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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