Radar systems, wireless systems, high frequency analog systems…all of these need to include measures to ensure signal integrity. With many high frequency systems, this can be difficult with microstrip or embedded microstrip routing on the surface layer. However, you can save yourself a lot of signal integrity headaches using coplanar waveguide routing.
Most design tools can be used to define coplanar waveguide routing with a ground pour feature. This allows you to easily define a coplanar waveguide on your board, or a grounded coplanar waveguide by defining a ground plane in an interior layer. The question remains: when should you use coplanar waveguide routing?
A coplanar waveguide is a copper arrangement where a signal trace is routed in parallel to two ground planes. The presence of the ground plane on each side of a signal trace provides natural shielding for the signal against interference from other traces on a board. A coplanar waveguide also comes in the grounded variety. The geometry is essentially the same, except there is another ground plane beneath the surface layer.
This is shown in the image below. Coplanar waveguide geometries. Compared to microstrip and stripline traces, placing a signal trace on the top layer with the ground pour on each side of the trace causes a signal to see lower radiation losses. This also reduces resistive heating losses as the signal hugs the side of the trace, rather than hugging the bottom of the trace near the rough interface with the substrate.
This means your signal will be stronger at the receiver end of the trace, and the shape of your signals will not be distorted as they travel along the trace. Because coplanar waveguides have a ground plane directly next to the trace, these parallel components can be mounted directly between the trace and the ground plane without placing routing through a via. Because coplanar waveguides require the use of ground planes surrounding the trace, you have less real estate available on the surface layer.
The cost of all that copper on the surface layer also drives up the board cost. You also need a relatively thick substrate, so you should keep the layer count low if you are using a standard board thickness.
Grounded Coplanar Waveguide with Coaxial Transition
There are closed-form equations for the impedance of a coplanar waveguide, but these formulas require evaluating elliptical integrals. One thing you will notice is that the impedance is more sensitive to the spacing between the signal trace and the ground plane than it is to the cross-sectional geometry of the signal trace.
Thankfully, you can find a number of calculators online for specific coplanar waveguide geometries, including a grounded coplanar waveguide. There is a great calculator on Sourcefourge that allows you to consider everything from your substrate dielectric properties and the frequency you will work with in your board. There is another issue with coplanar waveguides that relates to the plating used on copper to prevent trace corrosion. Electroless nickel immersion gold ENIG plating has higher insertion loss on a coplanar waveguide than on a microstrip.Wenin Basically, a coplanar waveguide CPW consists of a conductor separated from two ground planes, and everything is etched on one side of a dielectric on the same plane.
The dielectric should be thick enough to damp the electromagnetic fields inside the substrate. There is also a different type of coplanar waveguide which is having a ground plane on the opposite side of the dielectric, which is called finite ground-plane coplanar waveguide FGCPWor more simply, grounded coplanar waveguide GCPW. This kind of waveguide is preferred to microstrip when we are dealing with thick substrates.
However, using microstrip, will cause impedance variation over the frequency because of dispersion effect. On the other hand there will be non-negligible radiation loss for high frequencies.
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When using the grounded coplanar waveguide for such structures, the mentioned drawbacks will be addressed. In this study, we analyze the performances of a grounded coplanar waveguide, etched on a thick 0. The mesh of the structure is more accurate in the areas of the via and ports.
We can resort to the default mesh of HFWorks which recognizes the dimensions of the structures and applies a pretty convenient mesh. To simulate the behavior of this transition insertion and return loss at the desired frequency band; input and output matchingwe will create a Scattering Parameters study, and specify the frequency scheme in our case 20 frequencies in a fast sweep plan from 0. In an antenna simulation, radiation boundaries which are peculiar features of such a simulation have to be assigned to the radiation surfaces.
These surfaces truncate the air surrounding the antenna and somehow minimizes the reflection on these surfaces and define the extent of radiation measurements. The simulated study provides multiple choices and options to plot and to adjust the outputted results according to the user's desire. They also offer the exploitation of electrical parameters calculated in Scattering parameters simulations insertion, return losses The coplanar waveguide is composed of signal and ground conductors.
Each one could be assigned Signal and PEC materials during the step of assignment of solids and materials. Various 3D and 2D plots are available to exploit, depending on the nature of the task and on which parameter the user is interested in.
