[ntp:questions] backup PPS source
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:
>>> 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 192.168.59.64 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 192.168.59.64 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.
>> 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)
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.
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