Add README for SPARAMS MATLAB S-Parameter Toolbox

Added documentation for SPARAMS MATLAB toolbox including usage examples, properties, methods, and supported file formats.

README.md

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+# SPARAMS — MATLAB S-Parameter Toolbox
+
+A MATLAB class for reading, manipulating, converting, and plotting S-parameter data from Touchstone files (`.s1p`, `.s2p`, `.s3p`, `.s4p`). Supports up to 4-port measurements in **RI**, **MA**, and **dB/Angle** formats.
+
+---
+
+## Getting Started
+
+### Load from a Touchstone file
+```matlab
+s = SPARAMS('myfile.s2p');
+```
+
+### Create an empty object and assign data manually
+```matlab
+s = SPARAMS();
+s.setNumPorts(2);
+s.setZ0(50);
+s.f   = freq;   % frequency vector in Hz
+s.S11 = s11;    % complex S-parameter vectors
+s.S21 = s21;
+% etc.
+```
+
+### Copy an object (no pointer referencing)
+```matlab
+s2 = s.copyobj();
+```
+
+---
+
+## Properties
+
+| Property | Description |
+|---|---|
+| `f` | Frequency vector (Hz) |
+| `S11`, `S12`, ... `S44` | Complex S-parameters (up to 4-port) |
+| `Z0` | System reference impedance (e.g. 50 Ω) |
+| `numPorts` | Number of ports (1–4) |
+
+---
+
+## Methods
+
+### Setup & Configuration
+
+```matlab
+s.setFile('filename.s2p')      % Load a new file into an existing object
+s.setZ0(50)                    % Set system impedance manually
+s.setNumPorts(2)               % Set number of ports manually
+s.setFreqUnits('GHz')          % Set frequency axis units for plotting
+                               % Options: 'Hz', 'kHz', 'MHz', 'GHz'
+```
+
+### Accessing Data
+
+```matlab
+s11 = s.S11;                   % Get S11 as complex (R + jI)
+[S, fActual] = s.toMat(freq)   % Get full S-matrix at a specified frequency
+                               % (uses nearest frequency if exact match not found)
+```
+
+### Plotting
+
+```matlab
+s.plotdB('S11')                        % Plot a single parameter in dB
+s.plotdB({'S11', 'S21'})               % Plot multiple parameters in dB
+s.plotAlldB()                          % Plot all S-parameters in dB
+s.plotPolar({'S11', 'S21'})            % Polar plot
+s.plotSmith('S11')                     % Smith chart plot
+s.plotDispersion()                     % Dispersion diagram (for unit cells)
+s.plotDispersion('flip')               % Dispersion with flipped axes
+```
+
+### Parameter Conversions
+
+```matlab
+[Zin1, Zin2] = s.toInputImpedance()           % Input impedance at each port
+[A, B, C, D] = s.toABCDparams()               % ABCD parameters (all frequencies)
+[A, B, C, D] = s.toABCDparams(freq)           % ABCD parameters at one frequency
+[T11,T12,T21,T22] = s.toTParams()             % T-parameters (all frequencies)
+[T11,T12,T21,T22] = s.toTParams(freq)         % T-parameters at one frequency
+[Z11,Z12,Z21,Z22] = s.toZparams()             % Z-parameters
+[Y11,Y12,Y21,Y22] = s.toYparams()             % Y-parameters
+```
+
+### Manipulation
+
+```matlab
+s.renorm(Z0new)      % Renormalize to a new reference impedance
+                     % (also updates s.Z0 automatically)
+s.cascade(N)         % Cascade the unit cell N times (2-port only)
+                     % Note: overwrites original S-parameters
+                     % Use copyobj() first to preserve originals
+```
+
+### Export
+
+```matlab
+s.writeCSV('output.csv')    % Export S-parameter data to CSV
+s.writeSNP('output')        % Write a new Touchstone file
+                            % Extension (.s1p/.s2p/etc.) is added automatically
+                            % Format is GHz, RI
+```
+
+---
+
+## Static Methods
+
+### De-embedding
+Remove error networks from a measurement to isolate the DUT:
+
+```
+P1 <-- err1 <--> DUT <--> err2 --> P2
+```
+
+```matlab
+sDUT = SPARAMS.deembed(err1, measured, err2);
+```
+
+Returns a new `SPARAMS` object representing the DUT.
+
+### General Matrix Conversions
+
+```matlab
+Z = SPARAMS.s2z(S, Z0)              % S-matrix → Z-matrix
+S = SPARAMS.z2s(Z, Z0)              % Z-matrix → S-matrix
+[A,B,C,D] = SPARAMS.s2abcd(S, Z0)  % S-matrix → ABCD
+[S11,S21,S12,S22] = SPARAMS.abcd2s(A, B, C, D, Z0)  % ABCD → S-parameters
+```
+
+### Comparing Two Objects
+
+```matlab
+SPARAMS.plotAnydB(obj1, {'S11','S21'}, obj2, {'S11','S21'});
+```
+
+---
+
+## Example Workflow
+
+```matlab
+% Load a 2-port measurement
+s = SPARAMS('dut.s2p');
+s.setFreqUnits('GHz');
+
+% Plot S11 and S21
+figure;
+s.plotdB({'S11', 'S21'});
+title('DUT S-Parameters');
+
+% Get ABCD parameters and cascade 3x
+sc = s.copyobj();
+sc.cascade(3);
+
+% Export result
+sc.writeSNP('cascaded_3x');
+```
+
+---
+
+## Supported File Formats
+
+| Format | Description |
+|---|---|
+| `.s1p` | 1-port Touchstone |
+| `.s2p` | 2-port Touchstone |
+| `.s3p` | 3-port Touchstone |
+| `.s4p` | 4-port Touchstone |
+
+Data formats supported within files: **RI** (Real/Imaginary), **MA** (Magnitude/Angle), **dB** (dB/Angle).
+
+---
+
+## Notes
+
+- `cascade()` and `plotDispersion()` are intended for **2-port unit cell** measurements.
+- `renorm()` is fully implemented for **1- and 2-port** measurements.
+- `toZparams()`, `toYparams()`, `toABCDparams()`, `toInputImpedance()` require **2-port** data.
+- When using `cascade()`, it is recommended to `copyobj()` first, as the method overwrites the object's S-parameters.