[time-nuts] Papers on timing for lunar laser ranging

EWKehren at aol.com EWKehren at aol.com
Sun Jul 16 08:21:35 EDT 2017

As part of our GPSDO work Richard Mc Corkle and I tested multiple DAC's  
using hardware I developed and Richard wrote the code. We limited to 
affordable  and solderable. The LTC1655 was the clear winner because of linearity and 
 temperature, see attached. We tested dithering 20 bits and stacking two 
for  coarse fine, storing the test data from using the LTC2400 ADC. Limited 
our  choices to dither and bare use. We did this 5 years ago. Contact with 
Richard  has sadly stopped. I am very concerned, however we continue to use his 
 contributions on several projects with very good results. We use the LTC 
1655 on  Rb's because its resolution and range as is, is perfect. 
Five years later I know no better alternative
Bert Kehren
In a message dated 7/16/2017 5:09:08 A.M. Eastern Daylight Time,  
attila at kinali.ch writes:

On Sat,  8 Jul 2017 11:42:44 -0700
Tim Lister <listertim at gmail.com>  wrote:

> Forgive the ignorance, but why is there a large disparity  between ADC
> and DAC capabilities ?
> For example, Linear  Technology sell a 24 bit ADC for ~$7 but an 18 bit
> DAC is  $30-50...

Much simplified, it boils down to it being easier to measure  voltage
differences by averaging than keeping a voltage  constant.

E.g. in those >20bit ADC's you will usually find a  delta-sigma ADC,
usually 3rd to 5th order with a 1.5 to 5 bit ADC/DAC  inside. The ADC
and DAC can be laser trimmed to be in the order of 0.1% of  their
ideal values. With a few additional tricks you can get the most  of
the remaining non-linearity out. These tricks also help to  remove
errors due to DC-offsets in the signal path. But the  biggest
improvement comes from averaging over many "samples" to get  the
white noise out. If you look at the usual sample rates at  which
those ADC reach their "full" performance, it is around 
1-30  (output) samples per second.

On the other hand, on a DAC you need to  keep the output voltage
stable. You can do the same delta-sigma approach as  with the ADC
with much the same result, but you have one big problem:
it  is not easy to build an analog low pass filter that has a corner
frequency  down at 10Hz. This means, you have to work at a much higher
frequency to  have a low pass filter that can be realized (let's say 1kHz
if you are  building a discrete filter, higher if it's integrated).
But that means that  you have several orders of magnitude more (white) 

Additionally,  a lot of people expect to do a couple of 1000 samples
per second at least,  to have a usefull DAC. But that contradicts the
need to have a narrow band  low pass filter to get the noise out.

Attila Kinali
It is upon moral qualities that a  society is ultimately founded. All 
the prosperity and technological  sophistication in the world is of no 
use without that  foundation.
-- Miss Matheson, The Diamond Age, Neil  Stephenson
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