matlab-manage-sparameters

S-Parameter I/O and Visualization

Load, inspect, visualize, and export S-parameters using the sparameters object and RF Toolbox plotting functions.

When to Use

Loading Touchstone files (.s1p, .s2p, .s4p, .snp) into MATLAB
Extracting S-parameter values with rfparam
Plotting S-parameters with rfplot or smithplot
Computing group delay from S-parameter data
Interpolating S-parameters to a new frequency grid with rfinterp1
Changing reference impedance with newref
Exporting S-parameters to Touchstone format with rfwrite

When NOT to Use

Converting between network parameter types (S, Z, Y, ABCD) -- use matlab-convert-network-parameters
Cascading or de-embedding S-parameter networks -- use matlab-deembed-rf-cascade
Fitting rational models to S-parameter data -- use matlab-fit-rational-model
Amplifier stability or gain analysis -- use matlab-analyze-rf-amplifier

Workflow

Load — Construct sparameters from Touchstone file, raw data, or circuit object
Inspect — Check ports, frequencies, impedance; extract parameters with rfparam
Transform — Interpolate with rfinterp1, re-reference with newref
Visualize — Plot with rfplot, smithplot, or compute group delay
Export — Write to Touchstone with rfwrite

Core Object: `sparameters`

Construction

% From Touchstone file (most common)
s = sparameters('device.s2p');

% From raw complex data
s = sparameters(data, freq, Z0);       % data: N×N×K, freq: K×1 (Hz), Z0: scalar or vector

% From other network parameter objects
s = sparameters(yObj);                  % Y → S (uses yObj's impedance)
s = sparameters(zObj, 75);              % Z → S with explicit Z0

% From RF circuit objects
s = sparameters(circuitObj, freq);      % circuit, rffilter, nport, transmission line, RLC

Properties

Property	Description
`Parameters`	N×N×K complex array — indexed as `(row, col, freqIdx)`
`Frequencies`	K×1 vector in Hz (ascending)
`Impedance`	Reference impedance — scalar or per-port vector
`NumPorts`	Number of ports (read-only)

Per-Port Reference Impedance (R2023a+)

s = sparameters(data, freq, [50 75]);   % Port 1: 50 Ohm, Port 2: 75 Ohm

Gotcha: When NumPorts == numel(Frequencies), MATLAB cannot distinguish per-port from per-frequency impedance. Use reshape(z0, 1, 1, []) to force frequency-dependent interpretation.

Quality Checks

ispassive(s)                            % true if max singular value of S <= 1 at all freqs
iscausal(s)                             % true if network is causal (no output before input)
p = passivity(s);                       % max singular value per frequency (passive where <= 1)
sp = makepassive(s);                    % enforce passivity by clipping singular values to <= 1

All four work directly on sparameters objects (no rational fit required). passivity(s) returns a vector showing where violations occur — useful for diagnostics before calling makepassive. makepassive(s) modifies data point-by-point — for optimization-based passivity enforcement on a fitted model, see matlab-fit-rational-model.

IEEE P370 Quality Metrics (R2023b+)

[cm, rm, pm] = ieee370QualityCheckFrequencyDomain(s);
% cm = causality metric (0-100, higher is better)
% rm = reciprocity metric (0-100)
% pm = passivity metric (0-100)

Returns percentage scores per the IEEE 370 standard — quantitative grades beyond pass/fail booleans. Accepts a sparameters object, filename, or raw 3-D array.

Extracting Parameters

s21 = rfparam(s, 2, 1);                % Complex S21 across all frequencies
s21_dB = 20*log10(abs(s21));            % Convert to dB magnitude
s21_phase = angle(s21)*180/pi;          % Phase in degrees

Gotcha: rfparam returns complex values, NOT dB. Always use 20*log10(abs(...)) for dB magnitude.

Interpolation

fNew = linspace(1e9, 6e9, 500);
sInterp = rfinterp1(s, fNew);          % Interpolate to new frequency grid
sExtrap = rfinterp1(s, fNew, 'extrap'); % Allow extrapolation outside original range

Gotcha: rfinterp1 interpolates the real and imaginary parts of each S-parameter independently (like interp1), not magnitude and angle. This can produce artifacts near sharp resonances or rapid phase transitions.

Change Reference Impedance

s75 = newref(s, 75);                   % Re-reference from 50 to 75 Ohm

Gotcha: sparameters(filename, Z0) is not a valid call signature — it errors with "Too many input arguments." Always load first, then use newref to change impedance.

Export to Touchstone

rfwrite(s, 'output.s2p');
rfwrite(s, 'output.s2p', 'FrequencyUnit', 'GHz', 'Format', 'RI', 'ForceOverwrite', true);

Format options: 'MA' (magnitude/angle, default), 'DB' (dB/angle), 'RI' (real/imaginary).

Gotcha: 'DB' format writes -Inf for any zero-valued S-parameter (e.g., S12=0 in an isolator), producing a file that sparameters cannot re-read. Use 'RI' for lossless round-trip fidelity.

