Two different results in grapher for a Bode plot -- one's right, one's wrong. Why?

[flagstone3535]

I'm simulating a simple RLC band pass filter and looking at the output in grapher.

In the AC analysis setup screen, if I define the output as Vout/Vin and set up the vertical scale as decibel - the resulting graph is correct.

On the other hand, if I define the output as dB(Vout) - dB(Vin), or dB(Vout/Vin), or dB(mag(Vin))- dB(mag(Vout) using the add expression command and set up the vertical scale as linear -- the output is inverted (instead of a band pass filter -- the output looks like a notch filter and the magnitudes are wrong).

The second technique "used to" work - but truthfully I can't remember if this was in Multisim or in PSpice.

So, what am I doing wrong? 

[Mahmoud_W]

Hello,

When you set the vertical scale to linear in the AC Analysis window, only the magnitude of the output variable is evaluated to be drawn on the top graph whilewhether it is positive or negative is evaluated in the bottom graph, the phase graph (-1 = 180 phase shift).

When you choose a logarithmic scale, the 20*log of this magnitude is evaluated and it can be positive or negative according to whether this magnitude is greater or less than 1. By setting the vertical scale to linear and your output to an expression your basically drawing the magnitude of your expression (any of the three you mentioned below) on your top graph which will always be a poisitve value.

Also, in your graph when you enter any of these expressions notice that the phase information has been ommitted (bottom graph) and has been replaced by a constant 180 which represents the negative sign of the magnitude.

Hope this explains the results you were getting.

Mahmoud W

National Instruments