Microstrips are developed for the transmission of high- frequency signals on printed circuit boards. In the transmission of high-frequency signals and digital signals, the conductive path on the PCB plays a functionally decisive role.
Undefined traceimpedances cause phase shifts at high frequencies, and they can affect switching times during short-time rising edges. The problems that undefined impedances of traces can trigger cause functional impairments in high-speed logic. Microstrips and striplines are techniques used to design traces so that they have defined impedances.
Microstrips are flat, planar conductor paths for the transmission of high-frequency signals. They are used on printed circuit boards and in monolithic microwave ICs( MMIC). They were developed by ITT Labs in the 1950s. Microstrips consist of a conductive trace, the microstrip, deposited on the base laminate or a dielectric substrate, and the ground plane with the potential reference. The laminate separates the conductive trace from the ground plane and forms a capacitance with it. The dielectric above the trace can be air or a dielectric coating of a solder mask.
Characteristic values of microstrips
The impedance of a microstrip depends on the trace inductance, the effective dielectric constant and the capacitance. The latter is determined by the width of the planar trace, the thickness of the laminate and its permittivity. In practice, the impedance of a microstrip with a trace width of 200 µm located on a laminate with a thickness of 200 µm, which has a dielectric constant of 5, is between 40 ohms and 50 ohms. The trace thickness reduces the impedance only slightly.
If there is air above the trace, it is called a surface microstrip; if solder resist is applied above the trace, it is a coated microstrip. With a surface microstrip, the effective dielectric constant is lower than that of the dielectric laminate. Depending on the dielectric substrate and its thickness, microstrips can transmit frequencies between 20 GHz and 160 GHz.
The microstrip principle is also used in microstrip antennas or patch antennas.