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

[Joseph Gwinn]

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