On Sun, 10 Oct 2021 03:30:24 -0400, time-nuts-requ...@lists.febo.com wrote: time-nuts Digest, Vol 210, Issue 7
> Date: Sat, 9 Oct 2021 15:18:08 -0700 > From: ed breya <e...@telight.com> > Subject: [time-nuts] Re: HP Z3801A info needed - 1PPS interface > To: time-nuts@lists.febo.com > Message-ID: <61621520.9050...@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 _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.