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.
Just a thought,
Brooke Clarke, N6GCE
--
w/Java http://www.PRC68.com
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
http://www.precisionclock.com
Jim Miller wrote:
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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