[time-nuts] TCVCXO Adjustment

Adrian Godwin artgodwin at gmail.com
Sat Apr 14 11:10:00 EDT 2018


What if you iterated toward a suitable minimum-error setting, then looked
for cyclic corrections with a period of weeks to months. Once you start to
see that, choose the centre of the cycle and track it (or perhaps just
increase the time constant).

On Sat, Apr 14, 2018 at 2:37 PM, Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> The gotcha is that you do not have a calibrated adjustment. Put another
> way,
> there isn’t a perfect correlation between DAC bits and ppm. Each
> adjustment
> you make is subject to a bit of error. When you are trying to get within a
> ppm,
> your measurements are quicker, so the larger error ( percentage of step)
> may
> not be as big a deal. When you get close, it is likely to become a big
> deal.
>
> You could track all of your changes (month to month). The issue there is
> that the
> drift in the TCXO month to month is not likely to be the same. Sorting all
> of that out
> could be a bit nasty …..
>
> ====
>
> TCXO drift is not the only contributor to the accuracy of a time code
> generator.
> The other obvious one is setting it to the correct time in the first
> place.  If the
> objective is to compare data from different locations, getting it set may
> be as
> 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 ….
>
> Bob
>
>
> > On Apr 14, 2018, at 6:35 AM, Adrian Godwin <artgodwin at gmail.com> wrote:
> >
> > If you compare VCXO time with UTC or GPS once a month to an accuracy of
> 1s
> > (with NMEA or even a time signal and manual pushbutton) and make a
> > correction for the 2.5 million seconds that occurred since the last
> > correction, you'll be better than 0.5 ppm.
> >
> > Is that good enough ?
> >
> > On Sat, Apr 14, 2018 at 5:59 AM, Hal Murray <hmurray at megapathdsl.net>
> wrote:
> >
> >>
> >> kb8tq at n1k.org said:
> >>> The alternative is to plug a USB GPS into the mac and do a bit of code
> to
> >>> compare things. If you want to pass the gizmo around to your friends ….
> >> that
> >>> can be done. Pretty good ones are “sub $10” delivered.
> >>
> >> USB GPS gizmos generally don't have a PPS and the timing on the NMEA
> >> sentences is generally crappy.  (I may be biased by a few bad examples.)
> >>
> >> Does anybody have a list of ones known to work well?
> >>
> >> There is at least one GPS-USB with PPS, the Navisys
> >> GR-601W/GR-701W/GR-801W,
> >> They were hard to get retail.  Looks like idealez sells them Taiwan for
> >> $47
> >> for the 701W (Ublox 7), $50 for the 801W (Ublox 8) (plus shipping).  I
> >> haven't tried ordering through them.  Gary and/or Mark may have some for
> >> sale.
> >>
> >> ----------
> >>
> >> Plan B is a GPS breakout and a USB-serial breakout and a few wires.  4x
> >> less
> >> USB jitter if your USB-Serial chip is full-speed.
> >>
> >> I got mine from SparkFun:
> >> Venus GPS with SMA Connector
> >>  https://www.sparkfun.com/products/11058
> >> USB to Serial Breakout - FT232RL
> >>  https://www.sparkfun.com/products/12731
> >>
> >> ----------
> >>
> >> Google found this.  I don't know anything more:
> >>  https://www.zti-communications.com/z050-gps-dongle/
> >>
> >>
> >> --
> >> These are my opinions.  I hate spam.
> >>
> >>
> >>
> >>
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