[time-nuts] Ublox neo-7M GPS

SAIDJACK at aol.com SAIDJACK at aol.com
Tue Aug 19 17:23:29 EDT 2014


Hi Tom,
 
last time I looked at these I tried figuring out what they were doing. It  
is very hard to get measurement data, our TSC did not converge on their 
signal,  and looking at the output on a scope revealed only a bunch of crazy 
random phase  jumps. I guess one could use a counter to measure how many time 
pulses are being  sent in x seconds with x being a large number, or divide 
the output by 10  million and see how the pulse moves back and forth compared 
to the 1PPS UTC  output..
 
Since I don't know the exact algorithm being used, I said  
"adds/drops/extends/retards" in my previous email. I did not mean to imply that  the unit is 
doing all or any of those items. But that is exactly part of the  problem 
isn't it, there is no clear description of what exactly is happening in  the 
uBlox documents or the CW docs for that matter that I could find.
 
I for one would not use that output to drive a processor or other digital  
device directly, who knows what happens if the processor sees a 100ns, then 
a  110ns, and then an 70ns pulse if it is only rated at 10MHz and 100ns 
pulse-width  +/- a couple percent for example.. Without knowing the exact 
minimum phase  time period specification that could come out of one of these 
NCO's, one should  not properly use that signal in a digital design.
 
My initial concern was that this is time-nuts, and we should call a GPSDO a 
 GPSDO, and an NCO an NCO in my opinion. Nothing wrong with one or the  
other, but they sure are not the same thing - by 6 or more orders of  magnitude 
in phase stability. We usually are concerned here about parts per  trillion 
stability and accuracy, and now we are mixing things up that are  millions 
of times worse than one another..
 
bye,
Said
 
 
In a message dated 8/19/2014 13:08:52 Pacific Daylight Time,  
tvb at leapsecond.com writes:

Hal, as  long as you maintain long-term phase lock it's a disciplined 
oscillator. So,  yes, a carrier tracking WWVB receiver with sufficiently stable 
flywheel LO is  a WWVBDO.

Said, too-short or too-long 100 ns cycles is one thing. Still  ok for many 
applications. But tell me more about extra or missing pulses in  the 
ublox-7. That sounds like a show stopper to me.

/tvb  (i5s)

> On Aug 19, 2014, at 2:05 PM, Hal Murray  <hmurray at megapathdsl.net> wrote:
> 
> 
>  SAIDJACK at aol.com said:
>> its not a GPSDO though, not even a simple  one :)
>>  It does not discipline an oscillator. It generates  the output by
>> mathematically calculating how many phases it has to  add/drop in a 
second,
>> then   digitally  adds/drops/extends/retards the phase of the output 
clock to
>>  achieve an average of number of desired clock cycles.
> 
> Is  there something about the term GPSDO that says I have to do the "D" 
in the  
> analog domain rather than the digital domain?
> 
> I  agree that current technology doesn't give results that are useful for 
many  
> applications that currently use GPSDOs.  What if the clock ran  at a GHz? 
 10 
> GHz?  Sure, it would have spurs, but would it  be useful for some 
applications?
> 
> Is a GPSDO still a GPSDO if  the D/A driving the VCXO only has a few 
bits?  
> How many bits  does it need to be a real GPSDO?
> 
> Is a battery powered wall  clock listening to WWVB at 2 AM a WWVDO?  It's 
got 
> a pretty good  ADEV if you go out far enough.
> 
> -- 
> These are my  opinions.  I hate spam.
> 
> 
> 
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