Circuit designers classify power loss in microstrip lines into conductor loss, dielectric loss, and radiation loss.
The substrates with a high loss tangent contribute to higher dielectric loss, whereas low loss tangent substrates decrease dielectric loss.
The metal's conductivity, the skin effect, and surface roughness affect conductor losses in microstrip lines. The less conductive the metal, the more ohmic loss takes place in the microstrip line.
Microstrip lines are planar conducting strips
Microstrip lines are planar conducting strips laid over a dielectric sheet with an earthed conducting plane on the other side of the circuit. Microstrip lines have a quasi-TEM mode of propagation, making it easy to realize various lumped-elements. Their flexible layout is excellent, giving microstrip lines optimal versatility. While microstrips have desirable characteristics, when designing microwave or RF circuits, it is essential to note the different types of loss mechanisms in these types of lines.
The Different Types of Losses in Microstrip Lines
Circuit designers classify power loss in microstrip lines into conductor loss, dielectric loss, and radiation loss. These losses are dependent on:
- The loss tangent or dissipation factor of the dielectric material.
- The surface roughness of the conductor.
- The frequency of the signal through the microstrip line.
- The dielectric constant of the substrate.
- The height of the substrate.
The conductivity of the metal, the skin effect, and surface roughness affect conductor losses in microstrip lines. Microstrip lines will experience more ohmic loss with reduced conductivity. The skin effect is a phenomenon that increases the effective resistance of the microstrip line and boosts ohmic losses. As the surface roughness goes from low to high, the conductor losses also increase. So, the skin effect is higher in rougher conductor strips.
The dielectric substrate loss tangent drives microstrip line dielectric losses. The substrates with a high loss tangent contribute more to dielectric loss, whereas low loss tangent substrates decrease dielectric loss. Dielectric losses are directly proportional to frequency.
Dielectric losses are less than conductor losses when using dielectrics such as Alumina, AlN, or BeO. When silicon substrates are used in monolithic integrated circuits, the dielectric loss values are comparable to conductor loss values. This is because silicon substrates have lower resistivity, which makes dielectric and conductor losses almost equal. The material GaAs has higher resistivity, so using it as a dielectric substrate reduces dielectric losses.
Radiation losses in microstrip lines are overlooked when compared to ohmic losses and dielectric losses. Radiation losses depend on the dielectric constant, thickness of the dielectric substrate, and the microstrip line geometry. When the dielectric constant value is less, the concentration of the electromagnetic energy in the substrate decreases, and radiation losses increase. When using a high dielectric constant material, the electromagnetic field gets stored in the dielectric between the microstrip and the ground plane, which lowers radiation loss. The table below shows the dielectric constant of various dielectric substrates.
The dielectric constant of dielectric substrates
When circuit designers use magnetic materials, such as ferrites or garnets, as dielectric material, magnetic losses can occur in the circuit. These materials will cause an increase in magnetic loss near the material’s natural resonant frequency. The magnetic loss tangent and characteristic impedance undergo a rapid increase at the resonant frequency, and the magnetic losses increase accordingly. The conductor losses have a dependence on characteristic impedance. As the characteristic impedance increases at the resonant frequency, conductor losses increase along with magnetic losses.
When designing high-frequency RF and microwave circuits, it is crucial to account for the different types of losses in microstrip lines. Cadence's software can help in RF circuit design while mitigating losses in microstrip lines.