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Re: Crystal question



on 5/6/03 2:12 PM, William Leijenaar at pe1rah@hotmail.com wrote:

> I have tried this also with some crystals I have here, but I was not
> succesfull.
> 
> Maybe someone knows how to change a crystal frequency without distroying it
> ????

It's very difficult to do.  As others have said, the crystal blanks are cut
at a very precise angle in the quartz.  Depending on the type of crystal you
want you do AT cuts, SC cuts, etc.   If you take a piece of quartz, you can
cut through it in various angles, not just straight up or down.  Different
cuts give you different performance parameters such as stability, temp
tolerance, etc.

The thickness of a crystal primarily determines its frequency.  It's a
vibrational type of device.  The thicker the xtal the lower the frequency.
The crystals are lapped to their proper thickness.

The electrode on a crystal is not etched.  Rather it is sputtered on using a
vacuum sputtering machine (etching is a removal process).  Now as the
electrode is added, it decreases the frequency so crystal blanks are always
cut higher than the final frequency.  The size of the electrode then almost
determines the final frequency.

I say almost because then you have to add the holders and possibly some
solder to hold everything in place.  Those do slightly affect the crystal
frequency.

No where etching could come in would be to remove a small amount of the
electrode to raise the frequency slightly.  Now I doubt you have etching
equipment.  You could try ever so slightly removing some of the electrode
with an Exacto knife or very fine sand paper or emery cloth.  But you have
to be real careful.  It's not an exact science.

To lower the frequency, you can add a small amount of material to the edges
of the electrodes such as solder.

Of course, this is assuming you could get the cover off the crystal.  Most
are welded or soldered these days.  Additionally, most crystals today are
manufactured in a clean room and the atmosphere is evacuated out of the
crystal.  Proper crystal stability requires next to zero impurities.

I would not recommend doing any modifications to crystals except for in the
case where you just want to learn about them.  I would expect any equipment
performance to degrade and possibly become out of spec.  It's just too
difficult to do it all.

A crystal is just one part of the tank circuit in an oscillator.  Crystals
can be "pulled" with a capacitor either across the terminals or to ground on
the output.  It depends on how you are using it as either series or parallel
resonant.  Anyhow, you can pull a crystal a few PPM with a cap or a varactor
diode.  So I would do that.  You could also change some inductance in the
tank circuit and shift your oscillator frequency that way as well.  It's
just too hit or miss with trying to shift the crystal frequency.  Finally,
crystals themselves are rather cheap so it's not that costly to just get a
whole bunch with different values and play with them.

I hope this helps.

73,

Jon
NA9D

-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)

Citizen of the People's Democratic Republik of Illinois

Life Member: ARRL, NRA
Member:  AMSAT, DXCC

http://www.qsl.net/na9d   <- Updated on 1/22/03!!!

"A life lived in fear is a life half lived."


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