CalcDjordjevicSarkarApprox
==========================
```{function}  CalcDjordjevicSarkarApprox(varargin)
```
Full definition:
```{code-block} matlab
function [paramDebye, paramSarkar] = CalcDjordjevicSarkarApprox(varargin)
```

Approximate Djordjevic–Sarkar model with multi-term Debye model

Fits a wideband dielectric model using a single permittivity and loss tangent
measurement at one frequency. The Djordjevic–Sarkar [1] model is calculated from
these values and approximated using multiple Debye terms. The poles are placed
logarithmically between f1 and f2. One term per decade is used by default.

required name-value pairs:
- 'fMeas'     : Frequency of the measurement [Hz]
- 'epsRMeas'  : Relative permittivity ε_r at 'fMeas'
- 'tandMeas'  : Loss tangent tan(δ) at 'fMeas'
- 'f2'        : Upper corner frequency of the Djordjevic–Sarkar model [Hz]

optional name-value pairs:
- 'lowFreqEvalType': Low-frequency behavior:
                     0 = use 'f1' (default), typical Djordjevic–Sarkar
                     1 = use 'epsRdc'
- 'f1'             : Lower corner frequency [Hz] (required if `lowFreqEvalType` = 0)
- 'epsRdc'         : DC permittivity (required if `lowFreqEvalType` = 1)
- 'sigmaDC'        : DC conductivity [S/m], usually neglected, default: 0
- 'nTermsPerDec'   : Debye poles per frequency decade, default: 1
- 'plotEn'         : Enable plotting (0 = off, 1 = on), default: 0

output:
- paramDebye  : Struct with multi-term Debye model:
    .epsInf     – Permittivity at infinite frequency
    .deltaEpsT  – Array of Δε'_i values
    .wi         – Array of angular pole frequencies ω_i
    .sigmaDC    – DC conductivity [S/m]

- paramSarkar : Struct with Djordjevic–Sarkar model parameters:
    .epsInf     – High-frequency permittivity
    .deltaEpsT  – Total permittivity change (ε_r,DC - ε_inf)
    .m1         – log10(lower angular frequency)
    .m2         – log10(upper angular frequency)
    .sigmaDC    – DC conductivity [S/m]

example:

    [pDebye, pSarkar] = calcDjordjevicSarkarApprox( ...
        'fMeas', 1e9, 'epsRMeas', 4.2, 'tandMeas', 0.02, ...
        'f1', 1e6, 'f2', 200e9, 'plotEn', 1);

notes:
- Either `'f1'` or `'epsRdc'` must be specified depending on `'lowFreqEvalType'`.
- Debye model is matched to the imaginary part of the DS model, sampled at
  logarithmically spaced frequencies.

See also: AddDebyeMaterial, AddLorentzMaterial

[1] Djordjevic, Antonije R., et al. "Wideband frequency-domain
    characterization of FR-4 and time-domain causality." IEEE Transactions
    on Electromagnetic Compatibility 43.4 (2001): 662–667.

-------------------------------------------------------------------------
author: Tobias Ammann (2025)

Version:
v1.0  2025-06-30  Tobias Ammann  Initial release