In electromagnetic simulations performed in specialized tools such as CST Studio Suite, a port is a defined interface through which electromagnetic energy enters or exits the system. It serves as a connection point between the device and the external environment, enabling the excitation of the model or the measurement of its response. Ports represent the physical locations where signals, power, or fields would be applied or extracted in a real world setup, such as at the terminal of a cable, antenna feed, or waveguide. There are two primary types of ports based on their intended application: discrete ports and waveguide ports. In this article, we will discuss discrete port in detail.
Discrete Port (Lumped Elements)
A discrete port applies a localized lumped excitation inside the simulation domain. It excites the physical structure by applying voltage, current or impedance element. There are two types of discrete ports based on the domain used for applying for input excitation : discrete edge port and discrete face port.
Discrete Edge Port
A discrete edge port has two connection points (pins) that attach to the structure in the simulation. This applies input excitation along an edge. It’s commonly used as a feeding source for antennas, as a termination point for transmission lines and for modeling circuit component (resistors, capacitors or inductors) connections. Discrete ports are especially useful at low frequencies. At high frequencies when the size of port becomes significant part of wavelength, the S-parameters might not be accurate while using discrete ports. In this case, waveguide ports are more suitable. We will discuss waveguide ports in detail in the coming blogs. A discrete edge port is defined by selecting two end points in the physical domain. It can also be defined by selecting one point and one face in the domain.
Discrete Face Port
This is a special kind of discrete port which is supported by only few solvers like integral equation, transient and frequency solvers using tetrahedral meshes. When other solvers are selected, the discrete face port is automatically replaced by a discrete edge port. They apply input excitation on the surface. They are more flexible and can be positioned inside the simulation domain to excite a small region of a circuit, antenna feed or interconnect structure.
Discrete face ports can be defined by picking two edges or by picking an edge and a face as shown in the figure below.
Types Of Excitations
There are 3 types of discrete edge and face ports based on the type of excitation provided at the port, which are discussed further below.
Voltage Port
This port type represents a voltage source that excites the structure with a constant voltage amplitude, similar to connecting a battery or signal generator in a circuit. It provides fixed voltage to the system so that we can study how our design responds to this input. If we do not activate this port during a transient simulation, the voltage in the wire connected to it will be zero. During the simulation, we can keep track of the voltage signal applied at this port to see how it changes over time and how the system reacted to it.
Current Port
Similar to a voltage port, a current port provides a steady flow of electric current in a circuit. Instead of controlling the voltage, it controls the amount of current that flows into the electrical system. This helps us to see how a design behaves when a certain current is applied. During the simulation, we can record the current signal at this port to analyze how it changes over time and how the system responded.
Impedance Type (S-Parameter)
This type of port is modeled with a lumped element consisting of a current source with an inner impedance (default value = 50 Ohms). This port can excite and absorb power based on how the input signal behaves. The current source becomes active only when this port is chosen as an excitation source in a transient simulation. By default, the port provides 1 W of input power to calculate S-parameters. These S-parameters describe how much of the signal is transmitted, reflected, or absorbed by the circuit. We can also measure voltage across and current through the discrete port.
Final Thoughts
Discrete ports provide a flexible and compact method to excite or terminate electromagnetic energy within the simulation domain. They can be implemented as voltage, current, or impedance sources facilitating comprehensive analysis of circuit and antenna performance. Discrete edge ports and discrete face ports cater to different geometries and solver requirements, making them suitable for a wide range of low to mid frequency applications. However, careful consideration is required at higher frequencies as waveguide ports may provide more accurate results for S-parameter evaluations.
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