[ntp:questions] how do I lock in average frequency correction

David Lord snews at lordynet.org
Tue Feb 14 08:31:50 UTC 2012

Ron Frazier (NTP) wrote:
> On 2/13/2012 3:25 PM, Chris Albertson wrote:
>> On Mon, Feb 13, 2012 at 10:59 AM, Chuck Swiger<cswiger at mac.com>  wrote:
>>>>> You might be able to improve the stability of the crystal by 
>>>>> ensuring good
>>>>> airflow and cooling via HVAC as needed.  And I suppose you could 
>>>>> adjust the
>>>>> rate by changing the HVAC set-point, but I don't think the benefit 
>>>>> is worth it.
>> For temperature stability, I just finished building a fan controller.
>>   There is a temperature sensor on the end of a 18" cable.  Glue the
>> sensor onto anything you like.  Then when the sensor matches a set
>> point the fan comes on.    I think this should work as long as you
>> keep the set point above the room temperature.     I've not tried it
>> yet.  It is simple enough to make.  The TMP36 sensor outputs a voltage
>> of 10mV per degree.  That goes to one side of an LM311 comparator.
>> The lm311 switches a transistor that drives the fan.   It looks like
>> holding the sensor temp to +- 1/2 degree is easy.    Holding to only
>> 0.5 C is not hard and might help.       Actually this controller is
>> going on a Rubidium oscillator, not an XO.   but if it works well I'll
>> build a few more.
>>   The idea is about the same is an ovenized XO but slightly more crude.
>>   Just aim the fan at the part you want to control and insulate the
>> sensor from airflow.     I'll know if this work in a few weeks
>> Chris Albertson
>> Redondo Beach, California
> Perhaps a silly question, but, does the "tick" that drives the OS 
> software clock originate from the RTC or from the CPU master clock at 2 
> GHz or whatever?  Just trying to understand how this stuff works.

My most recent PC is from about 7 years ago. All the
systems that I've checked seem to have a 14.31818 MHz
crystal. Some of the older systems even have a jumper
to use an external source.


> Ron

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