[time-nuts] Ublox neo-7M GPS

Bob Camp kb8tq at n1k.org
Tue Aug 19 21:46:18 EDT 2014


Hi

If you have to womp up a MCU anyway, there is no reason to put in a delay chip. It’s easier / faster / more accurate to just do it all in the MCU. You have to write and maintain custom code either way. 

Bob

On Aug 19, 2014, at 7:53 PM, EWKehren at aol.com wrote:

> I recall when the LEA-M8F was announced that they mentioned a VCTCXO and  
> maybe I wrongly assumed that they used it for sawtooth correction they also  
> mention ability to control in addition an external OCXO. I previously 
> suggested  using saw tooth correction information to tune a TCXO but that would 
> require a  GPS module with sawtooth information and than it would be simpler 
> to just use a  PIC and delay chip. Still do not understand why no one took me 
> up on the offer  of chips and PCB. I guess time nuts like to talk about it 
> but not fix it. How  many receivers are out there.
> Bert Kehren.
> 
> 
> In a message dated 8/19/2014 5:51:30 P.M. Eastern Daylight Time,  
> kb8tq at n1k.org writes:
> 
> 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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