Troubleshooting Frequency Response Estimation

When to Troubleshoot

If, after analyzing your frequency response estimation, the frequency response plot does not match the expected behavior of your system, you can use the time response and FFT plots to help you improve the results.

If your estimation is slow or you run out of memory during estimation, see Managing Estimation Speed and Memory.

Time Response Not at Steady State

What Does This Mean?

How Do I Fix It?

Possible Cause	Action
Model cannot initialize to steady state.	Increase the number of periods for frequencies that do not reach steady state by changing the `NumPeriods` and `SettlingPeriods` properties. See Modify Estimation Input Signals. Disable all time-varying source blocks in your model and repeat the estimation. See Effects of Time-Varying Source Blocks on Frequency Response Estimation.
(Sinestream input) Not enough periods for the output to reach steady state.	Increase the number of periods for frequencies that do not reach steady state by changing the `NumPeriods` and `SettlingPeriods`. See Modify Estimation Input Signals. Check that filtering is enabled during estimation. You enable filtering by setting the `ApplyFilteringInFRESTIMATE` option to `on`. For information about how estimation uses filtering, see the `frestimate` reference page.
(Chirp input) Signal sweeps through the frequency range too quickly.	Increase the simulation time by increasing `NumSamples`. See Modify Estimation Input Signals.

After you try the suggested actions, recompute the estimation either:

At all frequencies
In a particular frequency range (only for sinestream input signals)

To recompute the estimation in a particular frequency range:

Determine the frequencies for which you want to recompute the estimation results. Then, extract a portion of the sinestream input signal at these frequencies using fselect.
For example, these commands extract a sinestream input signal between 10 and 20 rad/s from the input signal input:
```
input2 = fselect(input,10,20);
```
Modify the properties of the extracted sinestream input signal input2, as described in Modify Estimation Input Signals.
Estimate the frequency response sysest2 with the modified input signal using frestimate.
Merge the original estimated frequency response sysest and the recomputed estimated frequency response sysest2:
1. Remove data from sysest at the frequencies in sysest2 using fdel.
  sysest = fdel(sysest,input2.Frequency)
2. Concatenate the original and recomputed responses using fcat.
  sys_combined = fcat(sysest2,sysest)

Analyze the recomputed frequency response, as described in Analyze Estimated Frequency Response.

For an example of frequency response estimation with time-varying source blocks, see Effects of Time-Varying Source Blocks on Frequency Response Estimation

FFT Contains Large Harmonics at Frequencies Other than the Input Signal Frequency

What Does This Mean?

When the FFT plot shows large amplitudes at frequencies other than the input signal, your model is operating outside the linear range. This condition can cause problems when you want to analyze the response of your linear system to small perturbations.

For models operating in the linear range, the input amplitude A₁ in y(t) must be larger than the amplitudes of other harmonics, A₂ and A₃.

$\begin{array}{l} u (t) = A_{1} \sin (ω_{1} + ϕ_{1}) \\ y (t) = A_{1} \sin (ω_{1} + ϕ_{1}) + A_{2} \sin (ω_{2} + ϕ_{2}) + A_{3} \sin (ω_{3} + ϕ_{3}) + ... \end{array}$

How Do I Fix It?

Time Response Grows Without Bound

What Does This Mean?

How Do I Fix It?

Possible Cause	Action
Model is unstable.	You cannot estimate the frequency response using `frestimate`. Instead, use exact linearization to get a linear representation of your model. See Linearize Simulink Model at Model Operating Point or the `linearize` reference page.
Stable model is not at steady state.	Disable all source blocks in your model, and repeat the estimation using a steady-state operating point. See Compute Steady-State Operating Points.
Stable model captures a growing transient.	If the model captures a growing transient, increase the number of periods in the input signal by changing `NumPeriods`. Repeat the estimation using a steady-state operating point.