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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