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

EWKehren at aol.com EWKehren at aol.com
Sat Jul 8 17:30:20 EDT 2017

Having used Brooks Shera's GPSDO since 1998 with RB's never OCXO's I still  
am convinced it is the best out there. The problem was the DAC which is not 
 intended for this application. Brooks was working on a LTC1655 replacement 
but  cancer stopped that work.We use the 16 bit LTC1655 with very good 
results. You  ask about resolution and range. With 4 E-14 steps, range is 2.7 
E-9 very  acceptable for any Rb, with 1 E-14 the range is 6.75  E-10. With our 
FRK  test results and my age of 75 I will be glad if I have the opportunity 
to adjust  it once.
We use in our work gate arrays because it is easy to correct mistakes, but  
before getting to know Juerg I did a Brooks 100 MHz board with discrete 
IC's  presently still available from DigiKeys.
In a message dated 7/8/2017 2:43:01 P.M. Eastern Daylight Time,  
listertim at gmail.com writes:

On Fri, Jul 7,  2017 at 4:14 PM, jimlux <jimlux at earthlink.net> wrote:
> On 7/7/17  3:14 PM, Hal Murray wrote:
>> kb8tq at n1k.org  said:
>>> Consider that in 1974, I could buy a  nice new car for less than what a
>>> decent packaged 16 bit DAC  cost.  Go back into the 1960’s and you 
>>>  up
>>> into the “several new cars†  range. Even for  NASA projects cost 
>>> make it
>>> into the  equation ….
> Note that these papers are talking about  optical ranging to the 
> left on the moon by the Apollo  missions, but the actual work was being 
> recently (e.g. it's a  Microsemi 5071 Cesium clock)

Right, this is the "third generation" of  laser ranging. APOLLO started
in 2007, so given
the usual delays in  obtaining grant funding and purchasing, the tech
(pre-upgrade to
the  5071A) is going to be early 2000s tech.

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...

>> When was the first GPSDO shipped  as a commercial product?
> An interesting question - at least 20  years ago - XL-DC manual, Rev E, 
> 1997
>  http://glacier.lbl.gov/gtp/DOM/Support/xl-dc-manual.pdf

I also  found it interesting that the paper says that the GPSDO uses a
2000 sec  Kalman
filter. I've heard of Kalman filters being used for GPS navigation  but
not in timing use, although
I gather things like Thunderbolts use a  ~1000 sec loop constant - is
this the same form of
filtering or have  different forms of filtering become more popular and
Kalman filtering is no  longer
used ?

>> There is an interesting  tradeoff in GPSDO design.  With a specific DAC,
>>  you
>> can get finer steps if you reduce the tuning range.  Has  anybody built 
>> with a reduced range and a knob on the side to  adjust the center point 
>> that range?  You would have to  adjust that knob occasionally as the
>> crystal
>> you are  tuning drifted.
> My mid 2000s 10 MHz OCXOs from Wenzel  have both EFC and a manual 
> of some sort (I'm not sure  what's under the little cap on the side.. a
> trimmer cap or  something?)

Right. Apparently the DAC values have changed by  3500 over the ~11
years, which given the
1.2e-11 DAC steps would give an  accumulated change of 4.2e-8, agreeing
with the typical and
quoted  ~1e-11/day drift/aging for a good  OCXO.

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