[Milsurplus] big crystals

[Bob Camp]

Hi

The 100Kc calibrator crystals are a good example of making the  
resonator long in only one axis and very skinny in the other two. It  
works, but you give up a lot of performance doing so.

The resistance of those calibrator crystals is way high. Q should go  
up as frequency goes down. You can get about 2 to 2.5 million Q at 5  
MHz. That should translate to something over 100 million at 100 Kc if  
you didn't trade anything off. That also assumes that the Q*F law  
holds *way* past the point it falls apart. Even if you take a simple  
5 MHz crystal with a Q of 200K (air) to 500K (vacuum) as a starting  
point, the  <100K Q of the calibrator crystals is a step in the wrong  
direction.

When you start down to things like 8Kc tone bank crystals you see  
some *really* long thin pieces of quartz in tall vacuum tube  
enclosures. They are slow enough that you can watch them flex with a  
really good strobe light.

The crystal industry really isn't all that old. The number of  
crystals made commercially before 1939 was a tiny amount. WWII caused  
the industry to explode. The technology in 1945 was light years ahead  
of pre-war state of the art. Of course at some not to distant point  
the US crystal industry will be back to it's 1938 size, but that's a  
different story ....

Bob




On Oct 8, 2007, at 12:40 PM, Unserviceable but Repairable wrote:

> hey Bob, how do you 'splain the tall hc6u 100kc calibrator xtals
> found is so much 50s hamjobs?
>
> Think answer is vibration mode of quartz... & fm there I'm lost
>