Crystal load?

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I'm ordering a 2-pin crystal (8.86723 MHz) and need to specify the load.  

A little help calculating this, please?

http://www.tinyuploads.com/images/yr8Paq.jpg

Thanks!


Re: Crystal load?
On 12.2.13 10:27 , DaveC wrote:
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I'm pretty sure that the circuit does not function
as a crystal oscillator. Is it an own invention?

For a crystal, you should decide if the circuit
needs the series or parallel resonance. There are
both, pretty near each other. For details, get
e.g. the ARRL Handbook and read about crystal
oscillator basics.

--  

Tauno Voipio


Re: Crystal load?
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I double-checked the board layout; it is as I describe.  

The circuit is part of an existing video driver board that outputs ASCII text  
to a monochrome CRT monitor. I have substituted a sine wave generator in  
place of the crystal and the board works.  

The original crystal is gone (missing when I obtained the equipment) else I  
would have looked up the numbers on the original.  

I'm not asking for evaluation of the circuit's function -- that is confirmed  
-- only to help calculate the load on the crystal.

Thanks,
Dave  


Re: Crystal load?

DaveC wrote:
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   Can you draw the circuit around the crystal and ask the crystal
company for their help?

Re: Crystal load?
On Tue, 12 Feb 2013 22:56:28 +0200, Tauno Voipio

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Yup, makes no sense, DC or AC.


--  

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
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Re: Crystal load?
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Hmm... maybe this does? ::

http://www.tinyuploads.com/images/X4cOrL.jpeg

Sorry for the crummy first attempt...

Thanks!


Re: Crystal load?

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Looks like Series, Generally CL can be ignored. It would be PCB related.  
If you specify it, you can always ad it to the circuit.
The 10pf compensates for the propagation delay in the gates.

<http://www.foxonline.com/pdfs/xtaldesignnotes.pdf
<http://www.freescale.com/files/timing_interconnect_access/doc/app_note/AN1579.pdf>

Cheers  



Re: Crystal load?
DaveC wrote:

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I see you're having difficulties in determining the load..

If you are looking for an exact figure, I think you need to use
your signal generator via 100 Ohm R for example and a scope
to measure the drop in the circuit.

   First, test the scope probe by measuring the drop after the
R only, to make sure you know the exact cap value in your probe.

   Feed the circuit with this signal via the 100 ohm R, measure the
drop. Calculate the load and remove the scope probe load from the
results.

   The net results should give you a load that is going to be close
enough.. You may want to operate the circuit for a bit before taking
final values. The logic chips are going to shift a little.

  I'm guessing you'll end up with an approximate value that equals 8 pf.

  In the capacitor manufacture world, the common practice was to zero
beat a tuned circuit with a fixed frequency. You attach a test subject
to the post which were part of this tuned circuit. You then moved the
calibrated dial which was nothing more than a capacitor, to make it
zero beat again. A scope was used with the X,Y inputs for that nice
lissajous circle or spiral curves.

  In any case, this dial would give you the exact capacitance load.

Jamie


Re: Crystal load?
Isn't load capacity for the crystal a straightforward calculation?  

http://www.tinyuploads.com/images/X4cOrL.jpeg


Re: Crystal load?
On a sunny day (Wed, 13 Feb 2013 12:15:49 -0800) it happened DaveC

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Fir * sake use a trimmer.
Those 8.8.. 2xFc crystals are not MENT to be exactly on frequency.
They are normally used in a PLL, possibly with varicap
or some other reactance, to lock to incoming color burst.

If you do not do such a genlock, and want it free running,
then you need a trimmer, zero tc components (caps),
and for sure not a 2 inverter LSTTL oscillator, but a real one,
and maybe even an oven.

JFET makes nice oscillator.


Re: Crystal load?


"DaveC"  wrote in message  

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Load capacitance applies when the crystal is operated at its parallel  
resonance.  Your circuit operates at the series resonant frequency.  Tell  
the crystal manufacturer you want it to be series-resonant at 8.86723 MHz.




Re: Crystal load?

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By load they mean the total capacitance in parallel with the crystal.
Some of this will be the obvious circuit, and the rest will be the
inherent capacitance of wiring to the crystal.

Just take a reasonable stab by quoting say 20pf, and if you need
precision, then use a trimmer capacitor across the crystal to bring it
to the required frequency.

peter

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