[time-nuts] Driving clocks from 1pps
brooke at pacific.net
Thu Sep 4 17:49:21 EDT 2008
When driving an inductive load the time constant is L/R. This is very
different than for a capacitive load where it's C*R. So for inductive loads
you want to use as large a series resistance as possible consistent with the
voltage of your power supply and the current needed by the load to get the
fastest response. Note that a current source circuit can not be substituted
for the higher voltage and resistor. I learned this while researching stock
tickers, see: http://www.prc68.com/I/WU5A.shtml#Coil
When applied to clocks using a higher voltage and resistor results in faster
(crisper) and more reliable operation than using the minimum voltage and
resistance which sounds sluggish and is not as reliable.
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Murray Greenman wrote:
> In the light of the latest posts on driving clocks from 1pps, it sounds
> as though I'd better rethink what I was planning!
> I am in the middle of the design of a micro which uses a 10MHz crystal
> to provide a digital clock, but the time is kept in line via GPS using a
> 1pps NCO, which is steered digitally, rather than altering the 10MHz
> oscillator in GPSDO fashion.
> The plan was to provide two outputs (biphase) at 1pps to drive slave
> clocks, but in the light of the notes from Brooke and Chuck, I would be
> better off just providing a single output, and use a series cacacitor.
> Chuck, I would expect that the 1pps would need to be about 50% duty
> cycle, or at least have a pulse width of 100ms or so. I can imagine a
> clock driven from 1pps with a low duty cycle would sound quite
> Murray ZL1BPU
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