Impedance Matching

Why Impedance Match?

A question often asked by people new to the microwave field is, "what is so important about impedance matching?" The answer is that this is one of the very few known and reliable operating conditions (the others, which are harder to implement and are position-dependent, and for which no power transfer is possible, are the short and open circuit).

Efficient power transfer is possible with other source and load impedances at a single frequency, but the ability to measure and adjust to known conditions is too difficult to be reliable. The other advantage of the matched load condition is that it uniquely removes the requirement for a specific reference plane.

Also, the power-handling capacity of a transmission line is maximum when it is "flat", i.e., operating at low SWR. Lastly, it is important to be able to interconnect a number of different components into a system, and the only way that can be done reliably and predictably is by constraining the reflection coefficients of the various interfaces through impedance matching. Multiple reflections can result in group delay variations that can produce undesired intermodulation in broadband systems.

As we have seen, the S-parameter matrix is especially useful for transmission line and waveguide situations, because the various parameters are defined for matched conditions.

This is extremely helpful in measurement of active devices, which may not be stable with
source or load l l=1 characteristic of a short or open termination.

The greatest amount of engineering time is spent in searching for ways to provide efficient impedance matching, especially to active circuit elements, so it pays to know some of the many useful impedance-matching methods and their limitations. Microwave instruments for measurement of impedance by way of direct measurement or S-parameters are among the most widely used tools of the microwave engineer.

Publicado por Jahir Alonzo Linares Mora C.I: 19769430 CRF

Bibliografia: http://puhep1.princeton.edu/