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

[ed breya]

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