|Maxwell's Dream Help: What are transmission line parameters?||Back to Overview|
All parameters here are related to the transmission of high speed signals along the line:
This parameter describes, how fast a signal travels along a line. This parameter is calculated in nanoseconds per meter.
The capacitance is calculated between the signal trace and the ground planes. The capacitance is given in picofarad per meter.
In general, an inductance is defined for a closed current loop only. Here, in the triplate, it is assumed, that the current flows through the signal trace in one direction and back through the ground planes in the opposite direction. This is always true for high frequency signals. The inductance calculated here is the sum of the partial inductance of signal trace and ground planes.
This parameter is the most difficult to understand, but at the same time the most important one for high speed signaling. At high frequencies the signal travels along the signal trace as an electromagnetic wave. The form and strength of the electric field and magnetic field are determined by the geometry of the transmission line and the material properties of the dielectricum.
The signal voltage and the signal current now follow the electrical and magnetical fields. As a surprising result there is a fixed ratio of voltage and current of any signal wave travelling along the signal trace. It is not possible to inject a signal with any other ratio of voltage and current. If you would try to do so, this signal would be splitted into two signals: One signal with the fixed ratio would travel along the signal, the second signal would be reflected. Of course the sum of both signals would be the signal you tried to inject into the transmission line.
In the electronic world a ratio of voltage and current is called an impedance. In the case of transmission lines the fixed ratio of signal voltage and signal current is called "wave impedance".
The knowledge of wave impedance is important, if you want to design an electrical sender, which injects a signal without reflection loss into the transmission line. If you want to concatenate two transmission lines, knowledge of wave impedance is important also.
Maxwell's Dream calculates the fields first, then it calculates the transmission line parameters from the fields. For field calculation an Finite-Element Method is used.
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