[time-nuts] Re: HP Z3801A info needed - 1PPS interface

[paul swed]

I believe it is as thats the alternate strap on the z3801. Its also the way
z3801s arrives as RS 422.
Regards
Paul

On Sun, Oct 10, 2021 at 11:14 AM Joseph Gwinn <joegwinn at comcast.net> wrote:

> On Sun, 10 Oct 2021 03:30:24 -0400, time-nuts-request at lists.febo.com
> wrote:
> time-nuts Digest, Vol 210, Issue 7
>
> > Date: Sat, 9 Oct 2021 15:18:08 -0700
> > From: ed breya <eb at telight.com>
> > Subject: [time-nuts] Re: HP Z3801A info needed - 1PPS interface
> > To: time-nuts at lists.febo.com
> > Message-ID: <61621520.9050200 at telight.com>
> > Content-Type: text/plain; charset=utf-8; format=flowed
> >
> > I have the 1 PPS circuit working just fine. The pulse width is around 27
> > uSec, nice and flat and strong regardless of the termination. I can't
> > discern the rise time or prop delay yet.
> >
> > I discovered an interesting thing about the 1 PPS signals from the DB-25
> > connector. They are (or rather, one of them is) rather odd in voltage -
> > not PECL, except under certain conditions.
> >
> > I hooked up one of the 1 PPS outputs to the circuit, just with a pair of
> > wires. This gave me a chance to make some measurements out in the open.
> > The comparator circuit worked fine, and once I got a good view of the
> > result, I started looking into the details. The first thing I found is
> > that the quiescent "low" value of the "1 PPS_1-" (J3 P17) rests at about
> > 2.5 VDC - not PECL at all. The high side "1 PPS_1+" (J3 P9) seemed about
> > right, near 3.9 V. Uh oh - I thought maybe the port is damaged. I double
> > and triple checked the connections (they were right), then tacked some
> > wires on the number two port, pins 8 and 21.
> >
> > They behaved exactly the same, so probably normal - or both burned out
> > the same way. So, I figured there must be some logic to this big
> > asymmetry. It couldn't be terminations to ground, since the 2.5 V one
> > could only go lower, so differential is the only kind that makes sense.
> > I tried various values across the lines, and sure enough, the 2.5 V
> > level rose substantially with decreasing R, but did not reach a "proper"
> > PECL low level until the differential load was around 50 ohms. The high
> > side changed only a little, indicating it goes right to the output of an
> > ECL part - if it was reverse terminated it would have dropped much more
> > with the loading.
> >
> > So, it looks like these lines are connected to the outputs of ECL parts
> > (run as PECL), or maybe a simulation from some other kind of circuit. If
> > you picture each line being the emitter output, the high one is on most
> > of the time, and of proper level, You'd think the low one should still
> > hold at PECL low, at some current into its load, but it doesn't. It
> > could be that its load is made heavier, and to ground, on purpose,
> > drawing it down more. If it were terminated into a proper terminator
> > supply, it should be 2 V below Vcc, or 3 V in this case, so it couldn't
> > go to 2.5 V. Anyway, I understand what it's doing, but don't see why it
> > was made this way.
> >
> > Just in case, I checked these levels under different conditions - fresh
> > power-up, locked, and hold modes, to make sure the common-mode levels
> > aren't changed for external signalling of conditions. They were constant
> > in all conditions.
> >
> > Then I checked the signals on all the lines with a scope, directly
> > through coax. I tried a few different termination Rs, as shown below,
> > with the results.
> >
> > When the pulse goes active, the high side drops, and the low side rises,
> > to roughly the same as the DC levels, so only the terminator value and
> > end levels are needed to get the picture. Remember, these are
> > approximate, from eyeballing a scope trace flash once a second.
> >
> > Open circuit 3.9/2.5
> > 221 R 3.8/2.5
> > 100 R 3.7/2.6
> > 75 R 3.7/2.8
> > 47 R 3.7/3.2
> >
> > So, there's plenty of signal under all conditions, and I think it's just
> > a matter of picking a termination for whatever cable is used. I was
> > quite surprised by this oddity, but it seems to work fine with my
> > circuit no matter what.
> >
> > BTW the two 10 MHz outputs there are also described as "pseudo-ECL," so
> > I'd imagine they have the same characteristics. I'll take a look when I
> > get a chance.
> >
> > Ed
>
> Can this be RS-422 from a 5-volt source?
>
> .<https://en.wikipedia.org/wiki/RS-422>
>
> Joe Gwinn
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