[time-nuts] OCXO Retrace

Bob Stewart bob at evoria.net
Wed Feb 10 15:45:02 EST 2016


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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