RF MODULE

Microstrip on a Printed Circuit Board
Model Definition
20 | CHAPTER 2: TUTORIAL MODELS
This example calculates the impedance of a microstrip line on a printed circuit board.
It compares the impedance with that obtained from an analytical formula given the
dimensions of the micro strip and permittivity of the circuit board. This formula is
available as a script with the following arguments: width, height, height of conductor,
and permittivity of circuit board. The following command gives the analytical
characteristic impedance of the microstrip:
zmustrip(2e-2,5e-3,5e-4,3)
35.4780
Results
The analysis tests two terminations: lumped port termination and numerical port
termination. Both versions use a lumped port with 50 Ω cable impedance to excite the
signal into the microstrip. The numerical port version does a boundary mode analysis
on the terminating end, which results in almost no reflection from the termination of
the microstrip. The lumped port termination uses the calculated impedance as cable
impedance. This matching is less accurate and gives reflections, which interfere with
the impedance calculation, and therefore deviates from the analytical result. Table 2-1
below summarizes the result.
TABLE 2-1: RESULT OF THE IMPEDANCE COMPARISON
ANALYSIS MICROSTRIP IMPEDANCE
Analytical 35.48 Ω
Numerical port termination 36.44 Ω
Lumped port termination 39.91 Ω
Note that the analytical result does not represent the most accurate value because it
assumes infinite extension of the surroundings. This model is terminated by perfect
magnetic conductors, which influence the impedance of the microstrip. It is clear that
the numerical port has much less reflection compared to the lumped port. This is
important to consider when lumped ports are used as terminations.