Orcad PSpice help - plot axis in dB uA

[DHambley]

As many of the Orcad users may have noticed, the Orcad/PSpice forums are getting less helpful over the years. Maybe someone here knows the solution...

I'm analyzing an EMI filter using the FFT function in PSpice. (The FFT is done after a transient, not an AC sweep). I use the "add trace" function and tell it I want
20*LOG10(I*1E6)
I can't get the Y axis to display this and the display does not yeild the proper value. For example, if the FFT shows 50uA, when I add the trace (with the above equation) it should give me 34dB on the Y axis. Instead, it's some bogus value.
Anyone have experiance with this?
thanks

ps I'm not looking for the AC transient response with a 1VAC input. I have the actual inputs from the PWM model.

 

[VEBill]

What "bogus value" is it returning? Perhaps there's a clue as to what's happening in that mysterious value. One should be able to 'reverse-engineer' what's going on from a few examples.

 

[DHambley]

VEBill,
I didn't see a way to reverse engineer it after messing with several different ways of plotting in for an hour on thursday. I'm suspecting that PSpice may get fooled into combining both the AC and the DC components of the waveform before it calculates the log. The FFT should be on the AC components only.

So, you haven't seen a solution to this? Do you use Cadence PSpice?

 

[davidbeach]

I know nothing about PSpice, but why would you expect the DC component to be omitted when doing an FFT? The DC component is where all even harmonics come from.

 

[DHambley]

If anyone who knows how to get a dB scale in PSpice, please post the solution you know of. If people know nothing about PSpice or if people don't know what "FFT" means, it just wastes time posting.

 

[jimkirk]

It sounds like you're asking SPICE for something like FFT(20*LOG10(I*1E6)). The last time I used OrCAD SPICE (before my hard disk crashed) there's a button to click for FFT and another button you can click to get a logarithmic Y scale.

 

[DHambley]

jimkirk - perfect! When I typed in The FFT(20*LOG10(I*1E6)) function that you wrote, PSpice gave the correct value, then I just displayed the scale in log.
THANKS.