[LEAPSECS] Bloomberg announced its smear

Peter Vince petervince1952 at gmail.com
Tue Sep 27 16:30:21 EDT 2016

On 27 September 2016 at 20:24, Michael Shields via LEAPSECS <
leapsecs at leapsecond.com> wrote:
> On Tue, Sep 27, 2016 at 11:56 AM, Peter Vince <petervince1952 at gmail.com>
> > Thanks for your reply.  If there are no steps - how is it achieved?  Do
> > have a dedicated hardware clock(s) where the frequency of the master
> > oscillator is wound down a bit, and your servers synchronise to that?
> No, the atomic frequency standards and GPS receivers continue to tick
> SI seconds, and the smear is implemented entirely in software.  Maybe
> I am not understanding your question.  Why do you think it would be
> necessary to step?

Thanks Michael. I am very unclear how a time server works. It presumably
takes PPS pulses from a GPS receiver, and somehow synchronises a high
resolution internal clock to those pulses.  But in order to provide the
smeared time on to all its clients, it has to gradually increase the offset
of its clock to the incoming PPS pulse.  So every second your clock needs
to be 13.888888888888 microseconds later than last time with respect to the
PPS pulse.  But with 100ns granularity on the system clock (I presume it is
that same standard?), you can't do that accurately.  One way would be to
have a dedicated hardware clock whose master frequency you offset, but I
suspect your time server is a "standard" PC server?  Maybe you keep a high
resolution floating point counter running in the PPS synchronising program
and adjust your 100ns resolution clock to the nearest tick???

Warner: Thank you, yes, having an externally adjustable oscillator for your
clock would do the trick, but does that really happen?  I have been
assuming that it is a PC that is running the time service, but perhaps
there is some dedicated hardware that takes in GPS pulses and does indeed
have a slewable oscillator???

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