[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 1960â€™s and you
>>> into the â€œseveral new carsâ€ range. Even for NASA projects cost
>>> make it
>>> into the equation â€¦.
> 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
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,
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
>> There is an interesting tradeoff in GPSDO design. With a specific DAC,
>> 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
>> 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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