[ntp:questions] backup PPS source

David Lord snews at lordynet.org
Tue Apr 13 02:28:34 UTC 2010


David Lord wrote:
> 
> Hi
> 
> Latest experiment is to attempt to make a backup PPS source and
> I've now made a start with a watch xtal oscillator and divider
> that can be synchronised by a PPS source when present (except
> I've not yet wired up for synchronisation).
> 
>      remote           refid      st t when poll reach   delay   offset 
>  jitter
> ============================================================================== 
> 
> +GPS_NMEA(0)     .GPSb.           0 l   57   64  377    0.000    1.865 
>  0.381
> oPPS(0)          .PPSb.           1 l    -   16  377    0.000   -0.004 
>  0.002
>  PPS(2)          .OSCc.           1 l   10   16  377    0.000    1.859   
> 0.006
> -k6x400.home.lor 192.168.59.64    2 u    3   64  377    0.951    3.837 
>  0.447
> +me6000g.home.lo .MSFa.           1 u   29   64  377    0.905    3.190 
>  0.451
> -p4x2400c.home.l 192.168.59.64    2 u   34   64  377    0.832    3.078 
>  0.343
> 
> PPS(2) was time1 fudged to get it < 2ms.
> 
> I can see there being problems already with full rotation of trim
> cap giving about +/- 6 us/s variation and temperature changes
> have already shown a minor problem (south facing window, very hot
> day, rapidly going cold tonight). Of course putting the whole
> oscillator module in a temperature controlled box is in the plan.
> 
> Earlier this afternoon I had trimmer set so that offset change
> between poll periods was < +1us, but that started increasing and
> change has now reversed direction with offset over last hour
> having dropped from 1867us, steps at 6min intervals being:
>   -31 -43 -54 -61 -59 -60 -58 -60 -59 -68 -75 -76us
> ie total 0.63ms over last hour.

peerstats for yesterday:
           ident   cnt     mean    rms     max
127.127.22.0     5237   -0.003  0.039   0.217   GPSb pps
127.127.22.2     5236  -10.938  4.678  11.263   OSCc pps

I guess temperature variation over period has been < 15C
to > 25C.

That level of drift is much better than I'd expected so
I'll move on to next stage as soon as I have the temperature
control sorted (peltier device) and for tuning replace
trimmer with varicap and use a 10t pot to set frequency.


David




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