Re: Low jitter oscillators - special frequencies

From: Michael Black (et472_at_FreeNet.Carleton.CA)
Date: 02/07/05 

Date: 7 Feb 2005 03:43:10 GMT

Hal Murray (hmurray@suespammers.org) writes:
> Most special order crystal oscillator packages now have a
> PLL in them. When you order a special frequency, they take
> an unprogrammed unit off the shelf and program in the PLL
> parameters. Delivery in a few days.
>
> The problem is that you get the jitter specs of the PLL rather than
> the crystal. The jitter turns out to be important for high speed A/D
> applications. It's probably important in most RF work too.
>
> Anybody know a good place to get real crystal oscillators made to order
> in small volumes? I need 10 or 20 but the first try came up with
> a min order of 250. I'm expecting 10-12 week delivery. I can live
> with that.
>
> Many crystal/oscillator companies used to be in the business
> of making small runs to a specified frequency. How many are
> left?
>
>
You're complicating matters by insisting on an actual oscillator.
An oscillator is really simple, so there's not a lot of advantage to
buying an oscillator module, certainly not if they are giving you problems.

You aren't likely to get custom frequency oscillator modules, because
they will be too costly to manufacture unless you are buying large quantities.
The modules you've seen are a tradeoff, allowing a wide range of frequencies
without the manufacturing cost. But they have to use a synthesizer, because
otherwise they are dealing with crystal grinding, and that's where the cost
lies.

Just order crystals. Newcomers don't realize that up till thirty or
so years ago, except for a handful of frequencies, crystals were not a
commodity. They were ground on demand, and of course you paid for them.
The only mass-produced crystals were for frequencies that were used
a lot.

That hasn't really changed, the crystals you see in the catalogs are
mass produced because they are commonly used frequencies. But those
common frequencies are more numerous now, since digital devices caused
a rise in the use of crystals in electronic equipment.

So if the frequency you want isn't a common frequency, you send the specs
to a crystal manufacturer, and they grind a crystal for you.

  Michael

> Anybody familiar with the details of the PLL setup? I assume
> somebody did the math and figured out what frequencies they
> have to stock in order to cover all the target frequencies they
> might want to produce. How many different frequencies do they
> have to stock? What sort of ratios are they using in the PLL?
>
> When did everybody switch to PLLs?
>
> --
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>

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