As we are dealing with a transition of connection between two different transmission supports in this example, plotting the insertion and return losses sounds like an intuitive task. The following figure shows both the insertion and return losses of the considered filter:. The insertion loss appears to be very low within the frequency band and gives acceptable levels on most parts of the band; As for the return loss, it seem fluctuating but keeps good matching performance; Plotting the return loss on a Smith chart plot is more relevant when we deal with matching issues.
The electric field distribution at 8. In this example, we were able to discover how to arrange a study in pre and post simulation steps in HFWorks. The model being simulated in an S parameter simulation, shows the inner details of wave propagation.
It also outputs different electrical parameters in 2D, 3D plots and Smith charts. The transition between different types of RF signal carriers has always been an interesting issue in the RF community; thus, getting an optimized result for your application can be in the origin of great amelioration of your system performance.
Southwest Microwave, Inc.
Grounded Coplanar Waveguide with Coaxial Transition. Used Tools:. Simulation To simulate the behavior of this transition insertion and return loss at the desired frequency band; input and output matchingwe will create a Scattering Parameters study, and specify the frequency scheme in our case 20 frequencies in a fast sweep plan from 0.
Results Various 3D and 2D plots are available to exploit, depending on the nature of the task and on which parameter the user is interested in. Conclusion In this example, we were able to discover how to arrange a study in pre and post simulation steps in HFWorks. References Southwest Microwave, Inc.Objective: In this project, you will analyze different types of coplanar waveguides using the quasi-static simulation engine.
Ferma Lesson 9. In this tutorial you will construct and analyze more complicated types of multi-conductor transmission lines such as various coplanar waveguide structures with and without a dielectric substrate or conductor backing. A coplanar waveguide CPW transmission line consists of a center metallic strip located between two wide grounded metallic strips on the two sides.
All the three strips are collocated on the same plane. The width of the center strip also called center conductor is denoted by s. The gap between the center strip and the lateral ground strips on either side is denoted by w and is known as the slot width.
In practice, a dielectric substrate is used to support the three metallic strips. The dielectric substrate can be an infinite half-space or a layer of finite thickness. The simplest CPW utilizes an air substrate.
This can be realized using a very thin membrane and does have practical applications. The dielectric substrate may also have a PEC ground backing on its other side. Open the EM. Cube application and switch to EM. Start a new project with the following attributes:. Open the Domain Settings dialog and change the offset values in the various directions according to the following table:.
Note that the 2D quasi-static analysis will be performed on this sensor plane. The CPW structure has 10mm of clearance from the top and bottom domain boundary walls. In order to have adequate mesh resolution on the center conductor and across the slot gaps, you will choose a cell size of 0. This will put 20 cells on the center conductor and across each slot gap. To have square mesh cells of dimensions 0.
Open the Mesh Settings dialog and make these changes. Keep in mind that the number of cell along the X direction does not matter because you will run a 2D analysis only on the YZ plane.
At the end of the simulation, the output message window reports the computed values of the characteristic impedance and effective permittivity of the transmission line:. An effective permittivity of one is expected as your structure represents an air-filled homogenous multi-conductor transmission line supporting a dominant TEM propagating mode.
Two typical Coplanar Waveguides are diagrammed in cross section below. A dielectric substrate has metal layers patterned on top of it. When a metal layer is also present underneath the substrate, it is known as a Grounded Coplanar Waveguide. This metal layer is typically connected with vias to the metal layers above the dielectric. Although these metal layers are often called groundskeep in mind that there is a current flowing through these metal layers, hence the surfaces are not at a constant potential.
We will focus on the Grounded CPW case. The CPW is characterized by the metal trace layer thickness, tthe center conductor width, wthe gap, gbetween center and side conductors, and finally the dielectric substrate thickness, hfor the grounded case. One of the first quantities you should calculate, prior to any modeling, is the skin depth:.
This means that the electric fields and currents fall off as:where is the distance into the metal. The skin depth and the thickness of the metal layers will govern what kind of analysis you will need to do. If the skin depth and trace thickness are comparable, then it is necessary to include the metal domains themselves in the COMSOL model. On the other hand, if the skin depth is much smaller than the thickness, by at least a factor of tenthen the fields on one side of the metal layer do not significantly affect the fields on the other side.
In such cases, it is not necessary to model the interior of the metal layers; they can be considered boundaries of the modeling domain. Additionally, if the thickness, tof the metal layers is small enough, such that the thickness becomes negligible to the results, it becomes possible to model the metal traces as Perfect Electric Conductor PEC boundary conditions, as shown in the diagram below of the simplest model of a CPW.
