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

EWKehren at aol.com EWKehren at aol.com
Fri Oct 27 14:36:13 UTC 2017


Living in south Florida I have been through 8 hurricanes and uncountable  
thunderstorms, while being a time-nut. At no time a hold over, because of  
excellent power backup. Once a controlled power shut down in Miami because I 
had  to make a choice between window Air conditioner, refrigerators and 
coffee maker  after 3 days. Lab lost out. Yes holdover when disconnecting 
antennas for lab  work and modifications. Did in no way help. Ambient temperature a 
much bigger  problem.
In my humble opinion holdover is for commercial applications, it would be  
nice if we would spend more time discussing how to improve commercial 
products  not intended for time-nuts application. Lot of room for improvement.
Bert Kehren
 
 
In a message dated 10/26/2017 5:23:39 P.M. Eastern Daylight Time,  
aphid1 at comcast.net writes:

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 <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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