Crystal load?

I'm ordering a 2-pin crystal (8.86723 MHz) and need to specify the load.
A little help calculating this, please?
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Thanks!
Reply to
DaveC
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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.
Reply to
Tauno Voipio
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
Reply to
DaveC
Can you draw the circuit around the crystal and ask the crystal company for their help?
Reply to
Michael A. Terrell
Yup, makes no sense, DC or AC.
Reply to
John Larkin
Hmm... maybe this does? ::
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Sorry for the crummy first attempt...
Thanks!
Reply to
DaveC
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Reply to
Joe Chisolm
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.

Cheers
Reply to
Martin Riddle
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
Reply to
Jamie
The request was to help by looking at the circuit and provide a load value.
How to Google? I've done that. The search results are too non-specific to risk ordering a $50 crystal on.
Reply to
DaveC
Fox will make you an oscillator....
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Somebody else does this, too... Silabs maybe?
These are synthesized, programmed with internal eeprom or something.
Reply to
John Larkin
The OP is looking for a crystal to go into an existing oscillator.
Reply to
Tim Wescott
Why not hack in an oscillator? It would be dead-on frequency, without buying a custom-ground $80 crystal that may not work in that circuit.
Reply to
John Larkin
If I can find a 4-pin DIP oscillator at 8.86723 M I'd be tempted. But don't see such an animal...
Reply to
DaveC
Isn't load capacity for the crystal a straightforward calculation?
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Reply to
DaveC
On a sunny day (Wed, 13 Feb 2013 12:15:49 -0800) it happened DaveC wrote in :
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.
Reply to
Jan Panteltje
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
Reply to
Peter
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.
Reply to
Andrew Holme
I haven't looked at standard frequencies, but you might have better luck finding that frequency if you drop a significant digit or two. It is unlikely the original part was specified to 1 ppm, 100 ppm would likely do the job just fine. Didn't you say this was for a display? It will probably work fine with just five digits or even four digits of frequency specified. Is either 8.867 MHz or 8.868 MHz a common value perhaps? Really anything near 8.87 MHz should do the job.
Reply to
rickman
I just looked at digikey, they list 17.734475MHz and 35.46895MHz oscillators, but expensive and not in stock
-Lasse
Reply to
langwadt

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