[ntp:questions] Re: drifting on crystal

Marc Brett marc at fordson.demon.co.uk
Thu Jan 13 11:53:40 UTC 2005

On Thu, 13 Jan 2005 10:24:25 -0000, "David J Taylor" <david-taylor at invalid.com>

>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.
>An OCXO is a crystal oscillator assembly contained inside a temperature 
>regulated oven to keep it at a constant temperature.  The crystal itself 
>will be chosen to have a low temperature coefficient, but over a more 
>limited range than a non-ovened crystal.  I.e. a normal crystal might be 
>specified to be +/- 10ppm over 0C..45C, but one intended for oven 
>operation might be +/- 1ppm over 60-65C.  (I mage the figures up, by the 
>way).  Such characteristics may be achieved by a different cut of crystal.

The power consumption of an OCXO is typically 1 - 6 Watts, whereas a standard
quartz oscillator consumes microWatts.  They are physically quite large.

Since NTP gives more than enough accuracy for most people (even if the synch
interval is hours or days), it's a fair engineering tradeoff to leave out the
OCXO.  Most consumers would prefer lower power, thus quieter operation and a
smaller package, than hyper-accurate time.

Instead of OCXOs everywhere, I'd prefer to see a standard connector so consumers
can easily replace the crystal with a TCXO, OCXO, or one of those new tiny
atomic clocks if their application requires it.

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