[time-nuts] 1PPS for the beginner

Tim Shoppa tshoppa at gmail.com
Tue Aug 14 15:28:07 EDT 2018


With a scope it is super easy to look at PPS pulses that are microsecond to
a few milliseconds wide and tell you are triggering on the "leading edge".

Harder to decide at 50% duty cycle especially if you think it might have
been inverted along the way (almost all buffers are inverting).

Of course if you were 50% duty cycle you could figure out which edge was
which by listening to WWV. Oops not that topic again.

Tim N3QE

On Tue, Aug 14, 2018 at 2:52 PM, Bernd Neubig <BNeubig at t-online.de> wrote:

> Another question, a bit off the road of this thread:
> Is there any common practice for the duty cycle of the 1 PPS pulse?
> I have seen devices having extremely short 1 PPS pulses with duty cycles
> of a few percent or less and others with close to 50% duty cycle.
>
> Best regards
> Bernd
> DK1AG
>
> -----Ursprüngliche Nachricht-----
> Von: time-nuts [mailto:time-nuts-bounces at lists.febo.com] Im Auftrag von
> Bob kb8tq
> Gesendet: Dienstag, 14. August 2018 17:05
> An: Discussion of precise time and frequency measurement <
> time-nuts at lists.febo.com>
> Betreff: Re: [time-nuts] 1PPS for the beginner
>
> Hi
>
>
> > On Aug 14, 2018, at 10:33 AM, Ron Bean <time at rbean.users.panix.com>
> wrote:
> >
> >> A normal GPS module generates it \200\231s outputs off of a free
> >> running internal clock. Generally this is a TCXO in a
> >>  \200\234timing \200\235 GPS ( = one that puts out a rational PPS).
> >> This i bit counter intuitive, since you would *think* they phase lock
> >> the local source in the module. They don \200\231t mainly because it
> >> makes the math easier.
> >
> > Is there a need to make the math easier?
> > This is what we have microprocessors for.
>
> There is *always* a need to make the math easier if I’m quickly typing up
> an example. For proof I reference the hundreds of posts in the archives
> where I didn’t quite get it right :)
>
> Indeed, the real numbers could be just about anything. They are highly
> dependent on what’s in the specific module. In some cases, the clock
> involved is well over 300 MHz.
>
> Bob
>
> >
> > Thanks for the "hanging bridge" explanation. It would be nice if
> > someone had a time-nuts-glossary that included various terms that come
> > up on the list, like "hanging bridge", "spurs", "sawtooth correction",
> > "GPS allan ski jump", etc.
> >
> > Is there a time-nuts FAQ?
> >
> >
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>
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