[time-nuts] Driving clocks from 1pps

Brooke Clarke brooke at pacific.net
Thu Sep 4 17:49:21 EDT 2008


Hi Murray:

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.

Have Fun,

Brooke Clarke
http://www.prc68.com/P/Prod.html  Products I make and sell
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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
> different.
> 
> 73,
> Murray ZL1BPU
> 
> 
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