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

Jim Miller timenuts at jcmco.com
Wed Jun 22 17:40:33 UTC 2005


Hi,

I recently built an NTP reference clock using an Oncore VP on a  
FreeBSD 5.4 system running ntp v4.2.0-a.  ntptime shows an ever- 
increasing "jitter exceeded" counter value which seems excessive (at  
least to me).  Was wondering if any of you had used the VP in this  
role and if perhaps you saw the same behavior?  Or should I even  be  
concerned?

I have the VP's pps line connected through a 2N2222 configured as an  
inverter in front of a MAX232 in order to shift the TTL level into an  
RS-232 level.  The levels look fine on the 'scope when comparing the  
pps line on one channel against the level-shifted line on the other.

Some screenshots follow..

Thanks in advance!
Jim

% ntptime
ntp_gettime() returns code 0 (OK)
   time c6642065.3d34697c  Wed, Jun 22 2005 13:37:41.239, (.239081709),
   maximum error 3806 us, estimated error 3 us, TAI offset 0
ntp_adjtime() returns code 0 (OK)
   modes 0x0 (),
   offset 0.873 us, frequency 80.865 ppm, interval 256 s,
   maximum error 3806 us, estimated error 3 us,
   status 0x2107 (PLL,PPSFREQ,PPSTIME,PPSSIGNAL,NANO),
   time constant 4, precision 0.001 us, tolerance 496 ppm,
   pps frequency 80.865 ppm, stability 0.012 ppm, jitter 1.209 us,
   intervals 800, jitter exceeded 2438, stability exceeded 0, errors 11.
% ntpdc -c kerninfo
pll offset:           1.106e-06 s
pll frequency:        80.865 ppm
maximum error:        0.006806 s
estimated error:      3e-06 s
status:               2107  pll ppsfreq ppstime ppssignal nano
pll time constant:    4
precision:            1e-09 s
frequency tolerance:  496 ppm
pps frequency:        80.865 ppm
pps stability:        0.012 ppm
pps jitter:           2.126e-06 s
calibration interval: 256 s
calibration cycles:   800
jitter exceeded:      2438
stability exceeded:   0
calibration errors:   11
% ntpq -p
      remote           refid      st t when poll reach   delay    
offset  jitter
======================================================================== 
======
*GPS_ONCORE(0)   .GPS.            0 l   14   16  377    0.000     
0.002   0.004
+time.nist.gov   .ACTS.           1 u   24   64  377   34.147    
-0.185  31.367
-ntp0.usno.navy. .USNO.           1 u   39   64  353   89.195    
13.611  38.156
+navobs1.oar.net .USNO.           1 u   31   64  377   59.619     
1.030  24.527
navobs1.gatech. .INIT.          16 u    - 1024    0    0.000    0.000  
4000.00
+timekeeper.isi. .GPS.            1 u   16   64  377   76.212    
-1.565  49.556
224.0.1.1       .MCST.          16 u    -   64    0    0.000    0.000  
4000.00
%





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