[time-nuts] V standards

WarrenS warrensjmail-one at yahoo.com
Tue Dec 2 00:47:52 EST 2008


to Said
 Thanks for the information and response and pointing out some errors.

It sounds like the Mexico unit must not be runing a 100 SECONDS tracking
 loop as I used in my example. 

I did not really mean it to be the trick question it turned out to be.
I messed up on my seconds and minuties.  

As stated, the 12 bit Dac example assumed a 100 SECOND tracking time as 
I thought ULRICH meant with his "s" notation. So what did ULRICH's "100 s" mean?
With a 100 SECOND tracking time the noise on the GPS signal is close to 1E-9, 
so it appears that a 12 bit Dac would be a giant Overkill by almost 5 bits.  
A 100 second control loop just does not need anything nearly as fine as 
a 2.44E-11 step size to track a 1E-9 noise signal near prefect. 

The Root Allian noise of the GPS at 100 Minuties plot is about 1E-11.
So the question is: 
What is the minimun Dac resolution it would take so as not to degrade the 
noise performance by more than 3 dB for a 100 minute control loop?

BTW I've  head from others that a 2.44E-11 resolution Dac works OK for 
most applications.  More is better of course. Maybe there is some 
additional filtering going on or maybe it is they just can't measure short 
time period noise very well. That comes out to about 10 ps or so of 1 sec 
phase noise.Not many instruments are going to be able to measure that!
What is it you use using to measure the 2.44E-11 freq noise 
at 1 second? 

Warren 
********************************** 

Hi Warren,
 
as Bruce mentioned, you will want much more resolution than 12 bits to  
properly correct for tempco, and more importantly aging.
 
Having only 4096 steps for adjustment is definitely not enough, it will  give 
you a huge offset of 2.44E-011 per step for your 1E-07 OCXO.
 
That may not seem like much, but it actually is for a phase locked  system 
(2.44E-011 is 24.4ns over 1000s).
 
As an example, we have a unit running in Mexico that has an average EFC  
tracking range of less than +/-20 Microvolts over the period of a typical day,  
meaning the control voltage remains within a 40 microvolt window over the  
entire day. See the plots at:
 
   _http://resco.ucol.mx/Fury/gpsstat.htm_ 
(http://resco.ucol.mx/Fury/gpsstat.htm) 
 
If you only have 12 bits - 4096 steps, into a 5V control range, then  this 
gives you a step size of 1.22mV, a huge difference of 61 to 1 between the  
maximum control window range on this particular OCXO, and your minimum 12 bit  step 
size.
 
Your system would be constantly switching between two steps, and generate  a 
massive amount of phase drift.
 
Also, it is fairly simple to cascade DACs to get say 24 bits resolution out  
of two inexpensive 16 bit DAC's etc.
 
There was a recent Design Idea in EDN or Electronic Design (don't remember  
which one) where someone claimed 32 bit resolution out of two 16 bit dacs. That 
 claim is ridiculous of course due to noise and matching etc, but you can  
probably get 20 - 24 bits resolution and accuracy out of a cascaded system with  
some care.
 
bye,
Said
 
  
In a message dated 12/1/2008 14:58:47 Pacific Standard Time,  
warrensjmail-one at yahoo.com writes:

Doesn't this imply that a Dac with more than about 12 bits becomes  
an overkill at 100 seconds tracking when using a 1E-7 turning range OSC,  
and around 18 bits for the same Osc when tracking at 1000 Sec?  

**********************************
>Bruce Wrote: 
> "Sometimes its cheaper and more convenient to build  a high 
> resolution DAC than use a pot and a conventional DAC. ...

Very true, And don't forget also more fun, more challenging, and more better!
which all very import  requirements in this field, maybe more so than performance.

Assuming  100 to 1000 seconds are the typical max tracking times 
practical in the standard GPSDO's due to temperature changes, 
initial aging, etc  as stated by Ulrich in the posting  
http://www.febo.com/pipermail/time-nuts/2008-December/034823.html 
and IF IE-10 is the best you get from the GPS signal in these same 
time averages as stated on Page #3 of  
http://www.thinksrs.com/downloads/PDFs/Catalog/PRS10c.pdf

Doesn't this imply that a Dac with more than about 12 bits becomes 
an overkill at 100 seconds tracking when using a 1E-7 turning range OSC, 
and around 18 bits for the same Osc when tracking at 1000 Sec? 

WarrenS



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