Simulink Frequency Response

Simulink Frequency Response Estimation

Obtain frequency response data from Simulink models using simulation-based estimation (frestimate) when analytical linearization is not viable or as a validation tool.

When to Use

• Model contains hard discontinuities (PWM, switching, relay, quantizer) that linearize to zero or NaN
• Validating a linear model against a simulation based linearization of a Simulink model
• Estimating frequency response directly from simulation data

When NOT to Use

• Model linearizes cleanly with linearize — use simulink-linearize instead
• Working with measured data only (no Simulink model) — use System Identification Toolbox

Workflow

1. Define I/O  →  2. Choose Signal  →  3. Configure  →  4. Estimate  →  5. Fit (optional)
   (linio)          (PRBS/Sinestream)    (constraints)    (frestimate)     (tfest)

Stage 1: Define I/O Points

Determine I/O points for the estimation using this decision sequence. Use the first case that applies:

Case A — IO points can be inferred from prompt or model context:

Use the first sub-case that matches:

1. User specifies explicit I/O signals or blocks (e.g., "from r to y") → define linio points. All linio points must reference a block's output port. If a candidate block has no output ports (Outport, Terminator, Scope) → trace upstream to find the source block and port with model_read.

   io = [linio(sprintf("%s/InputBlock", mdl), 1, "input"); ...
         linio(sprintf("%s/OutputBlock", mdl), 1, "output")];
   

2. Root-level Inport/Outport blocks exist → use model_read at root scope (depth "0") to identify root-level Inport/Outport blocks, then define linio at those blocks (trace Outport blocks upstream to their source for the output linio point).

Case B — Cannot determine IO points:

If none of the above apply → do not guess. Ask the user which signals to use as estimation input and output. Present the available blocks/signals from the model to help them decide.

Pre-flight checks (before choosing a signal):

1. Verify I/O points are NOT at blocks without output ports (Outport, Terminator, Scope)
2. Check sample times at I/O points — both must match the perturbation signal rate, or both must be continuous
3. Consider whether the model has time-varying source blocks (Step, Ramp, Signal Generator, etc.) that could drive the system away from its steady-state operating point during estimation. If so, see Disabling Time-Varying Sources below.

Stage 2: Choose Perturbation Signal

Decision (follow in order):

1. Is broadband estimation sufficient (most cases)? → Use frest.PRBS (DEFAULT)
2. Is the model discrete? → Use frest.PRBS with Ts matching the I/O sample time, or frest.createFixedTsSinestream if per-frequency data needed
3. Do you need precise magnitude/phase at specific frequencies? → Use frest.Sinestream (continuous) or frest.createFixedTsSinestream (discrete)

Prefer frest.PRBS — it estimates the full frequency range in a single simulation. Sinestream simulates each frequency sequentially and is significantly slower for broadband estimation.

Stage 3: Configure the Signal

PRBS (default):

in = frest.PRBS(Ts=Ts, Amplitude=0.01, Order=10, NumPeriods=2);

If the I/O signal is discrete, set Ts to match the signal sample time. If continuous, set Ts to a value that provides sufficient temporal resolution. Choose Amplitude small enough to stay in the linear regime of saturations/nonlinearities.

Why PRBS first? A single PRBS simulation estimates the full frequency range at once. Sinestream simulates each frequency sequentially — for 30 frequencies with 8 periods each, this can take 10-100x longer. Use Sinestream only when you need precise per-frequency data (e.g., gain/phase margin at specific crossover frequencies).

Sinestream (continuous models):

in = frest.Sinestream(Frequency=logspace(-1, 2, 30), Amplitude=0.01);
in.NumPeriods = 8;
in.SettlingPeriods = 3;

The filtering constraint: NumPeriods - SettlingPeriods >= 3 when ApplyFilteringInFRESTIMATE = "on" (default). Violating this throws an error at estimation time.

