[LEAPSECS] Bloomberg announced its smear

[Warner Losh]

On Tue, Sep 27, 2016 at 2:30 PM, Peter Vince <petervince1952 at gmail.com> wrote:
> 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>
>> wrote:
>> > Thanks for your reply.  If there are no steps - how is it achieved?  Do
>> > you
>> > 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???
>
> Peter
>
>
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