multisim problem

Thread Starter

charmcello

Joined Jan 24, 2007
12
Anybody use Multisim out there? I'm trying to run a simulation using Channels A and B on the oscilloscope simultaneously. However, I get only one trace on the o-scope simulation display (the other one completely disappears) unless the two channels are on different volts/division. It doesn't matter which way they go. Just that if they're on the same vertical scale, only one waveform appears on the display. I've done this before and not had this problem... so I don't understand. Did some setting get messed up?
 

JoeJester

Joined Apr 26, 2005
4,351
Did you use the offset ... vertical position ... to have the ground for one trace on a different graticule than the other?

How do you have your scope settings ...

Signal to be measured? Frequency and Amplitude

Ch 1: volts per division, offset, AC or DC coupled
Ch 2: volts per division, offset, AC or DC coupled

Sweep settings: A+B, B delayed, Alternate

All things one needs to consider when measuring a signal.
 

JoeJester

Joined Apr 26, 2005
4,351
A graticule are those funny graduated lines you see on the scope. Very useful when you want to know the voltage, offset, and period of a signal.
 

Thread Starter

charmcello

Joined Jan 24, 2007
12
OK, let's see.... as far as I can tell, I don't have a voltage offset. If I did, that would be apparent when the second trace reappears when I change the volts/division on one of the channels.

I have the "AC" button pushed on the bottom of both channels.

I don't know what a sweep setting is. I don't see anything like that on the simulation display.

Thanks for your replies, by the way :)
 

JoeJester

Joined Apr 26, 2005
4,351
What are you trying to measure? Can you post your schematic and your oscilloscope pic ... like I did below?

I know I'm not using Multisim, but the oscilloscope will have similiar functions.
 

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Thread Starter

charmcello

Joined Jan 24, 2007
12
By-the-way, this is what happens when I simply change the vertical scale for Channel B. The second trace also appears if I change the vertical scale for Channel A, as long as the scales are not the same.

I don't understand why this is happening - I've run this kind of simulation a bunch of times and never had this happen.

I tried closing Multisim and starting over from scratch, drawing the circuit over again. Same problem. :confused: I don't get it.
 

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JoeJester

Joined Apr 26, 2005
4,351
I can see auto triggering causing a problem if you change CH A, you can change that to Nor or Sing ... Normal or Single Sweep.

You are triggering at 0 volt, not the best choice. .1 volts or anything other than zero will help stability ... especially when using real scopes. I'm not sure if that instability is observed with multisim virtuals.
 

Thread Starter

charmcello

Joined Jan 24, 2007
12
I tried triggering at 100 mV, at Nor, Sing and Auto. No change :(

by-the-way, is Multisim used in the real world, or is it only a hokey student software application?
 

Thread Starter

charmcello

Joined Jan 24, 2007
12
I tried placing the capacitor in SERIES with the load resistor, and I got a normal scope display, with two traces. Did I set the parallel circuit up wrong (or put the scope probe in the wrong place)?
 

Thread Starter

charmcello

Joined Jan 24, 2007
12
I just realized I've never tried this simulation with a parallel circuit before - except with the oscope only over the source (using only one channel).
 

Thread Starter

charmcello

Joined Jan 24, 2007
12
Oh, JoeJester! Where are you?? It seems we are so close to solving the problem!! :)
Well, in case I don't catch you again today, thanks for trouble-shooting with me! :)
 

Thread Starter

charmcello

Joined Jan 24, 2007
12
Duh. The source and load voltages have nearly identical amplitudes (because it's a purely parallel circuit, disregarding the small source resistance). That's why one trace seems to disappear when the vertical scales for the two channels are the same. The traces are superimposed!
Now I feel stupid!
 

JoeJester

Joined Apr 26, 2005
4,351
By the way, the "offset" I described in one of the earlier posts is labeled "Y position" on your virtual oscilloscope. If you had offset one to +2 and the other to -2, you would have seen both signals.
 
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