matlab-convert-network-parameters

Network Parameter Conversions

Convert between S, Z, Y, ABCD, T, H, and G parameter representations. Compute mixed-mode (differential/common) S-parameters and reduce or reorder ports.

When to Use

Converting S-parameters to Z, Y, ABCD, T, H, or G representations
Computing mixed-mode (differential/common) S-parameters from single-ended data
Reducing N-port networks to M-port with snp2smp
Reordering ports in S-parameter data
Computing voltage transfer functions with s2tf
Verifying conversion fidelity via round-trip tests

When NOT to Use

Loading or visualizing Touchstone files -- use matlab-manage-sparameters
Cascading or de-embedding networks -- use matlab-deembed-rf-cascade
Fitting rational models to S-parameter data -- use matlab-fit-rational-model

Workflow

Start with S-parameters — Load via sparameters (see matlab-manage-sparameters skill)
Convert — Use object constructors for clean impedance tracking, or array functions for raw data
Validate — Round-trip convert (S→Z→S) to verify numerical accuracy

Object-Based Conversions (Preferred)

Object constructors automatically handle impedance bookkeeping. This is the modern approach.

z = zparameters(s);                    % S → Z
y = yparameters(s);                    % S → Y
a = abcdparameters(s);                 % S → ABCD
t = tparameters(s);                    % S → T (transfer/chain scattering)
h = hparameters(s);                    % S → H (2-port only)
g = gparameters(s);                    % S → G (2-port only)

% Convert back
sRoundTrip = sparameters(z);           % Z → S (uses z.Impedance)
sCustomZ0 = sparameters(y, 75);        % Y → S with 75 Ohm reference

All parameter objects share the same interface: .Parameters, .Frequencies, .Impedance, .NumPorts.

Port Count Limitations

Parameter type	Port requirement
S, Y, Z	N-port (any size)
ABCD, T	2N-port only
H, G	2-port only

Array-Based Conversions

Use when you need raw numeric arrays without object overhead.

yData = s2y(s.Parameters, s.Impedance);
zData = s2z(s.Parameters, s.Impedance);
abcdData = s2abcd(s.Parameters, s.Impedance);
tData = s2t(s.Parameters);             % No Z0 needed for T-parameters

Mixed-Mode (Differential/Common) S-Parameters

Convert 2N-port single-ended to N-port differential or common mode. Critical for high-speed serial link analysis.

s4 = sparameters('connector.s4p');

% Differential mode (Sdd)
sdd = s2sdd(s4.Parameters);            % Default: option 1 port pairing

% Common mode (Scc)
scc = s2scc(s4.Parameters);

% Cross-mode (differential-to-common and vice versa)
sdc = s2sdc(s4.Parameters);
scd = s2scd(s4.Parameters);

Port Ordering Options — The #1 Source of Mixed-Mode Errors

For a 4-port network, the option selects which physical ports form each differential pair:

Option	Differential Pairs (4-port)	Description
1 (default)	(1,2) and (3,4)	Adjacent ports paired
2	(1,3) and (2,4)	First half paired with second half
3	(1,4) and (2,3)	Input ascending, output descending

sdd_opt2 = s2sdd(s4.Parameters, 2);   % Option 2 port ordering

Gotcha: Verify which option matches your VNA port numbering before converting to mixed-mode. Wrong port ordering produces subtly incorrect results that look plausible but are wrong.

Gotcha: Differential impedance = 2 × single-ended impedance. When wrapping s2sdd output in a sparameters object, double the reference impedance:

sdd = s2sdd(s4.Parameters);
sDiff = sparameters(sdd, s4.Frequencies, 2*s4.Impedance);  % 100 Ohm for 50 Ohm single-ended

Port Reduction and Reordering: `snp2smp`

Reduce N-port to M-port by selecting ports and terminating the rest.

Gotcha: Argument order is snp2smp(s, portList, Z0) — the port list comes before the termination impedance. Swapping them (e.g., snp2smp(s, 50, [1 3])) treats the scalar as a port index and errors.

% Extract ports 1 and 3 from a 4-port, terminate ports 2 and 4 with 50 Ohm
s2 = snp2smp(s4, [1 3], 50);

% Swap ports of a 2-port
sSwapped = snp2smp(s, [2 1]);

% Terminate with different impedances per port
s2 = snp2smp(s4, [1 3], {50, 75, 50, 75});

Transfer Function

tf = s2tf(s);                          % Voltage transfer function (option 1, default)
tf = s2tf(s, 60, 75, 2);              % Zs=60, Zl=75, option 2: Vl/Vs

Options: 1 = Vl/Va (default), 2 = Vl/Vs, 3 = power-wave gain.

Gotchas

H and G parameters are 2-port only -- hparameters(s) and gparameters(s) error for networks with more than 2 ports. ABCD and T require 2N-port (even number).
Mixed-mode port ordering is the #1 error source -- s2sdd, s2scc, s2sdc, s2scd accept an option argument (1, 2, or 3) for port pairing. Verify which option matches your VNA port numbering before converting -- wrong ordering produces subtly incorrect results that look plausible.
Differential impedance = 2x single-ended -- If single-ended Z0 is 50 Ohm, differential Z0 is 100 Ohm.
snp2smp argument order: ports before impedance -- snp2smp(s, portList, Z0), not snp2smp(s, Z0, portList). Swapping them treats the scalar as a port index and errors.

Conventions

Prefer object-based conversions over array-based — they track impedance automatically
Always verify round-trip accuracy (S→Z→S) when conversion fidelity matters
Use rfparam to extract individual parameters from any network parameter object