Why does signal modulate when using Opamp for a linear filter

I created a linear filter with the Opamp block from Simscape Electrical. Since this is a linear filter, there should be no nonlinear distortion. However, when I decrease the frequency below a certain number, the signal becomes modulated and is therefore not linear anymore.

Answer 1

[MathWorks Support Team]

When debugging this issue, follow these troubleshooting steps:

 

1. If the simulation stop time is set to 'inf', change this to have a time to stop so you can better see the transient response and the transition into steady state.
   
2. This modulation of filter out data could be one or both of:
   1. Poor solver convergence
   2. Nonlinearity effects from the op-amp
3. To rule out poor solver convergence, check the solver settings:
   1. Check that the max time step is shorter than the AC frequency
   2. Ensure 'Absolute tolerance' is not set to 'auto' and 'Auto scale absolute tolerance' is off
   3. Try tightening the relative and absolute tolerances
4. Replace the op-amp with the foundation library op-amp. This op-amp has no nonlinearity                                              
   
5. Run the simulation and check filter output
   
6. Put the nonlinear op-amp back and explore the internal op-amp integrator using 'sscexplore'
   
7. It can be seen that the op-amp is power rail limiting (+-15V) and is also slew-rate limiting.

In summary, the op-amp in the model is power rail limiting and also skew-rate limiting. The nonlinearity is the cause of the output waveform modulation.

1. In the Solver Configuration Parameters, change the absolute tolerance from 'auto' and uncheck 'Auto scale absolute tolerance'.
2. Replace the 'Band-Limited Op-Amp' block with the 'Op-Amp' block from the Foundational Library.