The tuning fork calibration method is the only one I had heard about.
However, the tuning fork calibration method does not work by doppler shift (a shift in frequency due to the velocity of an object). While the frequency of the tuning fork is very stable, the amplitude of the oscillation (velocity) is not (and is constantly decaying), thus doppler shift would be unreliable. The tuning fork work by shifting the phase of the reflected signal at the vibration rate of the tuning fork. In a classic radar gun, the reflected signal mixes with the outgoing signal in a diode at the third port of a circulator (first port is the radar gun oscillator, the second port is the antenna). I don't know if modern radar guns use superheterodyne receivers with a seperate L.O. and IF amplifier. If the tuning fork method works, there is another more controllable (and continuously operating) method that should work: Point the radar gun at an antenna designed to operate at the frequency of the radar gun. Attach a reflective phase modulator to the feed point of the antenna. The phase modulator is a piece of transmission line that has one end connected to the antenna, and the other end shorted at RF (a capacitor). Place a low capacitance diode 1/8 electrical wavelength from the shorted end. Feed an audio signal across the RF short circuit capacitor to provide current to turn the diode on and off. This will change the electrical length that the reflected RF travels by 1/4 wavelength or 90 degrees, which is the phase shift that will maximize the demodulated signal. The audio signal frequency to be applied would be equal to the doppler shift frequency of the velocity. This phase modulator would be most easily constructed in microstrip. Don Borowski Schweitzer Engineering Labs Pullman, WA, USA "Price, Ed" <[email protected]>@majordomo.ieee.org on 03/27/2003 07:45:36 PM Please respond to "Price, Ed" <[email protected]> Sent by: [email protected] To: 'Hjálmar Árnason' <[email protected]>, [email protected] cc: Subject: RE: Calibrating police radar guns >-----Original Message----- >From: Hjálmar Árnason [mailto:[email protected]] >Sent: Thursday, March 27, 2003 3:14 PM >To: [email protected] >Subject: Calibrating police radar guns > > > >Hi Forum > >I can recall back in November 2000 there was some discussions here in >this group on how to fight speeding tickets and many of you had good >advices. > >I'm about to start on a project which includes calibrating and >repairing >police radar guns. This will probably involve setting up a >semi-anechoic >chamber or OATS. I have access to a room which can be used to set-up >a chamber and want to restrict the set-up to the radar freq. >around 25 and >35 GHz. > >I would appreciate if you could give some advice and direct me to the >right websites to get information. I need both test equipment >and material >for the chamber. The budget is low so second hand equipment is my goal. >Anyone selling his set-up ?. > >Thank you kindly, > >Hjalmar Arnason >Reykjavik >Iceland >[email protected] > Hjalmar: The only calibration performed on police radar guns that I am aware of is verification of the Doppler shift response. This is done by whacking a factory-supplied tuning fork, and holding the tines in the radar beam. The vibrating tines reflect a Doppler-shifted signal, and the gun display counter is adjusted to read the appropriate speed. For the US market, the only tuning forks I have ever seen were designated for 55 MPH. To independently verify the calibration, you could measure the audio frequency of the tuning fork and relate that to peak velocity of the tuning fork tine. (I'm not certain, but, IIRC, the velocity of the tine should also be 55 MPH. Maybe we should dig out our high-school Physics book.) AFAIK, no measurements are made on such interesting things as RF power output, beam width, range, multipath rejection or rejection of undesired signals. Further, I don't know about Iceland, but in the USA, I would want "malpractice" insurance if I was "certifying" the performance of devices that regularly served as trial evidence. Who would want to enter a market like that? Regards, Ed Ed Price [email protected] Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA USA 858-505-2780 (Voice) 858-505-1583 (Fax) Military & Avionics EMC Is Our Specialty Shake-Bake-Shock - Metrology - Reliability Analysis This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
