kb8tq@n1k.org said:

When was the first GPSDO shipped as a commercial product?

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 one
with a reduced range and a knob on the side to adjust the center point of
that range?  You would have to adjust that knob occasionally as the crystal
you are tuning drifted.

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On 7/7/17 3:14 PM, Hal Murray wrote:

Note that these papers are talking about optical ranging to the
reflectors left on the moon by the Apollo missions, but the actual work
was being done recently (e.g. it's a Microsemi 5071 Cesium clock)

An interesting question - at least 20 years ago - XL-DC manual, Rev E,
from 1997
http://glacier.lbl.gov/gtp/DOM/Support/xl-dc-manual.pdf

My mid 2000s 10 MHz OCXOs from Wenzel have both EFC and a manual
adjustment of some sort (I'm not sure what's under the little cap on the
side.. a trimmer cap or something?)

Hi

Not in a commercial system ….

Bob

On Fri, Jul 7, 2017 at 4:14 PM, jimlux jimlux@earthlink.net wrote:

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...

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 ?

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.

Tim

Top unit is the GR 1164 synth sig gen.
I see what looks like an HP 105 but a slightly modified one.
Also the Sulzer.
Stuff on the left looks home brew.
Regards
Paul
WB8TSL

On Wed, Jul 12, 2017 at 3:27 PM, Tom Van Baak tvb@leapsecond.com wrote:

HI

Like the GR synthesizer above the GR frequency standard? Both are from the “dark gray” era rather than the later “light gray” period.

Not quite sure about the gizmo top left (next to the Austron) with the CRT? in the middle of the pannel.

Bob

The gizmo with a CRT is a Specific Products WWV receiver. I'm not sure exactly which model, some had the ability to show on the CRT the CRT phase between local clock and WWV via pips, others would show phase via Lissajous figures.

Tim N3QE

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

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) noise.

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

Hi:

The Quantic Timing GPS receiver makes use of patent 5440313which results in a 48 bit DAC.  How does this idea compare?
http://www.prc68.com/I/Q5200.shtml
https://www.google.com/patents/US5440313
PS the above patent cites 4582434 i.e. the Heathkit GC1000 HFDO.

--
Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html

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