[time-nuts] PPS jitter - Oncore VP w/NTP

Poul-Henning Kamp phk at phk.freebsd.dk
Wed Jun 22 14:39:08 EDT 2005

In message <42B9AEC8.7090603 at pacific.net>, Brooke Clarke writes:
>Hi Jim:
>I was using an Oncore VP and trying to set a Cesium standard.  As part 
>of the measurement the counter is set to average readings and often the 
>sigma (jitter) was very high (hundreds of ns).  This turned out to be 
>caused by multipath.  The fix is to change the elevation mask to a 
>higher angle.  Note when in position fixed (timing) mode only one sat is 
>needed.  The VP has a little over 50 ns of sawtooth so that's about as 
>good as it will get.

You won't get anywhere close to that with a interrupt timed serial
port DCD signal on a normal PC.

>> % ntpdc -c kerninfo
>> pll offset:           1.106e-06 s
>> pll frequency:        80.865 ppm

This is not indicative of good hardware.

What does
	sysctl kern.timecounter
say ?

>> maximum error:        0.006806 s
>> estimated error:      3e-06 s

This looks very typical for a interrupt signal.

The jitter warning thing is very likely if you load on the
machine which might disable interrupts for any amount of time.

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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