Electronics > Repair

FLUKE PM3394B sampling strange behavior

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cafi95:
Hi everyone,

I'm a French newone on that forum and I would have been glad to introduced myself  on the forum, but I haven't found where. So I do it on that thread.

I'm a former electronic  & TCM ingenier long years ago but I still work on electronic devices (vintage audio, ham radio, laser biological applications ... and lot of other stuff I generally design by myself).

So, recently, I've experienced a tricky issue on my Fluke PM3394B 4 channels combiscope (full options equiped)

I'm using that outstanding device from more than ten years without any problem.

Description of the issue :

Until MTB is not less than 250ns/div, everything is ok.

But below (so from 200ns to 2ns), the sampling rate appears to be much too slow.

On the trace, sampling dots can be clearly seen ...

In addition,  when a signal is active and the input mode is toggled to ground, it takes a lot of time for the trace to return to the ground state. It comes to that state dot after dot...

At the 2ns MTB setting, it needs more than 5s for the display to stabilize at a ground level.

Of course,  below 250ns/div, no single event can be displayed,  for the same reason.

Question : do you experience such a behavior or is it a scope issue  ?

To be noticed :

- I'm sure every parameter has the right setting.

- everything is nominal in analog mode.

Many thanks for any advice.

All the best.

Philippe  ;)

wn1fju:
Similar behavior on my PM3390B Combiscope.  It is because below 250 ns, it uses equivalent time sampling.  In other words, the scope constructs a picture of the input signal by accumulating the samples over many wave cycles.  Thus it takes time for the screen to change and the display to settle down.

You will note that it is not a time-base problem with your unit since the scope will work fine in analog mode below 250 ns/div. 

You can turn off the equivalent time sampling in the TB menu and you will see that the scope won't allow you to go less than 250 ns/div. 

A lot of the early digital scopes used equivalent time sampling.  Works great for repetitive signals only.

cafi95:
Many thanks for your answer   :-+

That was exactly my thoughts but I was'nt quite sure about it (and as a matter of fact, setting the real time sampling to ON, limits the TB to 250ns/div) ...

Fine !

The spec (and the user manual) are not very clear on that point.

So, the best to catch a single event of that speed is to set the TB at 250 ns/div and then, to magnify the trace ...

My incoming night will be peaceful  :=\

All the best. ;)

Philippe

HighVoltage:
Under 250 ns, the PM3394B scope goes into a special acquisition mode.
Maybe you can post a picture of how it looks.

 

cafi95:
[attachimg=1]

You're right.

On that example, channel 1 has just been grounded. Just notice that the other trace (still busy) is affected by the same behaviour. That's quite logical as the sampling is on both channels  ::)

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