[time-nuts] Re. DIY atomic "resonator"

John Ackermann N8UR jra at febo.com
Wed Apr 12 10:46:34 EDT 2017

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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