[time-nuts] Quartz Crystal Motional Movement

[Bob Camp]

Hi

The key trick that is often missed while watching it done is the need to set it (tune it) to operate at the
frequency you are looking for. In other words, if you are testing a 4 MHz crystal, you need to 
first tune the machine to 4 MHz. If you truly have a bag full of “mystery parts” that’s not an
easy thing to do :) The same thing applies to a whole raft of very similar machines. The 
reasoning is the same in all cases: You take out the effective load impedance of the device
by tuning it to “series” first.  In reality any oscillator you can tune up this way can be used
to get the motional capacitance. 

The second trick is the need to calibrate the resistance reading on the device. That may seem
easy, but it’s not quite as simple as you might think. Various resistors have some odd characteristics
even at 40 MHz. Carbon comp’s are truly weird. A good thin film resistor is the right thing to use.
Good luck finding one on most benches….

Finally, the motional stuff is done by popping a “load cap” in series with the crystal. The net result
is only as good as the accuracy of the setting on that cap. In most factories an inventory of “standard
crystals” is stored in some dark and dusty cabinet. When the meter comes in, the calibration is done such that
those crystals give the correct results. It may all be wrong (who knows …) but at least the factory
is producing stuff this week that matches up with what they made last week.

I’m sure I’ve forgotten at least ten other quirks about the device …. That’s why including a summary
of how it was used would be a really neat thing. 

Bob

> On Jun 25, 2016, at 9:33 PM, Tom Van Baak <tvb at LeapSecond.com> wrote:
> 
>> Hi
>> 
>> Might be a little more fun if it  showed the proper way to use the 150B (or any similar device). 
>> 
>> Bob
> 
> Hi Bob,
> 
> Can you tell those of us with those cool Saunders 150B crystal impedance meters how to do it right?
> 
> /tvb
> 
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