Fixed-Ts Sinestream (discrete models):

Ts = 0.01;
in = frest.createFixedTsSinestream(Ts, {wmin, wmax});
in.Amplitude = 0.01;
in.NumPeriods = 8;
in.SettlingPeriods = 3;

Use the cell syntax {wmin, wmax} for the frequency range — this auto-selects frequencies that are valid integer divisors of the sampling frequency. Do NOT pass an explicit frequency vector:

% CORRECT — cell syntax auto-selects valid frequencies
in = frest.createFixedTsSinestream(Ts, {wmin, wmax});

% WRONG — explicit vector (most frequencies violate integer-multiple constraint)
in = frest.createFixedTsSinestream(Ts, logspace(-1, 2, 30));  % Error

When to use Sinestream instead of PRBS:

• PRBS results are too noisy (high variance at individual frequencies)
• Need precise magnitude/phase at specific frequencies
• Very nonlinear system where broadband excitation causes intermodulation

Check simulation time: Always verify that the signal duration is practical before launching the estimation:

tFinal = getSimulationTime(in);
fprintf("Estimated simulation time: %.1f seconds\n", tFinal);

If tFinal is too big compared to Ts, use larger lower frequency bounds for estimation.

Stage 4: Estimate

Determine where to start the experiment. Choose one:

sysest = frestimate(mdl, op, io, in, opts);

op and opts are optional arguments. If op is not provided, the experiment will start at model initial conditions.

opts is a frestimateOptions object. Pass it when time-varying sources need to be disabled (see below).

The result is an frd (frequency response data) object.

Stage 5: Fit Parametric Model (Optional)

Only perform this step if a parametric model (transfer function, state-space, zpk) is required. If the goal is frequency response data only (e.g., Bode plot, gain/phase margins from frd), stop after Stage 4.

Convert the non-parametric frd to a parametric model:

sysFit = tfest(sysest, np, nz);
fprintf("Fit: %.1f%%\n", sysFit.Report.Fit.FitPercent);

Disabling Time-Varying Sources

Time-varying source blocks (Step, Ramp, Signal Generator, etc.) can drive the model away from its steady-state operating point during estimation. When this happens, the system does not remain near the operating point and the estimated response is unreliable — gain estimates can be off by orders of magnitude while executing without error.

When to disable sources:

• The model contains source blocks (other than the perturbation input) that change value during the estimation simulation
• Estimation results are implausible or don't match an expected linearization
• The time-domain response does not reach steady state at individual frequencies

How to identify and disable them:

Use frest.findSources to identify time-varying source blocks in the estimation path, then set BlocksToHoldConstant so they are held at their initial value during estimation:

srcblks = frest.findSources(mdl, io);
opts = frestimateOptions;
opts.BlocksToHoldConstant = srcblks;
sysest = frestimate(mdl, io, in, opts);

Note: frest.findSources requires model compilation. The perturbation input is not affected by BlocksToHoldConstant.

The Fallback Pattern

When linearize returns zero, follow this sequence:

% 1. Try linearize
sys = linearize(mdl, io);
if dcgain(sys) == 0
    % 2. Disable time-varying sources if present
    srcblks = frest.findSources(mdl, io);
    opts = frestimateOptions;
    opts.BlocksToHoldConstant = srcblks;
    % 3. Fall back to frestimate with PRBS
    in = frest.PRBS(Ts=Ts, Amplitude=0.01, Order=10, NumPeriods=2);
    sysest = frestimate(mdl, io, in, opts);
    % 4. Fit parametric model
    sysFit = tfest(sysest, 2);
end

Do NOT use manual block substitution (replace_block) as a workaround for zero linearization. The frestimate approach is generalizable to any discontinuous model without requiring domain knowledge of each block's averaged equivalent.

Key Functions

Common Mistakes

Conventions

• Always: Consider whether time-varying sources could drive the model from its operating point — use frest.findSources and BlocksToHoldConstant to disable them
• Prefer: frest.PRBS for broadband estimation — faster than Sinestream for most workflows
• Always: Use cell syntax {wmin, wmax} with frest.createFixedTsSinestream
• Always: Ensure NumPeriods - SettlingPeriods >= 3 when filtering is enabled
• Always: Place I/O points at rate-compatible blocks for multi-rate models
• Always: Call getSimulationTime — Validate signal duration before running. Long simulations relative to max solver step size should prompt redesign.
• Prefer: frestimate over manual block substitution for discontinuous models
• Prefer: tfest or ssest for fitting parametric models to frd results
• Prefer: Small amplitude — Keep perturbation small enough to stay in the locally linear regime (typically 1-5% of operating range).
• Never: Use replace_block as a general linearization workaround

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