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=== The Generic T-Line ===

[[RF.Spice A/D]] offers a passive device called Generic T-line with the keyboard shortcut “T”, which is a general purpose frequency-domain transmission line segment model. It is based on the native SPICE LTRA model, but with the following parameters:

The default parameters of the Generic T-Line are Z0 = 50 Ohms, eeff = 1, alpha = 0, and len = 10mm. A unit effective permittivity implies a TEM transmission line because √ε_eff = β / k₀, where β is the propagation constant of the transmission line and k₀ = 2πf/c is the free space propagation constant, with f being the frequency in Hertz and c = 3e8 m/s being the speed of light. A zero attenuation constant represents a lossless transmission line. The Generic T-Line device is indeed a two-port network with a 2×2 scattering matrix or four S-parameters: s11, s21, s12 and s22. Obviously, this is a reciprocal and symmetric network, i.e., s11 = s22, and s21 = s12.

Note that N-port networks in [[RF.Spice A/D]] have schematic symbols with 2N pins. Each pair of pins represents a port. In a similar way, the generic T-line has two ports and four pins. The pins are marked with plus and minus signs. For example, in the figure above, the pins P1+ and P1- together form Port 1. Normally, the negative pins are grounded, and the positive pins are connected to the other parts of the circuit.

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The physical transmission line types are characterized by their physical dimensions and material properties.

=== Physical Line Calculators and Designers ===

When you place a generic T-line part in your circuit, you have to specify its characteristic impedance (Z0), effective permittivity (eeff) and attenuation constant (alpha). In the case of physical transmission line parts like mircostrip, coaxial line or CPW, you specify the physical parameters of the line such as various dimensions and material properties. [[RF.Spice A/D]] then automatically calculates the necessary transmission line parameters at the time of simulation based on your physical data.

For every physical transmission line type listed above, the '''Device Manager''' of [[RF.Spice A/D]] provides a corresponding '''Line Calculator'''. The line calculators are accessible form the '''Tools Menu''' of the Device Manager. The line calculators take the substrate properties and the physical dimensions of a line types and calculate its characteristic impedance (Z0) and effective permittivity (eeff). The Line Calculator dialog also has an "Operational Frequency" input with a default frequency of 1GHz, which is used to calculate the guide wavelength of the transmission line at that frequency. In many practical applications, you need quarter-wavelength line segments. In that case, you must first calculate the guide wavelength of the transmission line as defined by &lambda;_g = &lambda;₀ / &radic;&epsilon;_eff, where &lambda;₀ = c/f is the free space wavelength at the operational frequency.

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You often need to design a 50&Omega; transmission line of a certain type. [[RF.Spice A/D]] provides ten physical transmission line design tools for:

* Twin-Lead Line

* Twisted-Pair Line

The above line designers ignore the conductor and dielectric losses and calculate the physical dimensions of the line for a given value of the characteristic impedance Z0. For example, given a substrate with thickness h and relative permittivity &epsilon;_r, the "Microstrip Designer" calculates the microstrip width in mm for a given value of Z0 (50 Ohms by default). Some line type like CPW and coaxial line have more than one dimensional parameter that can be varied. For example, CPW has slot width (w) and center strip width (s), while coaxial line has inner and outer conductor radii. In such cases, the line designer dialog provides radio button options to fix one parameter and vary the other.

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