The air region above the CPW can be truncated by either a PEC boundary condition, representing the metallic packaging, or a Perfect Magnetic Conductor PMC boundary condition, representing a surface along which no current can flow.
The Finding the Impedance of a Coaxial Cable model provides a similar example that goes into more details about setting up such models.
Now, such 2D models can quickly compute the impedance of the CPW, and give you an idea of the relative field magnitudes in the cross section. However, we are typically more interested in devices that have some variations to their structure that require a full 3D model.
This raises the question of how to excite the 3D Coplanar Waveguide model. Several different techniques are possible, but we will start by considering a CPW that can be modeled using PEC faces, where the trace thickness, tcan be considered negligible. One approach, diagrammed below, is to add several rectangular faces to the model, either normal or parallel to the plane of the CPW, that represent a probe tip.
These PEC faces act as a bridge between the two side conductors. A Lumped Port excitation is then applied to another rectangular face between the bridge and the center conductor. This Lumped Port applies a voltage difference between adjacent PEC faces note: the directions of the arrows in the figure are arbitrary; they are merely meant to show that there is an applied, sinusoidally time-varying, current flowing in the direction of the arrows.
This approach is quite straightforward, requiring only a minor modification of the model. Now, the above approach does require adding some extra structures to the model, so you may consider an approach that requires even less modification, as shown in the figure below.Microstrip vs. Coplanar Waveguides
The only difficulty with this approach is that it will require you to manually set; the Port Number to be the same in both Lumped Port features; the dimensions; and, most importantly, the direction. The direction of the Lumped Ports must be set so that they are either both pointing towards, or both pointing away from, the center conductor.
This approach introduces less extra structure into the model, but does require having two port features — that have to be manually set and point in the correct directions. It is also possible to extend the layout of the CPW and extend the side PEC planes to surround the center PEC strip, and then introduce an additional rectangle for the Lumped Port, mimicking a two-point probe, as shown here:. There are certainly other ways in which a CPW can be excited, but the above three approaches are the most common.
The differences, in terms of the solutions, should be small between all three approaches, but it is worth noting that all of these are meant to approximate an excitation, and the fields in the immediate vicinity of the Lumped Port will not be physically realistic. This is a local effect, the fields away from the excitation and quantities such as the computed impedance will be more accurate.The conventional open-wire transmission lines are not suitable for microwave transmission, as the radiation losses would be high.
At Microwave frequencies, the transmission lines employed can be broadly classified into three types. A coaxial line consists of an inner conductor with inner diameter dand then a concentric cylindrical insulating material, around it. This is surrounded by an outer conductor, which is a concentric cylinder with an inner diameter D. This structure is well understood by taking a look at the following figure.
The fundamental and dominant mode in co-axial cables is TEM mode. There is no cutoff frequency in the co-axial cable. It passes all frequencies. However, for higher frequencies, some higher order non-TEM mode starts propagating, causing a lot of attenuation. The width of the ground plates is five times greater than the spacing between the plates. The thickness of metallic central conductor and the thickness of metallic ground planes are the same.
The following figure shows the cross-sectional view of the strip line structure. The fundamental and dominant mode in Strip lines is TEM mode. The strip line has a disadvantage that it is not accessible for adjustment and tuning. This is avoided in micro strip lines, which allows mounting of active or passive devices, and also allows making minor adjustments after the circuit has been fabricated.
This can be understood by taking a look at the following figure, which shows a micro strip line. Micro strip lines are of many types such as embedded micro strip, inverted micro strip, suspended micro strip and slotted micro strip transmission lines.
In addition to these, some other TEM lines such as parallel strip lines and coplanar strip lines also have been used for microwave integrated circuits. A Parallel Strip line is similar to a two conductor transmission line. It can support quasi TEM mode.
The following figure explains this. A Coplanar strip line is formed by two conducting strips with one strip grounded, both being placed on the same substrate surface, for convenient connections. A Slot line transmission lineconsists of a slot or gap in a conducting coating on a dielectric substrate and this fabrication process is identical to the micro strip lines. Following is its diagrammatical representation. A coplanar waveguide consists of a strip of thin metallic film which is deposited on the surface of a dielectric slab.
This slab has two electrodes running adjacent and parallel to the strip on to the same surface. All of these micro strip lines are used in microwave applications where the use of bulky and expensive to manufacture transmission lines will be a disadvantage.
