Fascinating Quartz Crystal history! (Required reading)

beenthere

Joined Apr 20, 2004
15,808
The thought of tweaking slabs of quartz to resonate right on freq - especially given the state of test equipment available then - is mind-boggling. Nobody got to just toss an off-frequency crystal.
 

RiJoRI

Joined Aug 15, 2007
536
"The crystal units of the day were either X- or Y-cut plates mounted loosely between two metal plates which served as electrodes."

That explains the schematic symbol for crystals! I had wondered how they got that "box-in-a-capacitor" shape!

--Rich
 

Thread Starter

KL7AJ

Joined Nov 4, 2008
2,208
"The crystal units of the day were either X- or Y-cut plates mounted loosely between two metal plates which served as electrodes."

That explains the schematic symbol for crystals! I had wondered how they got that "box-in-a-capacitor" shape!

--Rich

Yipper, Skipper! If you ever get a chance to disassemble an FT-243 crystal (any old geezer ham has a drawer full of these) you will notice that the two plates have VERY slightly raised corners, perhaps .001 inch or less...just enough to feel. This allows the body of the crystal to "swing" between the suspension points. This is the normal "series" mode. There are also other vibrational modes such as thickness "modulation".
Since quartz is a good insulator in its normal state, you can't just poke a couple of wires into its side and energize it (though you CAN extract high voltage from it this way, if the "rock" is shock excited (hammered). So you need to couple the energy into and out of the rock with capacitance. This capacitor has some by-products, it creates an additional parallel resonant mode, which isn't normally a problem if you know about it. In fact, it can be used to great benefit in SSB filters and such.

Fascinating, wide ranging topic! (A lot more interesting than semiconductor silicon, in my opinion!)

eric
 

Thread Starter

KL7AJ

Joined Nov 4, 2008
2,208
The thought of tweaking slabs of quartz to resonate right on freq - especially given the state of test equipment available then - is mind-boggling. Nobody got to just toss an off-frequency crystal.
Spent a lot of my early ham years grinding crystals. Used a plate of glass and some MacCleans toothpaste! Imagine what that musta done to your teeth if you could grind quartz with it! :D
 

bertus

Joined Apr 5, 2008
19,944
Hello,

I also used to grind those crystals.
I used a stuff called JIF for it.
You could also change the frequency by writing on the crystal with a permanent marker.

Greetings,
Bertus
 

Mass

Joined Apr 9, 2009
26
I've heard stories about guys blowing cigarette smoke into an xtal enclosure to change the frequency too! Probably a few ppm at most.
 

Wendy

Joined Mar 24, 2008
21,840
You know, I'd love to cut a crystal myself sometime. I don't mean to earn a living or anything, just to get a taste of the tech. I'm somewhat of a hands on sort, and I think making an oscillator from the ground up (no pun intended, but I'll take it anyhow) would be interesting.
 
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