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

Brooke Clarke brooke at pacific.net
Wed Jun 22 18:32:40 UTC 2005


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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