[time-nuts] OCXO Retrace

Tim Shoppa tshoppa at gmail.com
Thu Feb 11 06:56:19 EST 2016


Bob, the issue I'm talking about, is how you jostle the divider chain
around when you do the phase lock.

You have 1PPS divided down from the OCXO, and 1PPS from the GPS, and in a
truly cold start they are completely random in phase and usually off by a
big chunk of a second.

If you don't reset the OCXO divider chain, you have to steer EFC to get rid
of that residual up to a second of phase. That can take a hugely long time
and I doubt you are doing that.

I think it's more likely, If you do reset the OCXO divider chain, the width
of the reset pulse or some other detail, is giving you some systematically
positive or negative residual phase to clear and I think you could be
interpreting this as retrace when really it's mostly just EFC steering to
get around the residual phase.

Tim N3QE

On Wed, Feb 10, 2016 at 3:45 PM, Bob Stewart <bob at evoria.net> wrote:

> Hi Tim,
>
> This is with the GPSDO that I designed.  It uses a PLL which is corrected
> for sawtooth.  The receiver is an LEA-6T.  I've noticed that a few of these
> OCXOs continue to retrace upwards in DAC movement, even after a number of
> days.  I haven't run any of them for more than a week, as I've built only a
> limited number of these units.  The normal situation is to retrace downward
> from the start, once initial warmup is over.
>
> Bob
>
> --------------------------------------------
> On Wed, 2/10/16, Tim Shoppa <tshoppa at gmail.com> wrote:
>
>  Subject: Re: [time-nuts] OCXO Retrace
>  To: "Bob Stewart" <bob at evoria.net>, "Discussion of precise time and
> frequency measurement" <time-nuts at febo.com>
>  Date: Wednesday, February 10, 2016, 2:15 PM
>
>  I think
>  what you are observing, is the detail of how the control
>  loop transitions from frequency locked to phase locked. And
>  this has more to do with the divider chain reset and phase
>  detector than anything else.
>  Note that several of the
>  hobbyist/E-bay GPSDO's (especially the "Chinese
>  GPSDO") do not have a phase locked mode, at best they
>  are frequency locked, and many of these have a consistent
>  bias based on the gating of the frequency counter (typically
>  their counter gating adds an extra count, meaning that when
>  they are frequency locked to GPS they are running slightly
>  slow).
>  Tim
>  N3QE
>  On Wed, Feb 10, 2016 at
>  2:17 PM, Bob Stewart <bob at evoria.net>
>  wrote:
>  I've
>  been running up a number of OCXOs lately, all Trimble
>  34310-T.  For most of them, the DAC moves in the negative
>  direction after lock.  But there are a few where the DAC
>  moves more positive.  Is this an indication of a different
>  cut of crystal, or is retrace direction more or less
>  random?
>
>
>
>  Bob - AE6RV
>
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