[time-nuts] Frequency from telco

Bill Janssen billj at ieee.org
Sun Sep 24 19:54:07 EDT 2006


Hal Murray wrote:

>Has anybody tried using the phone company as a frequency reference?
>
>Are the telco master clocks locked to GPS or national frequency sources?  Or 
>do they just use their own Cesium box?
>  
>
Yes to all of the above. It depends on the telco. All of them  "should" 
meet the 10E-11 requirement
over a "long" interval

>Is that a useful way to get a good clock without a GPS antenna?
>
>  
>
Probably, if you filter the jitter and wander with a reasonably good 
crystal oscillator

>I worked with T1 many years ago.  It wouldn't be hard to extract a signal to 
>feed to a PLL.
>  
>
You could lock an oscillator to the T1 rate and then divide that down to 
something like 8 or 4 KHz
and use that to discipline you "precision" oscillator

>Has anybody done that and collected data?  I'd expect lots of short term 
>jitter and wander but the long term should be pretty good.
>
>  
>
The telco's do that all the time. The worst wander I have seen was about 
200 NanoSeconds over
a few minuets time. There was a lot of data available at one time. SONET 
(fiber optics) had
a problem with step changes in phase on the payload.. Don't know how 
that was resolved

>Does anybody know how DSL works?  If I poke around with a scope in my DSL 
>modem/router, will I find a clock locked to the telco's master clock?
>
>How many different versions of DSL are there?
>
>
>  
>
I haven't worked in this field for about 16 years so anything I type is 
from that time frame.

Bill K7NOM





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