[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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