I investigated police radar stuff a long time ago, and for a while had
an old X band unit shaped just like a searchlight, with analog meter.
What I learned then was that even on the newer units, the tuning fork
was specified to provide an independent means to verify the accuracy of
the unit in the field.
I also remember hearing that the radar sales reps had a special gift
they would hand out to friends: a tuning fork marked "60 MPH" but that
rang at a lower frequency, so you could use it to "calibrate" the unit
to read higher than actual. I heard that it was popular among the
speed-trap crowd.
On 04/12/2017 09:25 AM, jimlux wrote:
On 4/11/17 11:09 AM, Mark Sims wrote:
Apparently fluorescent tubes continuously emit a lot of other
microwave signals. I once built a homodyne doppler "speed" radar kit
(used a coffee can for the antenna). The way you calibrated it was to
point it at a florescent tube and and adjust the reading to a specific
value.
--
That's not because the tube is emitting.. It's a target reflector
turning on and off at twice line frequency.
In most homodyne radars, you filter out the DC (the reflections from
stuff that's not moving), so anything that pulses on and off creates
nice output.
At 10.525 GHz, the Doppler is about 70 Hz/ (m/sec), 31 Hz/(mi/hr)
at 24GHz, 160 and 71 Hz, respectively
Most simple speed guns just have an audio frequency counter on the
output of the mixer diode(s). Older units use a Gunn oscillator, newer
ones use a DRO. Some have a pair of detectors so you can distinguish
motion towards and away.
The old "calibrate with a tuning fork" for police radar wasn't
calibrating the RF frequency (a 1000 ppm change of the gunn oscillator
isn't a big deal.. this is a "3%" kind of measurement) - it was
calibrating the audio frequency counter, which, in very early units,
used an RC timebase. (or an actual analog meter reading) I cannot
imagine a crystal oscillator bad enough that a tuning fork would be
better. - if XO based speed guns were checked with a tuning fork it's
for one of two reasons:
1) the purchasing requirement said "A tuning fork for calibration shall
be provided" (based on an older design)
2) it provides a "functional test" and you don't really care what the
frequency is, as long as it lights up anything reasonable
Homodyne/Doppler radars are fun
(http://home.earthlink.net/~w6rmk/radar10g.htm) and can form the basis
of a business that saves lives
(https://www.nasa.gov/jpl/finder-search-and-rescue-technology-helped-save-lives-in-nepal)
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