[ntp:questions] backup PPS source

David Lord snews at lordynet.org
Sun Apr 11 23:20:43 UTC 2010


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 
+GPS_NMEA(0)     .GPSb.           0 l   57   64  377    0.000    1.865 
oPPS(0)          .PPSb.           1 l    -   16  377    0.000   -0.004 
  PPS(2)          .OSCc.           1 l   10   16  377    0.000    1.859 
-k6x400.home.lor    2 u    3   64  377    0.951    3.837 
+me6000g.home.lo .MSFa.           1 u   29   64  377    0.905    3.190 
-p4x2400c.home.l    2 u   34   64  377    0.832    3.078 

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.

The temperature swings today have also has adverse effect on the
three peers shown that on a good day can all be < 500us.

Any hints/tips welcome, I'm also considering an oscillator using
an AT cut xtal but design for that is considerably more complex,
however it's then a possibility to convert the HF to be used as
system clock.

I also keep googling for rubidium sources but not found any
within my budget, cheapest I've found (new) starting at ukp 900.


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