  but does not
necessarily  help reject  other signal  sources.   Most other sources
will differ enough from the transmitted frequency that the value from
the frequency counter will be noticeably bogus.  Again, filtering may
be  used  to  reject  wide  differences but  I suspect  that a second
interfering  police  radar  could  be  close  enough  to  generate an
incorrect yet reasonable reading (not to mention an unfair citation).

--
Live: Phil Harbison, DataVision
Mail: 3409 Grassfort Dr., Huntsville, AL 35805
Uucp: {ihnp4,clyde,sdcsvax}!akgua!madhat!alvitar
Quot: "186,000 miles/second isn't just a good idea, its the law!"  [OMNI]