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Pasternack BlogPlanar transmission lines are used to carry a variety of analog, RF, and digital signals on insulative, planar substrates from kilohertz to hundreds of gigahertz frequencies. Planar transmission lines are constructed of one or more layers of conductive traces, possibly with adjacent conductive structures, with dielectric layers providing insolation between the conductive structures. Planar transmission lines are commonly fabricated using single or multilayer printed circuit boards, low-temperature co-fired ceramic (LTCC), high temperature co-fired ceramic (HTCC), and semiconductor fabrication technologies. In the case of PCB planar transmission lines, RF End-Launch Coaxial Connectors are often used to interface between the board-level transmission line and coaxial connectorized ports. Another common interface approach is to use a RF Probe Station With Coaxial Probes.
What Are the Common Planar Transmission Line Types?
There are several common types of planar transmission lines: stripline, suspended stripline, microstrip, coplanar waveguide, slotline, and imageline, of which there are also variants. Each type of planar transmission line exhibits different dominant transmission modes, max frequency, characteristic impedance range, and unloaded Q factor.
Types of Planar Transmission Lines
- Stripline: A “strip” conductor embedded within a dielectric substrate and sandwiched between two ground plans
- Suspended Stripline: A stripline that is suspended in air between the ground plans, with the air gaps being above and below the strip.
- Microstrip: A strip conductor on top of a dielectric substrate with a ground plane beneath the substrate.
- Coplanar Waveguide: a strip conductor with two ground planes parallel and on either side of the strip on the same dielectric substrate.
- Slotline: a slot separating two metal traces on the same plane of the dielectric substrate.
- Finline: A slotline that is rotated +/- 90 degrees inserted into the E-plane of a rectangular metal waveguide.
- Imageline: A dielectric slab waveguide with a strip of dielectric on a metallized plane.
Differences Among Planar Transmission Line Types
Typically, the outer, top, or bottom traces of a planar transmission line are grounded with the interior trace as the signal traces. These physical structures allow for the development of transmission line modes, namely transverse electromagnetic (TEM), transverse electric (TE), transverse magnetic (TM), quasi-TEM, longitudinal-section electric (LSE), and longitudinal-section magnetic (LSM), depending on the planar transmission line configuration. The electrical behavior of a planar transmission line depends on how the field lines are distributed through air and the substrate (dielectrics) and how the field lines are coupled to the signal and ground traces or metalization.
Dominant Modes of Planar Transmission Lines
- Stripline: TEM
- Suspended Stripline: TEM, quasi-TEM
- Microstrip: Quasi-TEM
- Coplanar Waveguide: Quasi-TEM
- Slotline: Quasi-TE
- Finline: LSE, LSM
- Imageline: TE, TM
Max Frequency (Typical)
- Stripline: 60 GHz
- Suspended Stripline: 220 GHz
- Microstrip: 110 GHz
- Coplanar Waveguide: 110 GHz
- Slotline: 110 GHz
- Finline: 220 GHz
- Imageline: >100 GHz
Characteristic Impedance Range (with substrate relative permittivity of 10)
- Stripline: 30 – 225 Ohm
- Suspended Stripline: 40 – 150 Ohm
- Microstrip: 10 – 110 Ohm
- Coplanar Waveguide: 40 – 110 Ohm
- Slotline: 35 – 250 Ohm
- Finline: 10 – 400 Ohm
- Imageline: ~26 Ohm
Unloaded Q Factor (with substrate relative permittivity of 10)
- Stripline: ~400
- Suspended Stripline: 600 @ 30 GHz
- Microstrip: 250 @ 30 GHz
- Coplanar Waveguide: 300 @ 30 GHz
- Slotline: 200 @ 30 GHz
- Finline: 550 @ 30 GHz
- Imageline: 2500 @ 30 GHz
Planar Transmission Line Variations
Planar transmission lines with “loose” field lines may also couple with nearby conductive structures on a substrate or any conductive housing or structures within close proximity. This can result in planar transmission lines exhibiting additional and undesirable spurious modes. Hence, there are planar transmission line types and variants that use tightly coupled grounded structures nearby, or even completely surrounding, the signal traces. Though these tightly coupled transmission lines tend to exhibit higher conductor losses, they exhibit lower radiation losses, better spurious mode suppression, and possibly higher frequency performance. The trade-off in greater grounding/shielding is the additional cost, weight, and a possible increase of performance sensitivity to substrate and metalization fabrication tolerances.
