Evaluating Permissive Change Compliance Paths for Potted RF Transmitter Modules
Adding potting compound to an unpotted RF module demands Class II permissive change re-testing if dielectric loading increases radiated spurious emissions.
Microstrip Impedance Mismatch
Meaning
Transmission line errors occur when the actual characteristic impedance of a printed circuit board trace deviates from the designed system impedance. A microstrip impedance mismatch causes signal reflections and increases return loss, which degrades the efficiency of the RF transmitter. This condition ceases to be the primary cause of signal degradation when the physical length of the trace is much shorter than the signal’s wavelength.
Reflection Coefficient
Signal energy is reflected toward the source when the electromagnetic wave encounters a change in the geometry of the transmission line. A microstrip impedance mismatch raises the voltage standing wave ratio along the trace, leading to decreased power delivery to the antenna. This mismatch is typically caused by incorrect trace widths, changes in dielectric thickness, or gaps in the reference ground plane.
Engineers use time-domain reflectometry to locate and measure these impedance discontinuities along the signal path.
Substrate Tolerance
Dielectric properties of the board material can vary due to manufacturing tolerances and moisture absorption. These variations cause the characteristic impedance to shift from the calculated target value.
Power Transfer
Power amplifier performance is degraded when the load impedance differs from the optimal design point. This degradation results in increased heat generation and reduced battery life in portable devices.