[time-nuts] I've been thinking about a GPS receiver experiment

Bob kb8tq kb8tq at n1k.org
Thu Oct 26 10:33:06 EDT 2017


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 <aphid1 at comcast.net> 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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