[time-nuts] Ublox neo-7M GPS

[Bob Camp]

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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