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From: jgk@osc.COM (Joe Keane)
Newsgroups: sci.electronics
Subject: Re: POLICE hand-held RADAR units
Summary: Ratio is relative to speed of light.
Message-ID: <4518@osc.COM>
Date: 15 Feb 91 20:34:26 GMT
References: <1991Feb12.035201.16098@nntp-server.caltech.edu> <2470014@hp-vcd.HP.COM> <1090.27b96a06@lrc.uucp> <1991Feb14.015812.14576@nntp-server.caltech.edu> <3498@casbah.acns.nwu.edu> <37578@netnews.upenn.edu> <12257@as0c.sei.cmu.edu> <1810@ole.UUCP>
Reply-To: jgk@osc.COM (Joe Keane)
Organization: Versant Object Technology, Menlo Park, CA
Lines: 22

The original poster is correct about comparing your speed to the speed of
light.  The ratio between the beat frequency and the original frequency gives
your speed relative to the speed of light.  Actually, the ratio is twice your
speed, but this obviously isn't important.

However, i don't agree with the statement about requiring parts per billion
accuracy.  The beat frequency is typically about 10^7 times lower than the
radar frequency.  But this doesn't matter, if you know both frequencies to
parts per thousand, then you know the ratio to parts per thousand too.

Note that we don't need to know either frequency, just the ratio.  Measuring
the ratio between frequencies can be done extremely accurately.  For example,
you can clock a counter at the radar frequency, or more likely some oscillator
which is multiplied to give the radar frequency.  By counting the number of
cycles between zero-crossings of beat frequency, we get the inverted ratio to
the closest whole number.  With this scheme it doesn't matter if the radar
frequency is off.  The ratio, and thus your speed, are still right.

So i don't think there are any theoretical limitations on accuracy.  What's
more important are the practical problems: interference by random signals,
components going out of spec due to heat or humidity.  These are the things
you should concentrate on if you want to beat the ticket.