[time-nuts] Mechanical clock sound pickup circuit

[Chuck Harris]

Let's be serious here.  Radar is the wrong part of the E-M
spectrum.  Use light.  It is cheap, easy to detect, and
there are plenty of reflections to go around.

Or better still, listen to the tic.  Then you don't even
have to open the case.... Which brings this full circle.

-Chuck Harris

Jim Lux wrote:
> On 12/14/15 9:12 PM, ed breya wrote:
>> This may be totally ridiculous, but maybe there's another way to get a
>> balance wheel signal. The X-band Doppler type microwave motion detectors
>> can pick up various object signals in free air from quite a distance, so
>> maybe up close there would be enough resolution and penetration of the
>> metal parts of a timepiece to get a usable signal in and out. It would
>> tend to accentuate the fastest part of any motion - the balance wheel in
>> this case. I can picture setting one up with the horn pointed at the
>> thinnest part, likely the watch face, from maybe a few inches away - or
>> whatever it takes to not overload the detector. The audio detector
>> signal (if sufficient) could then be processed in the same way as with a
>> microphone sound signal.
>
> As it happens, I have a fair amount of recent experience detecting small (<1mm)
> motions using radar.
>
> Yes, remarkably tiny holes will let enough signal in and out, but, it's going to be
> very, very position dependent. You have a lot of multipath in this kind of testing,
> and it's easy to wind up in a null zone.
>
> You might want to look for K-band (24GHz) units: the shorter the wavelength, the more
> phase shift you get from the tiny motion.  To put some numbers on it: at 3 GHz, a 1mm
> displacement gives you about 6-7 degrees; at 24 GHz, you're going to be getting 50-60
> degrees.
>
> You'll be wanting some form of homodyne detector (which has the nice property that
> the phase noise of the source cancels out, so you can have a pretty grungy quality
> oscillator).  The signal you're looking for, though, is phase shifts occurring at a
> 1Hz kind of rate.  Most of the cheap "motion detectors" have a high pass filter  (1
> m/sec at 3 cm wavelength is 66 Hz) and the amplifier chain is AC coupled.
>
> You'll need a good low noise amplifier with a low 1/f knee.
>
> For reference, a receiver gain of about 60 dB gives you a millivolt kind of signal
> from a 1mm displacement with 1mW at 3GHz from a 0.1 square meter target at 10 meter
> distance. You can scale to your situation.
>
>
> You'll probably want some way to subtract out the static baseline, so your high gain
> amplifier stages don't need enormous dynamic range. In my radars, I do this with an
> adjustable "leakage" path from Tx to Rx.  You could probably do it with a movable
> metal target next to your clock/watch and you adjust it for a null.
>
>
> You probably also want a I/Q output: if you think about the signal you're receiving,
> it's a slowly moving vector that spans a fairly small phase angle (because it
> combines a very large static response from stuff that's moving plus a little tiny
> moving component). If that vector happens to point at 90 degrees to your I axis only,
> then you're great: the variation shows up in the I axis. But if the vector happens to
> point parallel to the I axis, the motion is very small.
>
> With I/Q, you can  either do a arctan demodulation, or you can rotate the signal to
> make the variation largest (basically using the sin x=x approximation for small x)
>
>
>
>
>
>
>>
>> Ed
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