[time-nuts] TCVCXO Adjustment

[Chris Caudle]

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