[time-nuts] UBLOX LEA-5T Programming?

[Mike Cook]

> Le 9 oct. 2015 à 16:16, Clint Jay <cjaysharp at gmail.com> a écrit :
> 
> I am still learning and want to understand, if the PPS is good then why is
> the programmable output bad, as I understand it thus far, the PPS is
> derived from the same clock source or have I got that badly wrong?

Yes, the same clock, BUT the output signal rising edge is aligned with the nearest internal clock pulse leading edge, as is the 1PPS output, which gives rise to something called quantization error. If the configured frequency is an integral division of the cpu clock, then OK, but if it is not then this error is apparent. IIRC,  the on board cpu oscillator is 48MHz, and so an 8MHz output will be OK, but 10MHz does not divide evenly into 48Mz so you get significant jitter.  

There is a nice Ublox doc «  Timing_AppNote_(GPS.G6-X-11007).pdf » illustrating this. I don’t know if there was one issued for the 5T receiver, but the principle is the 
same. Google should find it.


> 
> On 9 October 2015 at 12:16, Bob Camp <kb8tq at n1k.org> wrote:
> 
>> Hi
>> 
>> Doing a GPSDO by locking to the awful 10 MHz output from any of these GPS
>> modules
>> is not going to work very well. Given the very long time constants
>> involved in a GPSDO
>> control loop, doing it without code is going to be pretty difficult.
>> 
>> A much easier approach:
>> 
>> Grab the GPS PPS and a scope. Use it to set the Lucent boxes on frequency.
>> Repeat the process
>> once a week.  The direct lock will flop around by (likely many) ppb. Your
>> manual set will probably keep it
>> under 1 ppb for a week.
>> 
>> Bob
>> 
>>> On Oct 9, 2015, at 1:39 AM, Perry Sandeen via time-nuts <
>> time-nuts at febo.com> wrote:
>>> 
>>> Hi,
>>> I have six of the older style (anodized solid aluminum case) Lucent
>> crystal oscillators that I’d like to turn into simple 10 MHZ output
>> GPSDO’s. I have some salvaged HP cases so I can have a linear PS and all
>> other stuff in one package.
>>> I’ve found two different choices of GPS relatively low cost units on
>> Ebay.
>>> One is a LUCENT M12+ GPS Timing Receiver W/100 Hz output  for  $19.99
>> Item 171886538138.
>>> The other is a UBLOX LEA-5T GPS module dev board 1PPS /USB/RS232/ntp ser
>> with two 1 PPS outputs for $49.90. Item 251785217093.
>>> IIRC, one output of a Ublox can be set to a 10 MHz.
>>> Can this model be set to have a 10 MHz output and is it easily done from
>> a PC? I have no coding ability.
>>> For more money I’d get a simpler phase detector circuit, which is all
>> I’m looking for. For less money I’d have to build more circuitry.
>>> A factor I don’t know is if the ublox is that much better/newer to
>> justify the price difference.
>>> I already have packed away two Oncore units with either a 1 KHz or 10
>> KHz output.
>>> Suggestions welcomed.
>>> Regards,
>>> Perrier
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> 
> 
> 
> -- 
> Clint.
> 
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