[time-nuts] The 5MHz Sweet Spot

[SAIDJACK at aol.com]

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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