[time-nuts] Ublox neo-7M GPS

Bob Camp kb8tq at n1k.org
Tue Aug 19 17:51:14 EDT 2014


Hi

They are constrained by the same basic TCXO issues that give you sawtooth correction. They do not use EFC to get the TCXO on frequency. With sawtooth they give you a word that lets you know what’s going on. With the NCO’s they often are doing very crude synthesis. They don’t put a $48 DDS chip in a $10 GPS module. If you put one on a spectrum analyzer, it’s not pretty ….

Bob

On Aug 19, 2014, at 5:46 PM, Said Jackson <saidjack at aol.com> wrote:

> Tom,
> 
> Btw part of my frustration with this is that we sometimes get calls from customers asking why they need our or others' GPSDOs for a couple 100 dollars when they can buy a CW or uBlox doing "the same thing" for a fraction of the cost.
> 
> Most of them come back to us after evaluating these NCOs and finding that its not the same thing.
> 
> You get what you pay for I guess..
> 
> Said
> 
> Sent From iPhone
> 
>> On Aug 19, 2014, at 13:01, "Tom Van Baak (lab)" <tvb at leapsecond.com> wrote:
>> 
>> 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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