[time-nuts] TCVCXO Adjustment

Bob kb8tq kb8tq at n1k.org
Sat Apr 14 17:30:08 EDT 2018


That’s one possible application. Jim’s VLBI in the back yard is another possible 
application. If this is aimed at “distributed VLBI” then the requirements are … errr …
pretty tight.


> On Apr 14, 2018, at 4:43 PM, Chris Caudle <chris at chriscaudle.org> wrote:
> On Sat, April 14, 2018 8:37 am, Bob kb8tq wrote:
>> big an issue as the TCXO. If it's a single location and the time is
>> arbitrary, then maybe not so big a deal.
>> If it's all arbitrary why worry about drift?
>> GPS on the board looks like a good thing to have to me
> The application is time stamping separate free running devices, in this
> case different video and audio recorders.  So the absolute time is
> arbitrary, but all the devices in use have to agree on the rate of time
> progression for as long as they are being used together.
> The typical requirement is that all the free running devices have timecode
> which will be aligned within one video frame, so ca. 33ms, at the end of
> the time of use.
> So for example, you are making some kind of video, you put all the
> timecode devices together and get their time synchronized, at which point
> they get separated and connected to various audio and video recording
> devices to output a timecode signal that the video and audio devices
> record along with their primary recordings, so that later you can line up
> the recordings from different machines and match same recording from
> different locations, angles, etc. and know they were from the same time. 
> You want the last work of the day to still be synchronized to within
> closer than 33ms, so the maximum time you want to be able to work without
> getting your timecode generators back together to synchronize defines your
> drift rate which defines your acceptable accuracy.
> From common specifications it seems that the commercial products converged
> on 24 hours as the  use time limit, so 33ms/24 hours -> 0.033s/86400s ~
> 0.4ppm
> Yes, in principle you could use an arbitrary clock rate as well as an
> arbitrary  starting time, but that could only work if all the devices were
> exactly the same rate, so if you have to adjust the devices anyway, and
> some may be coming from 3rd parties that you don't have access to prior to
> use, then the only practical approach is for everyone to calibrate their
> devices to standard rate.
> I'll let the original poster ponder on whether GPS on board is a good
> thing or not, but I think you cannot count on GPS being available in use
> (could be inside a steel building, or a steel reinforced concrete
> building, with no RF reception), so you would still need a local
> oscillator which could hold the rate tightly enough to guarantee less than
> 33ms of phase drift over the course of a day.  Maybe you could relax that
> to "working day" and say it's only over 12 hours, not 24 hours.
> What I think makes this potentially interesting to time-nuts is that the
> time requirements are pretty loose by time-nuts standards, but potentially
> some of the tricks that people come up with for getting ns level accuracy
> on hobby budgets could be applied to this to find a way for non-nuts (or
> at least not-yet-nuts) to get started on a really low budget.
> -- 
> Chris Caudle
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