These can also be stated as Open Electromagnetic Waveguides. A waveguide that is not entirely enclosed in a metal shielding, can be considered as an open waveguide. Free space is also considered as a kind of open waveguide. An open waveguide may be defined as any physical device with longitudinal axial symmetry and unbounded cross-section, capable of guiding electromagnetic waves. They possess a spectrum which is no longer discrete. Micro strip lines and optical fibers are also examples of open waveguides.
Types of Transmission Lines Advertisements. Previous Page. Next Page.Remember Me? With a CPS I draw a rectangular surface equal to the cross section of the CPS and I set a lumped port on it, with an integration line that goes from one strip to the other orthogonally to the parallel faces of the two strips. Now, with a CPW I cannot draw a rectangular surface within the outer metallic strip, since there is a conductor in the middle and the system provides me and error when I start the simulation.
So what should I do to define an excitation for a CPW? Please help me! You will need to create an impedance line between one of the side grounds and the center. Read the section in HFSS help on impedance calculations as there are additional considerations. After I finish I like to check the calculated Zo with another tool or a manual calculation just to make sure that I did not blunder.
Simon or Waddell are useful transmission line references. I assumed from your question that the CPW you are studying does not have a cover or additional groundplanes.
Your structure must be totally enclosed in that box. You'll find that helpful. Microstrip or coplanar waveguide?? Grounded coplanar waveguide with HFSS 2. Part and Inventory Search. Welcome to EDABoard. Design Resources. New Posts. Laser diode from printer 0. Keil code generated by Proteus vs STM32cube 2. Linear S21 in ADS s parameter simulation 3. Fully differential amplifier with simple CMFB scheme on the differential pair 3.
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Altium Designer problem in safe-mode 2. Chronology for understanding computer architecture 0. IGBT cheap welding machine mods 4.Objective: In this project, coplanar waveguide CPW transmission line segments with different types of termination are examined. In this tutorial you will learn how to construct coplanar waveguide CPW transmission line structures with different termination types.
You will learn about lumped devices like linear resistors and nonlinear diodes. You will examine temporal waveforms of different types for exciting your CPW structure and will investigate the transient response of your circuit.
You will also learn how to parameterize geometric objects using independent and dependent variables. Back to EM. Tempo Manual.
Tempo Tutorial Gateway. Download projects related to this tutorial lesson. Open the EM. Cube application and switch to EM.
Start a new project with the following attributes:. A one-port CPW structure is created in the project workspace. First, open the variables dialog and change the definitions of a number of variables according to the table below:. Note that most of the changes above were done to express the dimension variables directly in the project units just for convenience. Then, add a PEC rectangle strip according to the table below:.
Also, open the domain settings dialog and change the domain offset parameters according to the figure below:.
Modeling of Coplanar Waveguides
In this case, all the domain box faces are still PML walls. The geometry of your physical structure should now look like this:.
They are placed at one edge of the strip and extend out in opposite directions. The name refers to the spacing between the strip and the two lateral ground plane objects.
The wizard created the CPW source and automatically added a port definition to the navigation tree. For this project, you will define two temporal field probes and three near-field sensor observables according to the specifications below:.
Note that the coordinates of the field probe and field sensors have been chosen to measure the field at the center of the positive slot line.
Run an FDTD simulation of your transmission line circuit and visualize its field distributions. The E-field is almost zero everywhere except on the two slots.
The fields are uniform longitudinally along the two slot lines, meaning that a decent impedance match has been accomplished and there is little wave reflection that would cause a standing wave pattern. Note that the electric field at the open end is at its maximum while the magnetic field is at its minimum at that location. You can also clearly see the standing wave pattern of the fields along the transmission line. The distance between two consecutive field minima or maxima is equal to the half guide wavelength.
This can easily be verified from the 2D graphs of field distributions.
Types of Transmission Lines
Similar to the tutorial lesson 6, measure the distance between two field peaks in the graph. The figure below shows a spacing of According RF.
The excitation source in a FDTD simulation pumps up energy into the computational domain and sets the initial conditions of the boundary value problem.
By default, EM. Tempo uses modulated Gaussian waveform to excite all the sources. In the CPW source dialog and all other source dialogs, you can click the Waveform You can see a graph of the excitation waveform along with its mathematical expression and parameters in this dialog.