[time-nuts] yet another GPSDO design, or so

EWKehren at aol.com EWKehren at aol.com
Tue Jun 29 13:34:59 EDT 2010


Hi
for many 16 bits will do and one other alternative is do a design that will 
 also work with the AD 1861. I know it is hard to get but in the last year 
I  found a sizable volume on ebay, convinced them to lower their price, 
bought some  my self and posted it on time-nuts. With a group effort I am sure 
we can find  some sources.
There are also old audio devices out there that have them enbedded.
Bert
 
 
In a message dated 6/29/2010 12:54:37 P.M. Eastern Daylight Time,  
lists at rtty.us writes:

Hi

Some of the TI (Burr Brown) 16 bit parts are 1/4 lsb DNL  on > 98% of the
transitions. Most of the time you have a "coarse" DAC  that's at 18 bits.
Some of the errors are predictable and you can take them  out with a simple
training process. You won't easily get 24 bits, but 20 is  very achievable. 

20 bits is a nice round million to one factor. A 1  ppm EFC becomes a 1 ppb
step if everything is perfectly linear. Even with a  4:1 slope ratio it's
still quite small. With a more restricted EFC of  2.5x10^-8 you are at
1x10^-13 with the 4:1 slope ratio.   

Bob

-----Original Message-----
From:  time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of  Attila Kinali
Sent: Tuesday, June 29, 2010 12:42 PM
To: Discussion of  precise time and frequency measurement
Subject: Re: [time-nuts] yet another  GPSDO design, or so

On Tue, 29 Jun 2010 09:17:39 -0700 (PDT)
Stanley  Reynolds <stanley_reynolds at yahoo.com> wrote:

> If we lower  the size of each step to over lap more would this
> lower the  error ?  Software would adjust both converters at the
> cross over  point so neither would change it's full range at this point.
> Two 12  bit converters would form one 18 or 20 bit converter.

Yes, you can do  that. With good D/A converters, the non-linearity
is within 1-2LSB, so if a  full range of the fine D/A is more than,
lets say 4LSB of the coarse D/A  you can minimize the step in control
voltage, when you switch from one fine  D/A range to the next. But
it will be always there, you cannot get  completely rid of it.

There are two things that make this step  problematic:
1) If it happens automatically, it will result in problems  
when doing long term measurements using that GPSDO as reference.
When  doing the switch manually triggered (ie the GPSDO says that
it needs to  adjust the coarse D/A and does so only after the
user presses a button),  then unattended operation over long periods
is not possible  anymore.

2) Each adjustment of the coarse D/A is a non-linear  operation. It is
basically a step function excitation of the control loop.  With such
a non-linear element within the control loop, the software has  to
take either precautions during the period of the non linearity or
the  control loop has to be able to cope with such steps, without
starting to  oscillate.

> I guess taken to the extreme we could interweave any  number of
> converters so hopefully the errors would average out  :-)  64 times
> 12 bit converters making a 20 bit converter each  with an equal
> contribution at each step. 

Actually, the  less components you have, the better. Not only do
more components add more  non-linearity (it is very difficult to
average multiple components if they  arent R's and C's), the
whole circuitry involved produces also  noise.

Attila Kinali
--  
If you want to walk fast, walk alone.
If you want to walk far, walk  together.
-- African  proverb

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