[time-nuts] The 5MHz Sweet Spot
SAIDJACK at aol.com
SAIDJACK at aol.com
Sun Nov 3 15:29:23 EST 2013
Forgot one comment: the good parts' plot also shows a very nice crystal
retrace stabilization in the red EFC trace over about the first 6 days or so.
After that the crystal goes into it's long term crystal aging mode.
Retrace is one big reason why its best to let crystals run continuously..
In a message dated 11/3/2013 12:27:18 Pacific Standard Time,
SAIDJACK at aol.com writes:
Hi guys,
here are two plots of the same DOCXO product, one being jump-free over the
31 days test interval, another that had about 20 jumps in just ~4 days.
Needless to say the later OCXO went back to the vendor to be opened up,
and
the crystal replaced.
I have plots of units that were perfect for two+ days, then jumped like
crazy, and units that jumped for days, then stabilized and worked
perfectly.
It's all random when failures do happen.
Warren, frequency jumps are indicated by constant changes in EFC voltage
(red trace) after the jump happened. Most of these in the attached plot
are
frequency jumps, that cause an offset in phase (sometimes just some 10's
of
nanoseconds over minutes). Sometimes the frequency will "recover" and
that
can be seen by the EFC voltage going back to the initial voltage before
the jump happened. Phase jumps would just cause a small hop in EFC
voltage,
sometimes so small that it cannot be perceived.
Also, note that most of these jumps have 1mV to 2mV changes in EFC, which
for these oscillators would equal a frequency change of about 0.1 to 0.2
parts per billion frequency change. Very small, but on a GPSDO it leaves
a
huge footprint in the EFC voltage plot and the phase plot.
I wish every crystal we get would work as well as the one in this 31 day
test plot..
Bye,
Said
In a message dated 11/3/2013 11:57:50 Pacific Standard Time,
warrensjmail-one at yahoo.com writes:
Said Jackson posted:
>Crystal jumps are the biggest menace facing users of
crystals/oscillators
>today.
Are you including both phase jumps and frequency jumps together?
Is one more or likely to happen than the other?
Is it mostly a jump that effects just the phase or freq, or is there
everything in-between, jumps that effects both phase and freq at the same
instant in time also just as likely?
We all know each effects the other, but that is only over time,
instantaneously and over short time spans phase and freq jumps are
separate
things and maybe from different causes.
A true phase jump causes only a one cycle freq error and a true freq
offset
jump does not cause an instantaneous phase jump.
If the main causes of random freq jumps and random phase jumps are from
different things, then with a high speed, high resolution detector,
I wonder if knowing which event has really occurred, that then some
correction compensation could be applied that does not effect the other.
An Oversimplified example;
A Phase lock loop does not care what the instantaneous freq is, and a
true
Freq Lock loop does not care what the phase difference is.
With a DDS, one can change the freq without causing a phase step or it
can
cause a phase step, without causing a freq offset.
With two variables (instantaneous phase and freq offset control) and two
unknowns (instantaneous jumps in either), couldn't one apply a correction
to
the right place for any random step error that occurred?
It would depend if the errors are caused by true independent random fast
jumps or just slowly drifting interacting changes.
ws
*******************
Bob, et. al.,
Lots of opinions in this discussion, but none of it discusses the
elephant
in the room affecting todays' vendors:
Random crystal instability versus manufacturing techniques.
I can buy oscillators from multiple vendors that have -115dBc at 1Hz or
better and noise floors of -182dBc. That technology is well understood
and
has been mature for a very long time and to me its boring. Recently
Ulrich
Rhode even had a great article in the Microwave Journal detailing how
exactly to build one of those units.
But what does it help me to have -115dBc if the darn thing jumps 50ppt
every
two to three days??
Crystal jumps are the biggest menace facing users of crystals/oscillators
today and so far I have never been given a reasonable explanation from
any
of the vendors out there what causes it and how to avoid it or how they
plan
to address it.
In fact no vendor we know tests for it to levels of sub-ppt over days
which
is what is necessary for any disciplined application as disciplining will
clearly show even the smallest crystal jumps. Almost every vendor will do
a
frequency test only, where a phase test would be needed.
Users of crystals/oscillators are left with doing an exhaustive yield
test
during burn-in to find bad crystals. We test our boards for 3 days and
more
to weed out jumpy crystals, and its a pain and very expensive to have to
do
this on finished goods as rework is in order for units that fail.
The results are staggering, some vendors consistently have jumpy product,
others consistently have excellent product, all have at least occasional
batches that are worse to far worse than standard deviation. Some are so
bad
that one batch may yield 95% and the next batch of the same exact product
will only yield 50% or less!
I think this is the area of Quartz processing that has the least amount
of
research invested into it, and as anyone that has seen their Z38xx unit
jump
up and down in phase can attest to its a menace and can ruin one's day. I
wish there were something besides yield testing that can be done to avoid
manufacturing and shipping bad crystals to integrators. BVA seems to be
one
of those solutions, but how many BVA's have we seen in products that cost
$400 retail??
Bye,
Said
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