[ntp:questions] Re: drifting on crystal

Richard B. Gilbert rgilbert88 at comcast.net
Thu Jan 13 15:40:14 UTC 2005

Brad Knowles wrote:

> At 6:33 AM +0000 2005-01-13, Marc Brett wrote:
>>  That's the hardware cost.  How many watts does it consume?
>     No more than the low-quality oscillator that is actually included 
> by the hardware vendor.
>>                   why would an engineer include a component with 
>> "oven" in its
>>  name if he didn't have to?
>     There is no actual oven included with the crystal.  The issue is 
> that the OXCO has gone through a more rigorous testing process which 
> includes testing in an oven, and has been certified to work to certain 
> tighter performance standards than your average everyday crystal, and 
> either actively compensates for temperature variations or is 
> relatively immune to them.
>     When CPUs are made, at first no one knows how fast a given chip 
> will run.  It has to be tested first.  Ones that are stable up to a 
> slower speed are sold at a lower price, while ones that are stable to 
> higher speeds will be sold for higher prices -- even if they were made 
> from the same block of silicon, and in fact were sitting right next to 
> each other when the masks were being laid down.
>     Same deal for crystal oscillators.  Those that pass the more 
> stringent tests are sold for more money, those that don't are sold for 
> less.  But other than the passing of the tests (or failure to pass), 
> there are no human-discernible differences between the two 
> components.  One will get painted a different color than the other, so 
> that the manufacturer can tell the difference between them, and know 
> which one to sell for which price.
>>                             Also, how many people really need that 
>> level of
>>  accuracy?
>     Many people could benefit from it.  Indeed, most everyone who uses 
> a computer could benefit from having a better clock, even if they 
> don't know it.
>>          If hundreds of millions of PCs are consuming watts they 
>> don't need,
>>  that's wasteful on a pretty grand scale.
>     There's no waste of power here.  There is short-sightedness on the 
> part of the PC manufacturers, and a willingness to cut off their nose 
> to spite their face.  That's all.

Sorry to differ but an OCXO is an Oven Controlled Crystal Oscillator.   
It's actually built and operated inside a tiny electric oven rather than 
depend on the vagaries of "room temperature".  The temperature is kept 
just high enough that temperature excursions in the external environment 
will not materially affect the internal  temperature.   The power 
consumption is trivial but the extra expense is not justified by the 
needs of the average computer owner/user.

There is also something called a Temperature Controlled Crystal 
Oscillator or TCXO.  I'm not certain exactly how it differs, other than 
the name, but the basic idea is the same; to increase the frequency 
stability by stabilizing the temperature.

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