Hi,
On 10/06/2016 08:38 PM, Bob Camp wrote:
Hi
One very simple experiment:
Take a HP that has been off power for a year or so. Fire it up and watch it’s
predictions
of holdover accuracy. Many of them will go through a “zero” time estimate at
one or
two days. At three or four days they are struggling to hit spec (10us). The
reason is
pretty simple. The OCXO warmed up and went through an inflection (reversal in
direction).
They estimated across the inflection, got zero and passed that on ….
Indeed. The Z3801A does a least-square fit and then tries to maintain
that. If done at the wrong time it will be wildly off. I don't remember
the details, but I think I recall that you can trigger the
re-calibration routine which is what you want to do to drive it in the
right direction.
Least-square fitting isn't all that magic and doesn't really require
lots of memory, if you do it properly. You just need the oscillator to
heat up and settle before you attempt to do anything involving long
time-constants. Usually it's not the core algorithms, but the heuristics
that needs to work well.
Cheers,
Magnus
Bob
On Oct 6, 2016, at 1:32 PM, Bob Stewart <b...@evoria.net> wrote:
said: "The somewhat amazing holdover estimates on the HP units are one example of
this problem. It does not take much testing to quickly realize that they are far more
often wrong than right on a unit that has been on power for less than a few weeks."
Thank you Bob. These two sentences clear it all up. Silly me thinking that
the HP units could actually project aging from minimum data. I can sample
every hour and always have the past 24 hours to look at. In fact, it might
even be better to have multiple days in the queue and take a simple average for
projection. Naivety has been my biggest foe in this project.
Bob
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GFS GPSDO list:
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From: Bob Camp <kb...@n1k.org>
To: Discussion of precise time and frequency measurement <time-nuts@febo.com>
Sent: Thursday, October 6, 2016 12:24 PM
Subject: Re: [time-nuts] Measure GPSDO stability with minimum resources?
Hi
As normally used, the term “aging” means the long term drift in the frequency
of an OCXO.
It is independent of the temperature effects and things like retrace, warmup,
and voltage
stability. It is rare that there is any impact on the aging of a properly
designed OCXO from
drift of the oven temperature.
Any time you try to estimate (or measure) aging on an OCXO, you will have a
hard time
separating it from the other things that affect the frequency of the
oscillator. The somewhat
amazing holdover estimates on the HP units are one example of this problem. It
does not
take much testing to quickly realize that they are far more often wrong than
right on a unit
that has been on power for less than a few weeks.
Bob
On Oct 6, 2016, at 1:11 PM, Chris Albertson <albertson.ch...@gmail.com> wrote:
Wouldn't "aging" be the change in the temperature v. frequncy graph over
time? It is hard to hold temperture constant so maybe record the
function at several different times.
This is one big advantage of using a microprocessor inside the GPSDO, It
can log internal data to either a SD card or use a USB link to a PC to be
recored in log files. Even my "simple as possible" GPSDO push data to a
PC over USB. It is easy to glue a temperature sensor to "whatever" and
log it
On Wed, Oct 5, 2016 at 10:14 PM, Hal Murray <hmur...@megapathdsl.net> wrote:
j99har...@gmail.com said:
Unless the oscillator is still warming up, 5 minutes or even 60 is way
too
short a time to look at aging. For aging, you will want to look at the
change in DAC values over several days at least.
I think it's worse than that. You have to hold the temperature constant,
and
maybe even the power supply voltage. A probe on the crystal can might
allow
you to correct for temperature.
--
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Chris Albertson
Redondo Beach, California
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