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

[Bob Camp]

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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