Crystal load?

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!

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.

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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

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

On Tue, 12 Feb 2013 22:56:28 +0200, Tauno Voipio
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Yup, makes no sense, DC or AC.

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Hmm... maybe this does? ::
http://www.tinyuploads.com/images/X4cOrL.jpeg
Sorry for the crummy first attempt...
Thanks!

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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

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

On Tue, 12 Feb 2013 12:27:56 -0800, DaveC wrote:
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http://lmgtfy.com/?qtls04+oscillator+crystal

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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.

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Fox will make you an oscillator....
http://www.foxonline.com/xpressomain.htm
Somebody else does this, too... Silabs maybe?
These are synthesized, programmed with internal eeprom or something.

On Tue, 12 Feb 2013 21:37:38 -0800, John Larkin wrote:
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The OP is looking for a crystal to go into an existing oscillator.

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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.

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a
If I can find a 4-pin DIP oscillator at 8.86723 M I'd be tempted. But don't see such an animal...

On 2/13/2013 2:38 PM, DaveC wrote:
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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.

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buying
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I just looked at digikey, they list 17.734475MHz and 35.46895MHz oscillators, but expensive and not in stock
-Lasse

rickman wrote:
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Not if it is a video clock, to a fixed frequency monitor.

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I learned a little more about this circuit.
It's a proprietary video card card from a piece of German offset printing equipment.
I used a sig. gen. in place of the crystal. The circuit generated a video signal without any data (just sync pulses). The video card is separate from the system processor, so being on the bench the card has no data to display.
Viewed on a scope, varying the 8.867 MHz frequency doesn't change the video signal at all.
I'm guessing (with my limited understanding of how video works) that the 8.867 runs the dot clock, basically how fast the dots are shoved out of the data bus into the video generator IC.
So, how critical *is* this frequency?
Thanks, Dave (not an EE)

On a sunny day (Wed, 13 Feb 2013 19:33:37 -0800) it happened DaveC
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Likely not, that 8.8 is 2x Fc and used for PAL color generation. You need a much higher dot clock for video, I used 18MHz in my video card design, for 15625 lines and 80 characters per line. If it only does 40 characters / line then it COULD be 8.8 MHz.
Simple check: Measure horizontal frequency from that card, it should be 15625 Hz. If it changes when you change the 8.8MHz, then it is the dot clock. Else it is only for color.
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See above. If it is for color it needs to be 2x443... you know.
What is the video controller chip? Look up its datasheet!

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it's hard to say. it depends on the monitor. try a 9MHz crystal it's within 2% of the target , you might need to tweak the vertical and/or horizontal hold adjustemnt on the monitor.
http://www.mouser.com/ProductDetail/ECS/ECS-90-S-4X/?qs=sGAEpiMZZMsBj6bBr9Q9aWDZfF25lWfiN%2f2lvWpfTc0%3d