[ntp:questions] backup PPS source

David Lord snews at lordynet.org
Tue Apr 13 08:41:11 UTC 2010


Dave Baxter wrote:
> In article <82i36jF3r8U1 at mid.individual.net>, snews at lordynet.org says...
>> 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.
>>
> 
> Take care, varicap diodes have significant tempco's too, and pot's can 
> become very noisy after a while, even if you don't physicaly mess with 
> them!   We have lots of trouble with multi turn presets that set High 
> Voltage PSU levels here at the salt mine.  Seems a common thing, as I 
> remember similar things when I worked at Tek UK years (cough... decades) 
> back.

Perhaps a fixed slightly wrong frequency from the oscillator and
multiplier/divider PLL for frequency adjustment?

or even an old fashion beehive trimmer cap or plunger type,
neither of which seem to be listed in electronics catalogs.


David




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