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

[Jim Harman]

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