There are also varying complexities associated with fabricating each transmission line type and their variants. For instance, planar transmission lines with only surface traces on a single layer, such as standard coplanar waveguides or slotlines may be lower cost and more easily manufactured than microstriplines that require two layers of metal or grounded coplanar waveguides with metallized vias connecting the surface ground traces to the bottom ground layer.
Common Stripline Variants
- Suspended Stripline
- Bilateral Suspended Stripline
- Two Conductor Stripline
Common Microstrip Variants
- Suspended Microstrip
- Inverted Microstrip
- In-box Microstrip
- Trapped Inverted Microstrip
Common Coplanar Waveguide Variants
- Grounded Bottom or Common Bottom Coplanar Waveguide (GBCPW or CBCWG)
- Grounded Coplanar Waveguide (GCPW)
- Coplanar Strips
- Embedded Coplanar Strips
Common Slotline Variants
- Antipodal
- Bilateral
Common Finline Variants
- Unilateral
- Bilateral
- Antipodal
- Strongly Coupled Antipodal
- Insulated
Difference Between Stripline & Microstripline (Microstrip) Planar Transmission Lines
A strapline planar transmission line is a conductive signal path sandwiched between two dielectric layers that are then sandwiched between two conductive layers. A microstripline, or microstrip, is like a stripline with the top dielectric and conductive layer removed. In this may, microstrip planar transmission lines can be fabricated on the surface of PCBs or other planar circuit technologies. However, striplines are necessarily buried under a layer of dielectric and conductor which necessitates a secondary layer fabrication step or a multilayer circuit board fabrication process.
As a stripline’s field lines are entirely contained within conductive structures, striplines exhibit true TEM mode transmission line operation. As microstrip planar transmission lines have open field lines at the top of the microstrip, these planar transmission lines exhibit quasi-TEM mode transmission and enhanced radiation loss compared to striplines. Moreover, any material placed near enough to the conductor of a microstrip line will change the behavior of the microstrip, which requires some care during assembly and packaging to ensure there is adequate spacing between a microstrip and nearby structures.
Though more expensive and complex to fabricate, stripline’s tend to exhibit better transmission characteristics, especially for very high frequency RF signals or high-speed digital signals. Moreover, microstrip transmission lines have less intrinsic shielding from outside interference, and microstrip runs may also unintentionally act as antennas. Microstrip are generally less complex to design and fabricate and require less board layers, which could be advantageous if an application is highly space constrained or limited in the number of board layers that can be used. Depending on the location of a stripline and the complexity of the circuit, these planar transmission lines may also require blind vias, buried vias, or both. This is less likely with a microstrip transmission line, as they are usually located on the surface of a circuit.
Resources
- Jarry, Pierre; Beneat, Jacques, Design and Realizations of Miniaturized Fractal Microwave and RF Filters, Wiley, 2009 ISBN 0-470-48781-X.
- Edwards, Terry; Steer, Michael, Foundations for Microstrip Circuit Design, Wiley, 2016 ISBN 1-118-93619-1
- Wanhammar, Lars, Analog Filters using MATLAB, Springer, 2009 ISBN 0-387-92767-0
- Rogers, John W M; Plett, Calvin, Radio Frequency Integrated Circuit Design, Artech House, 2010 ISBN 1-60783-980-6
- Maloratsky, Leo, Passive RF and Microwave Integrated Circuits, Elsevier, 2003 ISBN 0-08-049205-3
