19.500 Mhz Oscillator/Crystal (or 39.000 Mhz)

The ones I'm familiar with are programmed at the factory or distributor. After that, you are supposed to think of them as normal parts.

There is one catch. They have a PLL inside so there will be more junk on the spectrum than a crystal cut for the exact frequency that you want. In the digital world, that would be a bit of jitter. It's probably not enough to worry about unless you are using it for something like an A/D where jitter turns into error.

You go to a crystal manufacturer, and have them grind a crystal to your frequency and specs (ie they will grind it to fit the oscillator you are using).

It's only fairly recently that crystals have become commodity items. Initially, all crystals were custom ground, with the rare exception being something like 100KHz crystals for frequency calibrators.

When WWII ended, that changed things a bit, since so many crystals were manufacturered for the war that for decades there were lots of crystals already ground, useful so long as you could live with what was available, or find one close enough to frequency that you could grind it yourself (the cases back then were phenolic and were screwed together, making it very easy to open the case and get the quartz blanks out to do some mild grinding). So at least for hobby purposes, a lot of things were built around those available crystals, you'd find the cheap crystal first and if it wasn't quite where you needed it, you'd change the design to fit it.

Color TV came along, and that meant color subcarrier crystals were manufactured in large quantities, useful if you could figure out something else to use them for. CB came along, and that caused a lot of crystals to be manufactured before demand, so you could drop by your local store and get a pair of crystals for another channel off the rack.

Then digital ICs came along. That did two things. First, it was often simpler to use crystals for clocking than an RC circuit, besides the obvious of frequency stability. If you can get crystals cheap, they can be cheaper than the multiple components of an RC oscillator, and of course it means fewer parts on board. Second, once you had a digital IC of fairly good integration, the frequency of the crystal often didn't matter, you could divide down or whatever to get it.

So that brought a number of popular frequencies to the catalogs, really odd looking frequencies until you started dividing them down and saw what lower freuquencies they'd generate. So a new set of commodity crystals came to the catalogs, those that popular ICs were designed around.

A lot more digital equipment came along, and it just became so common that the commodity crystals kept coming. Add more crystals to the catalog to generate baud rates, add more crystals for popular tone dialer ICs, and so on.

Anyone who came to electronics in recent years or decades might believe that you just ordered what crystals were in the catalog, the number that can be ordered off the shelf has greatly increased. But then they wonder how they are supposed to get a frequency that isn't in the catalog.

Nothing really has changed, except there are more frequencies common enough to warrant making crystals for that frequency as a commodity part. You still have to get a crystal ground to frequency and specs if you can't get it out of the catalog.

Michael

Michael Black schrieb:

But if you need only very small quantities (or just samples), it's quite comfortable (and cheaper) to use those programmed oscillators.

Tilmann

en

This might be wishful thinking but did they manufacture oscillators with dual clocks?

Any recommendations on where I can economically order a small quantity of 10 to 25?

Derek