[time-nuts] 60 Hz graphs

Hal Murray hmurray at megapathdsl.net
Mon Sep 12 06:22:04 UTC 2011

Here is a typical 2 day view.

Note that there are 3 interesting events visible on the frequency graph.  
More on those later.


I think this is a generator dropping off-line, or something interesting at 
the main power production level.

That happens at about 4 hours on the above pair of graphs.  On the offset 
graph, it's just a tiny glitch.  It's much easier to spot in the frequency 

Here is another similar event:

It took 0.07 hours (4 minutes) for the frequency to get back to normal.
During that time, the system lost 13 cycles or over 0.2 seconds.


Here is a typical extra cycle event.  This one is actually 2 cycles.

It's about 16 hours on the above graphs.

Here is another extra cycle:

This is about 27.5 hours on the above graphs.


Here is a time step on the machine collecting the data.

I was downloading a huge file.  That added a lot of queuing delay on my DSL 
link.  After a while, ntpd decided the delayed version was the correct time 
and shifted to it which produced a 1/4 second offset.  After another hour the 
download finished and ntpd shifted back to the correct time.

1/4 second at 60 Hz is 15 cycles.  1/4 second out of 10 seconds at 60 Hz is 
1.5 Hz.

[I have good time locally.  I left that laptop running off network time 
servers so I could see things like this and/or other glitches I didn't 


This looks like a glitch in the PPS data collection code.  I think it got the 
new time-stamp with the old count.

My guess would be a missing lock or disable interrupts.  I've only seen one 
of these over roughly 2 months.  The extra cycle events are more like 1 per 

These are my opinions, not necessarily my employer's.  I hate spam.

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