Opinion ID: 1710982
Heading Depth: 1
Heading Rank: 2

Heading: Calibration

Text: Three methods are available to check the calibration of the MARK VI traffic radar unit; the use of a test car, the use of a manual tuning fork, or the use of an electronic tuning fork.
A test vehicle, such as a patrol car equipped with an accurately calibrated speedometer, may be used to check the calibration of the MARK VI. The radar unit is set up on the roadway to be checked, taking care that beam angle is minimized as previously described. The test vehicle is driven through the beam at a set speed usually at the speed limit of the roadway. The operator checks the meter reading to see that it agrees with the speedometer reading of the test car.
A special tuning fork can be used to check the calibration of traffic radar. The special tuning fork is not to be confused with the forks used to tune musical instruments. The radar check fork is specially calibrated for checking a radar unit. The fork is tuned to vibrate at a frequency equal to the doppler frequency for some set speed, usually at 60 M.P.H. The speed check value will be stamped into the handle of the fork. The calibration fork is used as follows: Aim the antenna head upward away from any targets. Strike the fork on a plastic or wooden object to get it vibrating. Hold the vibrating fork in the radar beam approximately 4 in front of the radar head. Be careful not to move the fork while checking the meter reading. Check the meter reading; it should read the same speed as stamped on the fork handle. If it does not, the radar unit must be recalibrated as described in the operator manual. The use of the fork, as outlined above, modulates the radar beam so that the receiver is supplied with a signal equivalent to the normal doppler signal. The radar unit thus sees a signal equal to that coming from a vehicle traveling at a speed exactly equal to the speed stamped on the fork. When using a manual fork, care should be taken not to strike the fork hard enough to dent or damage it. A typical 60 M.P.H. manual tuning fork will have an accuracy of ± 0.2 M.P.H.
The MARK VI contains a built-in electronic tuning fork system. The heart of the system is a tiny precision tuning fork that is kept in oscillation by an electromagnetic drive coil driven by a transistor oscillator circuit. The tuning fork and drive system are hermetically sealed in a metal case to prevent humidity and barometric changes from effecting the fork frequency. A pick-up coil near the fork provides the doppler signal that is used to calibrate the radar circuit. Pressing the calibrate button turns on the electric tuning fork and applies its accurate calibration signal to the radar amplifier circuit. The frequency of the electronic fork is set for 60 M.P.H. If the meter does not read exactly 60 M.P. H., the radar unit must be calibrated as outlined in the operator's manual. The calibration signal from the electronic tuning fork is accurate to ± .2 M.P.H. at 60 M.P.H. Whenever any of the above methods are used to check calibration, a record should be made of the time, date and equipment used.