{{Note | Proper grounding of the [[Transmission Lines|[[Transmission Lines|transmission lines]]]] device is critical for a successful simulation.}}
[[File:tline2.png|thumb|350px| The schematic symbols of the Generic Open Stub (left) and Generic Short Stub (right) devices.]]
For more information about physical transmission line discontinuity devices, please refer to [[Glossary_of_Physical_Transmission_Lines_and_Components|Glossary of Physical Transmission Lines and Components]].
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== Physical Line Calculations and Design ==
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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). Or you may accept the default values Z0 = 50, eeff = 1, and alpha = 0. 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. In that case, RF.Spice automatically calculates the necessary transmission line parameters based on your physical data at the time of simulation. Oftentimes, you may want to design a 50Ω transmission line of a certain type, or calculate and compare the characteristics of several transmission line types. Another practical need in RF design is to quarter-wavelength line segments. In this case, you must calculate the guide wavelength of the transmission line as defined by λ<sub>g</sub> = λ<sub>0</sub> / √ε<sub>eff</sub>, where λ<sub>0</sub> = c/f is the free space wavelength at the operational frequency.
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== Physical Line Calculators ==
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The Device Editor of [[RF.Spice]] provides ten line calculators and ten designer tools for all the transmission line types listed in the above table. These tools are accessible form the RF Menu of the Device Editor. 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. Some of the line types, microstrip, stripline, coaxial line, twin-lead line and twisted-pair line, have loss [[parameters]]: dielectric loss tangent and metal conductivity. For these lines, the line calculator calculates the conductor attenuation constant (α<sub>c</sub>) and dielectric attenuation constant (α<sub>d</sub>), both in Neper per meter (Np/m). Note that the total attenuation constant is the sum of these two: α = α<sub>c</sub> + α<sub>d</sub>. Also, you can convert these values from Np/m to dB/m using the relationship: 1Np = 8.6859dB.
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<table>
<tr>
<td>
[[File:tline6.png|thumb|320px| Microstrip Line Calculator.]]
</td>
<td>
[[File:tline10.png|thumb|320px| CPW Line Calculator.]]
</td>
<td>
[[File:tline12.png|thumb|320px| Coaxial Line Calculator.]]
</td>
</tr>
</table>
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== Physical Line Designers ==
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For every physical transmission line type, [[RF.Spice]] also provides a designer tool. The designer ignores conductor and dielectric losses and calculates 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 ε<sub>r</sub>, 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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<table>
<tr>
<td>
[[File:tline7.png|thumb|320px| Microstrip Line Designer.]]
</td>
<td>
[[File:tline11.png|thumb|320px| CPW Line Designer.]]
</td>
<td>
[[File:tline13.png|thumb|320px| Coaxial Line Designer.]]
</td>
</tr>
</table>
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== Analyzing and Designing Physical Coupled Lines==
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In the case of coupled microstrips and coupled striplines, it is the even and odd mode characteristic impedances (Z0e and Z0o) that really matter. The Coupled Microstrips Calculator and Coupled Striplines Calculator find these two impedances for the given strip width and strip spacing. They also calculate the even and odd mode effective permittivities, which are typically different. The system characteristic impedance of the coupled line is calculated from the formula: Z<sub>0s</sub> = √( Z<sub>0e</sub> . Z<sub>0o</sub> ). To calculate the guide wavelength, the average of the two effective permittivities is used. In addition, the coupling coefficient of the coupled line is calculated in dB from the formula: C = ( Z<sub>0e</sub> - Z<sub>0o</sub> ) / ( Z<sub>0e</sub> + Z<sub>0o</sub> ).
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The Coupled Microstrips Designer and Coupled Striplines Designer, on the other hand, find the values of the strip width and strip spacing for given values of the even and odd mode characteristic impedances.
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<table>
<tr>
<td>
[[File:tline8.png|thumb|320px| Coupled Microstrips Calculator.]]
</td>
<td>
[[File:tline14.png|thumb|320px| Coupled Striplines Calculator.]]
</td>
</tr>
<tr>
<td>
[[File:tline9.png|thumb|320px| Coupled Microstrips Designer.]]
</td>
<td>
[[File:tline15.png|thumb|320px| Coupled Striplines Designer.]]
</td>
</tr>
</table>