[time-nuts] Re: Returnee, interested in Lars GPSDO

[Bob Camp]

Hi

Since you only get one sample per second, no matter what you do, why bother with
the 4046 at all? 1 us will roll over at 1 ppm off frequency ( which is very possible with
an OCXO ). You still need “something else” to get things close. 

Bob

> On Dec 22, 2022, at 7:52 PM, Jim Harman via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> You will notice that Lars' design divides the 10 MHz down to 1MHz then uses
> the R-S flip flop phase comparator in the HC4046 chip with the 1 pps from
> the GPS to make a pulse every second that goes high on the rising edge of
> the PPS and goes low on the next rising edge of the 1 MHz. A simple diode -
> R-C circuit produces a voltage proportional to the pulse width and thus the
> phase difference, and an A/D converter in the Arduino measures this voltage
> each second.
> 
> The HC4046 phase comparator with the 1 MHz gives it a lock-in range of 1
> usec, which helps it acquire and maintain a lock.
> 
> On Thu, Dec 22, 2022 at 1:32 PM Bob Camp via time-nuts <
> time-nuts at lists.febo.com> wrote:
> 
>> Hi
>> 
>> In a GPSDO, you are comparing 1 pps signals. The typical PLL chips really
>> aren’t made for doing
>> this sort of thing. There are several very simple ways to do an adequate
>> job of measuring the time
>> difference between the two PPS signals.
>> 
>> One of the reasons for wanting to read the time difference directly is to
>> incorporate the sawtooth
>> correction output from your GPS module. The other big reason is the
>> difficulty of coming up with
>> R/C filters that get you out to the hundreds of seconds range ….
>> 
>> Bob
>> 
>>> On Dec 22, 2022, at 12:22 PM, john.haine--- via time-nuts <
>> time-nuts at lists.febo.com> wrote:
>>> 
>>> Hello everyone, I have just re-joined the list after being on
>> pendulum-nuts
>>> for a while (where I still am!).  I was impelled to join as I'm thinking
>>> about a new clock timer, I have dabbled in the past with an OCXO and one
>> of
>>> TVB's picPETs.  I just got some new surplus OCXOs and the thought
>> crossed my
>>> mind of disciplining it to GPS - why not start another project?
>>> 
>>> 
>>> 
>>> Tom directed me to a number of websites with GPSDO designs and the "Lars
>> DIY
>>> GPSDO" looks interesting being relatively simple and Arduino based.  One
>>> thing about it does puzzle me though and I thought I'd seek your
>> collective
>>> wisdom.  It uses a 4046 PLL chip just for its RS-flipflop type phase
>>> comparator which then drives a simple time interval comparator (TIC).
>> What
>>> surprises me is that it doesn't use the 3-state phase/frequency detector
>> -
>>> in principle this has the benefit of automatically frequency locking.
>> This
>>> is the type of PD I have used in synthesisers in the past.  Is there a
>>> reason why the unit works this way, can anyone advise please?
>>> 
>>> 
>>> 
>>> Have a great Christmas everyone, and Happy New Year.
>>> 
>>> 
>>> 
>>> *     John Haine
>>> 
>>> 
>>> 
>>> 
>>> 
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> 
> 
> -- 
> 
> --Jim Harman
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