[time-nuts] Re: pps pulse timestamp device

[Thomas Abbott]

Juan, For a quick and cheap measurement I've used the TDC7200 alone with a
10 MHz reference.
It only measures from the PPS to the next clock edge (it can't resolve the
100 ns ambiguity) but it's ideal for comparing GPS PPS to Lab 10 MHz. GPS
doesn't move more than 50 ns between pulses, and the TDC's 55 ps resolution
is quite good enough for GPS.

Use the $15 Mikroe TDC Click board
<https://www.digikey.com/en/products/detail/mikroelektronika/MIKROE-4770/15280756>
with two SMA connectors.
  GPS PPS connected to the Start input.
  10 MHz goes to the Stop input.  (Must be a 3.3 v square wave, it doesn't
work with a sine clock).
The TDC specifies a minimum Start-Stop time of 12 ns. In practice I've
found that with a periodic Stop signal, it wraps very cleanly with no bad
behaviour. ie. Stop is properly gated, so it either captures the stop at ~6
ns, or it skips that one and captures the following stop at ~106 ns.
Nothing in between. This is fine for GPS PPS which you can easily unwrap.
Trigger the TDC as soon as you're finished processing one measurement,
it'll wait for the next PPS.
The Mikroe has an on-board TCXO, quite good enough over these short
intervals.

I've used it on breadboard with a Pi Pico and micropython. For a slightly
more solid solution there's also a "hat" for the full size raspberry pi,
that carries two or three click boards.
Everything after the 100 ns still depends on the clock of course.


Thomas


>
> > -------- Original Message --------
> > Subject: [time-nuts] pps pulse timestamp device
> > Date: 2024-02-11 7:37 am
> > From: juan--- via time-nuts <time-nuts at lists.febo.com>
> > To: time-nuts at lists.febo.com
> >
> > Hi all,
> >
> > I am looking for a cheap device (e.g. Teansy, micropython board, etc)
> that would help me measure with ns resolution some pulses at one event per
> second rate (gps pps).
> >
> > It would need quite a stable clock such that any jitter would be
> attributable to the pulse signal.
> >
> > Any ideas what is the best way around this nowadays?
> >
> > Thanks,
> > Juan.
>
>