Fascinating Quartz Crystal history! (Required reading)

Discussion in 'General Electronics Chat' started by KL7AJ, Jul 20, 2009.

  1. KL7AJ

    Thread Starter Senior Member

    Nov 4, 2008
    http://ericnichols.net/Crystal History.htm

    Not many people know just how tenuous our supply of quartz was during WW2...and how crucial that was to the outcome of the war!

    Required reading for every electronics tech and engineer.

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    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.
  3. RiJoRI

    Well-Known Member

    Aug 15, 2007
    "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!

  4. KL7AJ

    Thread Starter Senior Member

    Nov 4, 2008

    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!)

  5. KL7AJ

    Thread Starter Senior Member

    Nov 4, 2008
    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
  6. bertus


    Apr 5, 2008

    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.

  7. Mass


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


    Mar 24, 2008
    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.