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When you purchase a crystal to use as a clock, the company is asking you exact frequency to set it at.

I remember needing a crystal that was 4x the NTSC color carrier and they were asking me how many digit precision I needed.

What is the process to 'set' the crystal output so precisely?

Marcus Müller
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Thomas
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  • Choice of material and grinding or selection. – winny Apr 28 '18 at 13:20
  • The precision question was asked to me by DigiKey; it would seem odd that they would process the part mechanically to make it fit the specs. – Thomas Apr 28 '18 at 13:22
  • They would either pick the right component, or they would actually ask the manufacturer to do that. Also, there's programmable digital VCOs, but these are far more than just a crystal... – Marcus Müller Apr 28 '18 at 13:24
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    @Thomas - presumably they have already processed parts with varying levels of precision. 4 x NTSC colour burst frequency (or equivalent for PAL colour bursts) is a very common crystal that can be purchased at a wide variety of precisions. For example, this is the frequency of the crystal used in the original IBM PC and clones of it, so was mass-produced for many years for this purpose. – Jules Apr 28 '18 at 13:26
  • There is are two variables, the tolerance at room temp and the AT temperature curve which is calibrated in ['] minutes or 1/60th of a degree that affects the slope of the 3rd order polynomial temperature sensitivity and total swing over a defined range. – Tony Stewart EE75 Apr 28 '18 at 13:47
  • It's clear from the accepted answer that you're using the wrong words in the question. Anyone answering your literal question would be wrong, so I'm voting to close this as unclear. Maybe it could be salvaged if you ask about an actual product. – pipe Apr 28 '18 at 19:26
  • I agree that it is a bit ambiguous since I didn’t know what kind of product it was; I remembered that we needed a crystal and digikey was asking me the precision; I was wondering if it was programmable or adjusted through some other process. In a way both answers are accurate. While we can close this as unclear I think the answers from Dave and Ali are bringing informative content. – Thomas Apr 28 '18 at 21:13
  • Possibly of interest: https://youtu.be/b--FKHCFjOM –  Apr 29 '18 at 02:10

3 Answers3

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Coarse tuning is done in the grinding process, where the thickness of the quartz wafers is established. I once visited a manufacturing facility; the grinder operators were using ordinary shortwave receivers to monitor the broad peak of electrical noise that the batch of wafers emitted due to the grinding action. When it reached a point slightly higher than the target frequency, they would stop.

Fine tuning is done during the plating process that puts the electrodes on the wafer; adding metal brings the frequency back down. This process is much more precise, and each crystal can be individually calibrated.

Dave Tweed
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I believe there could be some confusion. When you use a crystal as a "clock", you need "clock generator" for this. To get a real quartz crystal tuned to customer's specification, the order must be for many thousand units. It is very likely that OP means "programmable crystal oscillator", there are thousands models, see Digi-Key for examples.

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These devices are made in the standard SMT ceramic cases, 4-pins, 3x5mm or smaller, just as the usual fixed-frequency crystal oscillator chips. However, the programmable ones have some base oscillator, and a PLL circuit, with ability to externally program its parameters to almost any desired frequency. The typical output range is 1 Mhz to 110 Mhz. The chips usually have programmable non-volatile memory, re-programmable maybe only two or four times, and frequency can be specified down to 10-digit accuracy or something.

When purchasing, you can specify your own frequency (that's where they can ask for this), or you can purchase a PC-based programmer for this, and program you values.

Ale..chenski
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  • I think that’s exactly what it was because the digikey guy asked me about the exact frequency just before they shipped the parts – Thomas Apr 28 '18 at 18:19
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Since you said it was a 4x NTSC crystal, it was likely a standard part made in bulk. How they select the precision there is an issue of binning.

For this process, the factory produces a batch and then tests the crystal frequencies. Due to random process variations, the frequencies will have a distribution (a spread) across the ideal frequency.

If you order a highly precise crystal, they will pick it out of the stock of crystals lie close to ideal. They will charge you more for this. If you can tolerate a wider variance, they will sell you those that lie further away at a discount.

For example, this crystal datasheet says you can order the product in tolerances of ±10, 20 or 30 ppm.

This is common in a wide variety of processes, e.g. LED color and brightness, CPU speeds/number of cores, DRAM & FPGA speeds, etc.

user71659
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