# Difference in phase response between circuit simulation and MATLAB

I have the following low pass filter: simulate this circuit – Schematic created using CircuitLab

When simulating through Circuitlab, I get the following phase response (I think you should be able to get this by simulating the linked circuit, If I embedded it correctly): This looks consistent with what I directly calculated the phase response to be: $$\ \phi = arctan(\frac{-2 \omega RC}{1-\omega ^2 (R C)^2}) \$$. The full transfer function is $$\ H(\omega) = \frac{1}{1- \omega ^2 (R C)^2 + 2 j \omega R C} \$$. (Both resistors and capacitors are the same).

However if I use the bode plot function in MATLAB, I get the following phase response: I'm given that the second plot is correct but I'm not sure why there's a difference in the simulation result and MATLAB result. Possibly I made some errors in setting up the simulation but I've used component values chosen by the solution to this design question.

I've also tried using calculators like Desmos to directly plot the phase response but it looks different from both the simulation and MATLAB result, but a bit more like the simulation.

Did I make an error in the transfer function, circuit simulation setup, etc?

MATLAB code:

R = 3900;
C = 0.0102*10^(-6);
H = tf(,[R^2*C^2, 2*R*C, 1]);
bode(H);

• Why don't you plot both over the same frequency range for a start. Jul 16, 2020 at 9:21
• @Andyaka I updated the plots. The MATLAB version does not include the step at ~180 kHz.
– knzy
Jul 16, 2020 at 9:36
• The step is irrelevant; -180.0000 degrees is +180.0000 degrees. You are being swayed of target by the step - it doesn't actually exist. Jul 16, 2020 at 9:41