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

[jimlux]

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)