Coaxial connector to microstrip transition optimization

February 18, 2014 //By Dr. Jaakko Juntunen, AWR Europe
Coaxial connector to microstrip transition optimization
This example briefly discusses how a coaxial connector to microstrip transition can be optimized using AWR’s Analyst™ 3D finite element method (FEM) EM simulator.

When comparing measurement results to simulated ones, the transition (Figure 1) from the measurement hardware’s coaxial cable to the signal trace of the DUT is commonly assumed to be ideal. But in reality, this “ideal” condition results in data mismatch (simulation data =/ measurements) at higher frequencies.

Figure 1: SMA connector is used to connect measurement hardware coaxial cable to the PCB, and converting the coaxial mode signal into microstrip mode propagating towards the device under test.

This ideal assumption is implicitly made as soon as the simulation input/output ports are on the trace, e.g., on the microstrip or coplanar waveguide (Figure 2). While there are multiple methods that can be used for ensuring the transition has as little reflection as possible, this specific example highlights the approach of using a full 3D FEM EM model for the connector. Doing so enables the treatment of the transition’s parasitics in an analytic fashion and moves us away from the “ideal” and toward reality.

Figure 2: Typically the simulation port is directly on the microstrip, neglecting the discontinuity represented by the connector in Figure 1, and thus systematically distorting the simulated versus measured results.

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