Reduce ohmic losses by using a good electrical conductor with minimum resistance and maximum conductivity.
Use a dielectric substrate of a low loss tangent and high dielectric constant to reduce dielectric losses and losses due to leakage currents.
Grounded coplanar waveguides and the hybrid combination of microstrip and coplanar waveguides, called coplanar launched microstrip configurations, are effective at reducing the radiation losses in transmission lines.
Losses are responsible for low signal integrity, poor efficiency, and reduced gain in RF circuits
RF and microwave circuits are prone to various losses such as ohmic losses, dielectric losses, radiation losses, and losses due to leakage currents. This is problematic, as losses are responsible for low signal integrity, poor efficiency, and reduced gain in RF circuits.
Most losses in RF circuits are influenced by factors such as the geometry of the transmission line, dielectric substrate, or frequency. When designing circuits, RF engineers should carefully consider how to best reduce losses in their transmission lines, as there are several modifications that can be implemented to achieve such a reduction. In this article, we will discuss how to reduce losses in transmission lines, particularly ohmic, dielectric, and radiation losses.
How to Reduce Losses in Transmission Lines
In RF and microwave circuits, most transmission lines are either microstrip transmission lines, stripline transmission lines, or coplanar waveguides. The losses associated with each of these transmission line types differ, and their performance also varies.
When sending critical data at a high bit rate or in high-speed communication systems, these losses become critical, and signal losses can bring disproportionately high financial losses. So, how does one reduce these losses in transmission lines? It is necessary to optimize the transmission line format to avoid downtimes and increase the reliability of the system. The first step of optimization is to modify the transmission lines of the circuit so that ohmic, dielectric, radiation, and leakage current losses are minimized.
Reducing Ohmic Losses
The ohmic losses in a transmission line are dependent on the type of metal, finish of the metal conductor, and operating frequency. When high-frequency signals are passed through a transmission line, they tend to occupy only the outer surface of the conductor due to the skin effect. When adjacent transmission lines carry high-frequency signals, they are also subjected to the proximity effect.
For transmission lines in RF circuits, a solution to this is to use a good electrical conductor with minimum resistance and maximum conductivity. Usually, copper traces used in RF circuits are characterized by width and thickness. The finish on a transmission line matters when reducing ohmic losses. A smooth conductor surface shortens the propagation path and minimizes loss.
The grided format of transmission lines is another possible way to reduce ohmic losses. A tapered transmission line with varying conductor widths can offset ohmic losses to some extent. Routing a transmission line narrower than its ground plane is also effective at reducing resistance and ohmic losses.
Reducing Dielectric Losses
The dielectric constant of a material influences the dielectric loss in a transmission line. The dissipation factor or loss tangent value of the material plays an important role in reducing dielectric losses. The lower the dissipation factor, the lesser the dielectric loss in the transmission line.
Alumina, silicon, polytetrafluoroethylene, and FR4 (epoxy laminate material) are commonly used dielectric materials in RF circuits. One method to achieve low-loss dielectric material is through a change in epoxy throughout the glass weave. Hybrid stack-ups are also effective at reducing dielectric losses. Blending FR4 and low-loss dielectrics can produce a hybrid stacking up solution. Losses due to leakage current can also be reduced by using dielectric material of high permittivity.
Reducing Radiation Losses
Radiation loss in a transmission line means RF energy is being lost as radiation. The thickness of a substrate influences the value of radiation loss. Due to this loss-height direct relationship, thinner substrate materials of high dielectric constant demonstrate less radiation. Grounded coplanar waveguides and the hybrid combination of microstrip and coplanar waveguides, called coplanar launched microstrip configurations, are effective at reducing the radiation losses in transmission lines.
It is important for RF circuit designers to have a clear idea of how to reduce losses in transmission lines. With this understanding, designers can optimize transmission lines to reduce ohmic losses, dielectric losses, and radiation losses. Cadence’s software helps designers optimize transmission line geometry.