Why does it give oscillations when Orcad doesnt and it shouldnt?

[Battelle]

I have identically created a circuit in Orcad's pspice and in multisim.  Orcad does not yield oscillatory results, while Multisim does.  I have tested this circuit in the lab and there should be 0 oscillations, it should arrive at steady-state.  What is wrong with Multisim?

[lacy]

If you don't post your circuit we have no way of knowing  what type of circuit you have and more importantly we no have basis for analysis on which to draw a conclusion. If you wouldn't care to post it, then maybe we may be of more assitance.

[Battelle]

Sorry here is the circuit.

[lacy]

Unfortunatly, I don't have Multisim 10 (right now I can't fork over the cash). I do have Multisim Power Pro 2001. I hate to impose, but if you could possibly post a picture of your circuit I can then enter it into my version and see if I get the same results. This would be very kind of you do to this. I want to help as much as I can.

[Battelle]

Picture

[Maxish]

Hello Battelle,

Based only on the symbols you see on schematic capture, it is not very convincing to say that you have an identical circuit because it may very well be that the model parameter of your components are different. For example, the parameters for diode 1n914 may not have been modeled by the same person/company and so it may differ. A much more convincing case that shows disparities between simulators would be to compare the results of the same netlist (or equivalent netlist since each vendor may have its own varient of circuit and model syntax).

I have looked at your circuit and I can't make any conclusions based on what you provided. I have stripped out virtual instruments and replaced pots with resistors so as to make the underlying netlist more or less compatible with generic SPICE. I exported this netlist and ran a simulation on Pspice and two other vendor's simulators. In Pspice, I got the following strange error:

INTERNAL ERROR -- Invalid Number in device C52, Divide

In the two other simulators, I got results identical to Multisim, probing collector node of Q5 (where your scope was hooked up).

So, right now I have no reason to believe that Multisim is doing something wrong.

I can help you further if you provide more information, in particular your Pspice design file.

Thanks,

[Battelle]

The node of interest is the collector of Q5.  I have attached just the front half of the 'equivalent circuits' (b/c while I have the full version of multisim, I only have a demo of orcad).  A screen shot of pspice, multisim, and the actual multisim file.  Would you also mind explaining how to export a multisim netlist into pspice?

Thanks,

Battelle

[Maxish]

Battelle,

C51 is your problem. At 1uF, the output does not appear to oscillate.

To export, go to transfer->export netlist. You can load that in the pspice text editor program and append some SPICE-like simulation commands. Multisim supports most pspice syntax and devices but the converse is not true - you will see many errors, each of which has to be dealt with separetely. I can help you go from pspice to Multisim, but I will need a good premise to help you go in the other direction.

Thanks

[lacy]

O.K. I have simulated your circuit with my Multisim 2001 Power Pro and her are my findings.

If you have built this circuit and it works fine the way you have it, then it stands to reason that your design and component values are correct. Then it has to be something in the simulator. I found that when I go to the Simulate Tab>Default Instrument Setting and click Set to Zero in the dialog box your circuit oscillates wildly. When it is set to Automatically Determine Initial Conditions then everything works just fine with no oscillations.

Apparently, given the above information, that the other programs you have tried this on are automatically determining intial conditions therefore they do not show the oscillations.

Try going into the Simulate>Default Instrument Setting and in the dialog box at the top click on Automatically Determine Initial Conditions and this should cure the oscillations and hopefully give you the results you are looking fior.

I hope I was helpful to you. Have a nice day

[lacy]

O.K. I have simulated your circuit with my Multisim 2001 Power Pro and her are my findings.

If you have built this circuit and it works fine the way you have it, then it stands to reason that your design and component values are correct. Then it has to be something in the simulator. I found that when I go to the Simulate Tab>Default Instrument Setting and click Set to Zero in the dialog box your circuit oscillates wildly. When it is set to Automatically Determine Initial Conditions then everything works just fine with no oscillations.

Apparently, given the above information, that the other programs you have tried this on are automatically determining intial conditions therefore they do not show the oscillations.

Try going into the Simulate>Default Instrument Setting and in the dialog box at the top click on Automatically Determine Initial Conditions and this should cure the oscillations and hopefully give you the results you are looking fior.

I hope I was helpful to you. Have a nice day