[time-nuts] Low-long-term-drift clock for board levelintegration?

Bob Camp lists at rtty.us
Tue Feb 21 17:16:24 UTC 2012


Hi

The only saving feature on a local LAN is that you get to run a *lot* of
data if you wish to. That helps you average things out. You can indeed get
well below 1 ms in that case.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Chris Albertson
Sent: Monday, February 20, 2012 9:57 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Low-long-term-drift clock for board
levelintegration?

Even with a very minimal local Ethernet where the path is the same for
every packet you still have variable timing.   There are queues and
buffers.  Also it is not so easy to measure the time a network packet
arrives at a computer.

The serial port is the best hardware for timing.  The DCD pin is
directly tied to a hardware interrupt that has as low a latency as you
will find on the PC.   These is nothing like this hardware interrupt
in the Ethernet controller.

There is a way around this, some Ethernet controllers can time stamp
the data packets in hardware.  This can be used by PTP for timing that
is almost as good as the serial port.

So you add the uncertain timing because of queues to the un-abilty to
accurately determine when the data pack arrives and you are stuck in
the millisecond level

On Mon, Feb 20, 2012 at 6:02 PM, Bob Camp <lists at rtty.us> wrote:
> Hi
>
> My understanding is that the path delay is what is being monitored.
>
> Bob
>
>
>
> On Feb 20, 2012, at 7:56 PM, Brooke Clarke <brooke at pacific.net> wrote:
>
>> Hi Bob:
>>
>> It was my understanding that the receiving station knows the path taken,
is that the case? Or are you saying even when it's the same path the time
delay has large variations?
>>
>> Have Fun,
>>
>> Brooke Clarke
>> http://www.PRC68.com
>> http://www.end2partygovernment.com/Brooke4Congress.html
>>
>>
>> Bob Camp wrote:
>>> Hi
>>>
>>> Since the network is being measured, I suspect that using it as the time
reference would present a basic problem.
>>>
>>> Bob
>>>
>>>
>>>
>>> On Feb 20, 2012, at 2:07 PM, Jim Lux<jimlux at earthlink.net>  wrote:
>>>
>>>> On 2/20/12 10:31 AM, Bob Camp wrote:
>>>>> Hi
>>>>>
>>>>> Simple answer:
>>>>>
>>>>> A rack mount cesium standard is as good as you can get. Figure on 15
ns per day of drift. That gets you to 15 us in 1000 days. You may or may not
get one that drifts that little, a lot depends on  little details. For>   3
years, consider an ensemble of cesiums. At $50K each cost can go up pretty
fast.
>>>>>
>>>>> There is nothing out there that will do better stand alone for less
money. Either you get a sky view or you change the budget....
>>>>>
>>>>> Bob
>>>>>
>>>>>
>>>>
>>>> But he *does* have the ability to have an external network connection..
So the real question is whether one could come up with a better-than-NTP
scheme to get the 100 (or 10) microseconds.
>>>>
>>>> I suspect that with sufficiently controlled network paths (which might
be doable in a widescale deployment) and a tweaking of the NTP stuff, you
might be able to get it to work.
>>>>
>>>> 100 microseconds out of a day is 1E-10, which is actually a fairly
liberal drift spec for an OCXO or even a TCXO. (which are things like
ppb/day)
>>>>
>>>> Maybe there's a particular time of day (or day of week) when network
uncertainties are minimal, or can be bounded.  So what you really need is a
onboard oscillator that is good at "carry over" between periodic calibration
checks.
>>>>
>>>>
>>>> One question about the 100 microsecond spec.. is that a worst case at
any instant, or is it an average spec over a day (so a consistent diurnal
variation would cancel out)
>>>>
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-- 

Chris Albertson
Redondo Beach, California

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