Visualization

`rfplot` — Rectangular Plots

rfplot(s);                              % All S-parameters, magnitude in dB
rfplot(s, 2, 1);                        % S21 only
rfplot(s, [1 2], [1 2]);               % S11, S12, S21, S22
rfplot(s, {[2 1]; [1 1]});             % S21 and S11 specifically
rfplot(s, 'diag');                      % Diagonal only (S11, S22, ...) — reflection coefficients
rfplot(s, 'triu');                      % Upper triangular portion

Plot types via the plotflag argument:

rfplot(s, 2, 1, 'db');                 % Magnitude in dB (default)
rfplot(s, 2, 1, 'angle');              % Phase in degrees
rfplot(s, 2, 1, 'abs');                % Linear magnitude
rfplot(s, 2, 1, 'real');               % Real part
rfplot(s, 2, 1, 'imag');               % Imaginary part

Multi-panel layout with tiledlayout:

figure;
tiledlayout(2, 2);
nexttile; rfplot(s, 1, 1);       title('S11 (dB)');
nexttile; rfplot(s, 2, 1);       title('S21 (dB)');
nexttile; rfplot(s, 1, 1, 'angle'); title('S11 Phase');
nexttile; rfplot(s, 2, 1, 'angle'); title('S21 Phase');

`smithplot` — Smith Chart (R2017b+)

Prefer smithplot over the legacy smith function.

smithplot(s, 1, 1);                    % S11 on Smith chart
smithplot(s, [1,1; 2,2]);             % S11 and S22 together

With name-value customization:

sp = smithplot(s, 1, 1, ...
    'TitleTop', 'Input Match', ...
    'LegendLabels', {'S_{11}'});

Gotcha: Use smithplot for modern workflows with name-value customization. The legacy smith function lacks these capabilities.

Group Delay

gd = groupdelay(s);                    % S21 group delay for 2-port (default)
gd = groupdelay(s, 2, 1);             % Explicit S21
% gd is in seconds — multiply by 1e9 for nanoseconds

figure;
plot(s.Frequencies/1e9, gd*1e9);
xlabel('Frequency (GHz)'); ylabel('Group Delay (ns)');
title('S21 Group Delay');

Gotchas

rfparam returns complex, not dB -- Always use 20*log10(abs(rfparam(...))) for dB magnitude. rfparam itself returns raw complex values.
sparameters(filename, Z0) is invalid -- Cannot pass reference impedance at load time. Load first, then use newref(s, Z0).
rfinterp1 interpolates real/imag independently -- Not magnitude and angle. Can produce artifacts near sharp resonances or rapid phase transitions.
'DB' format in rfwrite writes -Inf for zeros -- Zero-valued S-parameters (e.g., S12=0) produce -Inf in DB format, which sparameters cannot re-read. Use 'RI' for lossless round-trip fidelity.
Per-port vs per-frequency impedance ambiguity -- When NumPorts == numel(Frequencies), MATLAB cannot distinguish per-port from per-frequency impedance. Use reshape(z0, 1, 1, []) to force frequency-dependent interpretation.
Use smithplot, not smith -- The legacy smith function lacks name-value customization. smithplot is the modern replacement (R2017b+).
Non-ASCII characters in Touchstone comments cause read errors -- Degree symbols, em-dashes, or other non-ASCII characters in comment lines (starting with !) cause sparameters to error. Fix with setTouchstoneReader('IgnoreComments', false) before reading (R2024b+). The second argument is a persistence flag -- use false to avoid the setting persisting across MATLAB sessions. Restore default behavior with setTouchstoneReader('ParseComments', false).
File extension does not need to be .snp -- The Touchstone 2.0 reader (used by sparameters) accepts any file extension, not just .s2p/.s4p/.snp. Non-standard extensions (.txt, .dat, etc.) produce a warning ("The extension .xxx is unrecognized. Attempting to read the file using the Touchstone format.") but succeed if the file content is valid Touchstone. This is a Touchstone 2.0 relaxation -- Touchstone 1.0 required .snp extensions.
.amp files not supported by sparameters -- AMP files (bundling S-params, noise, power data) are not readable by sparameters or nport. Use the legacy rfckt.amplifier with read(rfckt.amplifier, 'file.amp') to access AMP data.

Legacy `extract` Function

The extract function belongs to the legacy rfckt/rfdata API. It returned S-parameter data (and other quantities like noise figure, group delay) from legacy objects. Common issue: it returns cached data and may appear to give identical results for different inputs if the object was not re-analyzed after property changes.

Modern equivalent: Use rfparam(s, i, j) for complex S-parameter extraction and s.Frequencies for the frequency vector. Other quantities have dedicated functions: groupdelay(s), noisefigure(...), etc.

Scope Note

RF Toolbox handles circuit-level S-parameter analysis (loading, plotting, cascading, de-embedding, fitting). It does NOT provide RF propagation modeling (path loss, fading channels, ray tracing). Propagation is in Communications Toolbox (propagationModel), Phased Array System Toolbox (fspl, gaspl), and Antenna Toolbox (txsite, coverage, raytrace).

Conventions

Use tiledlayout/nexttile for multi-panel figures (not subplot)
Always label axes with units (GHz, dB, ns) and include figure titles
Use rfparam to extract parameters — not squeeze(s.Parameters(i,j,:))

Quick Reference: Sample Touchstone Files

RF Toolbox ships with example files for testing:

File	Description
`default.s2p`	Active 2-port (amplifier-like, non-passive)
`passive.s2p`	Passive 2-port network
`default.s4p`	4-port for differential/mixed-mode analysis
`lnadata.s2p`	Low-noise amplifier data
`measured.s2p`	Measured device data