Oscilloscope problems

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Froskoy

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Hi,

Whenever I try and measure a constant DC voltage on my oscilloscope (HP 54501A 100MHz digitizing oscilloscope), it always takes about three sweeps of the screen to get up to the actual voltage (will upload video of necessary).

Is there a way to get a straight line to go to the voltage you want immediately?

I think its something to do with the trigger but don't think it's possible to turn the trigger off.

Any ideas?

Thanks very much,

Froskoy.
 
What settings is the scope at? What is the circuit the scope is connected to? You've really not provided us with any information to answer your question you've only stated the problem you have.

It's kind of like going to a car mechanic and telling him it's making a clunking noise and expecting him to tell you the parts and labor cost up front without even looking under the hood.
 
Sorry,

connected directly across a 12V DC power supply.

Channel
5V/div
Offset 0V
DC

Timebase
200ms/div
delay 0s
reference right
window off

Trigger
Edge trigger: Auto
Edge (from options edge, pattern, state, delay, tv)
Level 5V
Holdoff 40ns

Video of what is happening:

YouTube - oscope problem

I would like the line to go straight to 12V, not go to 12V after about 20s or so.

switching to a lower timebase helps a bit but does not solve the problem.
 
Maybe you need some load on your supply. try the test with a 100K resistor and 100 picofarad load ie. add a few load parts on circuit. I dunno, also speed up your scope, no need to go that slow.
 
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Your Youtube video shows very strange behavior. It goes to the final voltage in steps. Does the 'scope seem to work normally for AC waveforms?
 
Thanks very much for the reply. That's exactly the problem.

It works fine for AC waveforms.

Thanks again.
 
The time the scope takes to fill its display buffer is tiny compared how long it takes to display the buffer. It is in some sort of "holdoff while I display the content of the RAM" mode. This is why it is a really bad idea to use a digital storage scope where a dirt-simple analog scope would do There are a zillion ways to get garbage out of a digital scope...
 
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I don’t know if this applies to your scope or not. The two scopes that I have, have a special probes that is used for DC. The Hi frequency probe may have caps and resistor in them. When I used Hi frequency probe and attempt to measure DC it only measures a very low level DC and it take time to get to that max value.
 
Digital 'scopes can occasionally give strange results (much less so with the latest designs) but I have never seen one exhibit that sort of behavior, so perhaps there's a problem with the unit.
 
I think Mikebits might be on to something. If the load on the supply is extremely light (such as when connected to an oscilloscope) it takes the supply longer to stabilize as there is effectively no load, load it down with a power resistors that's 5% of the supplys rated amperage and test it again. It may also be a perfectly normal power up sequence for that power supply.

So more than likley it's not the scope but you're observing normal but unknown to you behavior of the power supply.
 
Thanks all for your generous help. It is very much appreciated.

MikeMl said:
The time the scope takes to fill its display buffer is tiny compared how long it takes to display the buffer. It is in some sort of "holdoff while I display the content of the RAM" mode.
Click to expand...

Thanks MikeMI. This got be looking through the settings on the scope and I found under display there was an option set to "avg" and when I set this to "off" everything was how it should be.

Thanks though Mikebits, crutschow, Tedfred and Sceadwian - I'm sure I'll have these problems again in the future and will now know how to fix them.
 
I use a 500 MHz hp 'scope much like that one.

I was about to suggest turning the averaging off when I saw that you had found it. The averaging is occasionally useful but often gives strange results.

You could try the scroll mode that is useful when the timebase is that slow.
 
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