Terrific points. There are so many levels of sophistication.
My own experience is with catastrophic signal loss on the reference.
Determining degradation on your primary reference can present
challenges. I once designed a device that compared three Cesiums
and switched the reference within one cycle if the amplitude of the
Cesium that was acting as the reference changed or the zero crossing
(10 MHZ) was a few nanoseconds out of spec relative to the other two
Cesiums. Nowadays they create ensembles of Cesiums and use them to
steer their timing systems while the Cesiums are steered by GPS.
Sophisticated Kalman filters are used to steer the signals based on
the properties of the signal sources.
The Microsemi 4145 Ultraclean Oscillator is designed for
catastrophic signal loss and freezes the DAC that controls the BVA
oscillator. This works well because even the DAC is ovenized. It
will also go into holdover if the input reference drifts too quickly.
It is pretty easy to make a simple temperature controlled box to
house your temperature sensitive references. Just provide lots of
insulation and control it at a temperature higher than your highest
expected ambient.
I never measured the temperature of oscillators and used the
information to compensate holdover but it makes sense with a
specific oscillator and enough run time to collect the data
to categorize the oscillator for temperature and ageing. This is
easier to accomplish when the DAC is is directly controlling the
oscillator. Since I prefer analog control loops, it could also be
done when the analog loop controls the oscillator if the DAC tracks
the analog loop control voltage. A comparator compares the DAC
output to the analog loop voltage. The DAC is adjusted to track and
thereby characterized so that it can be set to the correct value
when switched to holdover. As Bob pointed out this may or may not be
the last value of the DAC depending on the mode of failure of the
reference signal.
As Bob points out, there are very sophisticated ways of doing
temperature compensation today. As an example of his point, I was
told that the Microsemi CSAC (chip-scale atomic clock) uses
temperature compensation at many places in the design to achieve its
performance specs. I imagine that is the current ultimate in
temperature compensation for commercial products!
Bob M
On 10/26/2017 8:33 AM, Bob kb8tq wrote:
Hi
Most GPSDO’s do some sort of “slew” to an average DAC value when they go into
holdover.
Freezing at the last value is not (in general) a good idea. Often things
degrade before there
is a dropout. Your final DAC value may not be a good one to maximize holdover
duration.
Some setups try to “learn” temperature or aging. That gets fed into the DAC
when in holdover.
The value of this depends a lot on the quality of the training process.
Separating this and that
input to get a good value for a specific parameter is rarely done with good
accuracy. The exception
to that rule are oscillators that have a large TC or a very high drift rate. In
most cases those are not
the ones you pick for a GPSDO.
Bob
On Oct 25, 2017, at 7:46 PM, Bob Martin <[email protected]> wrote:
The holdover state is a DAC set to the last value of the analog control
voltage that adjusts the oscillator frequency. Some designs
use an analog control loop and switch the DAC into the control loop.
Others use the DAC to set the control voltage at all times. This can result in
a steps in the control voltage (output frequency).
I've used both methods and prefer the latter.
Bob M
On 10/25/2017 5:30 PM, Mark Sims wrote:
No, you set up an oscillator so that is why you have that problem.
I hooked the two rubidiums together just to see what would happen. It pretty
much did what I expected... chaos... the time-nut equivalent of a naughty
schoolboy putting a microphone up to the speaker of the public address system.
I't's a tough job, but somebody gotta do it ;-)
No, not really. The rubidium would be the real hold-over clock.
Symmetricom calls the disciplining state where it can't lock to the 1PPS signal the
"holdover" state. It's sort of like a GPSDO holdover state. Their discipline
firmware does let you set the time constant and damping values. I tried a little playing
around with them, but never found any settings that worked consistently well with the
LEA-5T.
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