HP 54121A Cal Problem in TDR Section of Cal

Started by Hawaii596, 10-30-2009 -- 07:26:06

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Hawaii596

10-30-2009 -- 07:26:06
I'm doing a 54121T (54120B mainfrm with 54121A 20 GHz test set).  All was going fine until I got to the Chan 1 TDR section.  I even pulle the USAF TO, Army TB and OEM Svc Manual for 54121A and instructions were identical.

In a nutshell there is the part at the beginning where you have a characterized 50 Ohm termination. You set up in the REFLECT mode and to the autoset part (sorry I'm at home and don't have the procedure in front of me). 

Expected results are 0 mV and I get 200 mV (which is the high side of the little square wave.  I then try moving forward to the precise edge find, and it can't find the edge - no edge found.

From tinkering, it looks like even with delay set to 16 nS, there is an improper delay that makes it so at minimum delay, I am stuck too far along (timewise) on the TDR's step square wave.

I even pulled a spare (not recently cal'd) - just the 54121A test set from the store room, and got identical results.  Next thing I'm doing is to pull the whole spare system to see if it's related to that big interface cable or to the mainframe. 

Before I dive into the alignment procedure, I wanted to see if anyone has specific experience with this and may know if it is a common problem, has Agilent dropped the test from the procedure, does it sound like a repair problem, or does it sound like some recognizable incorrect thing I may be doing.  I am going lock step verbatim with the TDR cal instructions.  I've run the section at least a half dozen times to make sure I am staying lock step.

Any thoughts highly appreciated.
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind."
Lord Kelvin (1824-1907)
from lecture to the Institute of Civil Engineers, 3 May 1883

Bryan

#1
11-10-2009 -- 15:35:34
I have just completed the test on my 54121, when I install the term at the end of the adapter (I'm testing with the little 3.5 f-f installed) and turn on the step I have about a nanosecond from the left side of the display to the step (1nS/div, delay 16 nS, Delay Ref at Left).  When you say autoset are your not referring to the Auto-Scale on the front panel instead of auto level set in the V Markers menu.  It's kind of a booger but I didn't have any problems as you describe.  I'd suggest you get a good default (one key power up) and set up a single screen, turn off ch 2, 3 & 4, then go to network, selct reflect and activate the step & ch 1 key and you shold see it.   

Hawaii596

#2
11-10-2009 -- 18:57:37
I finally figured it out, and no thanks to the procedure or slim pickings for manuals available.

The procedure did not help and the manual didn't help.  I did the factory reset, autoscale, and everything else.

Except... it turned out to be a channel skew adjustment.  The end user had mal-adjusted the channel skew adjustment so far on channer 1 (the TDR channel) that the scope couldn't correct for it.  It made the  square wave in the internal step function think it was supposed to be at the HIGH side of the wave when  it was at the LOW side, and the leading edge of the pulse was way off to the "left" of screen.  I reset
the channel 1 skew back to zero, and voila, everything was fine.  A second unit came in a couple of days later that didn't have the problem (giving me at least confidence my test setup was working).

I can't believe a factor preset didn't clear out that skew, and no mention was made in the manual or procedure.
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind."
Lord Kelvin (1824-1907)
from lecture to the Institute of Civil Engineers, 3 May 1883

dminesinger

#3
11-10-2009 -- 20:33:44
Quote from: Hawaii596 on 11-10-2009 -- 18:57:37
I finally figured it out, and no thanks to the procedure or slim pickings for manuals available.

The procedure did not help and the manual didn't help.  I did the factory reset, autoscale, and everything else.

Except... it turned out to be a channel skew adjustment.  The end user had mal-adjusted the channel skew adjustment so far on channer 1 (the TDR channel) that the scope couldn't correct for it.  It made the  square wave in the internal step function think it was supposed to be at the HIGH side of the wave when  it was at the LOW side, and the leading edge of the pulse was way off to the "left" of screen.  I reset
the channel 1 skew back to zero, and voila, everything was fine.  A second unit came in a couple of days later that didn't have the problem (giving me at least confidence my test setup was working).

I can't believe a factor preset didn't clear out that skew, and no mention was made in the manual or procedure.


If I was a betting man, I'd bet your notes on this unit now have something about this for the future.

PapaBear
PapaBear

*'It's the Veteran, not the reporter who has given us the freedom of the press.'
'It's the Veteran, not the poet, who has given us the freedom of speech.'
''It's the Military who salutes the flag, who serves beneath the flag, and whose coffin is draped by the flag, who allows the protester to burn the flag.'

Bryan

#4
11-11-2009 -- 13:45:58
Thanks for sharing your resolution of the issue, fortunatly for me few people here know how to work it beyond what a cal procedure tells them to do.
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