[time-nuts] Re: NTP servers

[folkert]

RPI does allow GPIO PPS input
Also the RPI4 does PTP.

On Tue, Nov 23, 2021 at 12:53:02PM +0200, Avamander wrote:
> Speaking of Raspberry Pi's as time servers, does anyone here know of a nice
> single board computer that supports both Ethernet hardware timestamping and
> GPIO PPS input?
> 
> On Tue, Nov 23, 2021 at 12:49 PM McFail Troll <time.isanapp at gmail.com>
> wrote:
> 
> > > If you have a good local NTP setup, you can measure how good other
> > servers are, or more likely how good the network connection is.  NTP
> > assumes the network delays are symmetric.  Often, that's not true.  So "how
> > good" turns into measuring network (a)symmetries.
> >
> > Yes this is what I'd like to do eventually. Trying to get a Raspberry pi +
> > gps setup going first though.
> >
> > On Mon, Nov 22, 2021 at 5:42 AM Hal Murray <halmurray at sonic.net> wrote:
> >
> > > time.isanapp at gmail.com said:
> > > > but I am curious to see if I could sync up with some of you guys who
> > > seem to
> > > > have some pretty cool set-ups.
> > >
> > > GPS has taken over the time-distribution business.
> > >
> > > If you want to use GPS, there are 2 ways to go.
> > >
> > > You can get a simple GPS receiver and plug that into your PC.  For decent
> > > time, you need a PPS signal and a real serial port, not USB.
> > >
> > > The other approach is to get a GPSDO -- GPS Disciplined Oscillator.
> > > That's a
> > > box with a GPS receiver and a good crystal and some software.  It
> > smoothes
> > > out
> > > the rough edges in the timing from the GPS signal and/or keeps going
> > > (holdover) when the GPS signal fades or dies.
> > >
> > > GPS is very very good in the long term but noisy in the short term.
> > Short
> > > means seconds to minutes.  Long means days/months.  A GPSDO will get rid
> > > of
> > > most of the short term noise.
> > >
> > > There are/were several models available at relatively low cost after they
> > > were recycled from cell phone towers.  HP Z3801A and Trimble Thunderbolt
> > > are popular.  "GPSDO" will get lots of hits on eBay.  I don't know how
> > good
> > > the recent ones are.
> > >
> > > Just reading the info available can be fun if you like that stuff.
> > >   http://www.realhamradio.com/GPS_Frequency_Standard.htm
> > > The Z3801A manual is a good read.
> > >   http://www.leapsecond.com/museum/z3801a/097-z3801-01-iss-1.pdf
> > > Time sink warning!
> > >
> > > GPS works much better if you have a good antenna location.  Modern GPS
> > > receivers are sensitive enough that they sometimes/often work indoors,
> > > maybe better on a window sill, maybe even better if the window faces
> > > south.  Just because it's working now doesn't mean it won't glitch often
> > > enough to cause problems.  Mumble.  Try it and see what happens.
> > >
> > > ---------
> > >
> > > There is actually a 3rd way: buy a box that does it all.  But that's not
> > > much
> > > fun, at least for most of the people on this list.
> > >
> > > ----------
> > >
> > > > there are plenty of good stratum 1 NTP servers open to the public (e.g.
> > > > NIST's servers),
> > >
> > > If you have a good local NTP setup, you can measure how good other
> > servers
> > > are, or more likely how good the network connection is.  NTP assumes the
> > > network delays are symmetric.  Often, that's not true.  So "how good"
> > turns
> > > into measuring network (a)symmetries.
> > >
> > >
> > > --
> > > These are my opinions.  I hate spam.
> > >
> > >
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Folkert van Heusden

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