So far in this chapter we have addressed techniques for the reconfigurability of more or less discrete circuit elements. Another vein in the area of reconfigurability?namely, the MEMS microswitch array ?addresses the reconfigurability of distributed microwave components (i.e., of the very metal traces, or patterns, that would otherwise define the interconnection transmission lines and tuning stubs of microstrip-based microwave circuits). The fundamental enabler of this paradigm, the microswitch, is shown in Figure 4.9. The microswitch is a cantilever beam?type structure that can be arrayed in two dimensions with an interelement pitch of 100 mm. For implementation on a fused silica substrate (er = 3.8), this size corresponds to 1/20th the wavelength at 100 GHz or 1/200th the wavelength at 10 GHz, so that no issues of line-length quantization are elicited .
By addressing the two-dimensional array, where each microswitch may be thought of as a pixel, any given metal pattern image can be defined on the substrate, particularly as it is appropriate to a matching or tuning network. For example, Figure 4.10 shows the microswitch array-based tuning for a reconfigurable power amplifier, in which both the input and output matching networks are reconfigured to retune the optimum frequency of the amplifier.
Impedance matching is one of the fundamental steps in the design and production of an RF/microwave circuit [15, 16]. In low-noise amplifiers (LNAs) and power amplifiers, properly tuned input/output matching networks are crucial to meeting required noise figure and power efficiency requirements.
In the production of low-volume MICs, or hybrid (discrete) circuits, it is the rule to manually tune the circuits until the desired performance levels are met; but this activity becomes too time consuming and expensive at millimeter-wave frequencies, not to mention impractical for high-volume applications and even impossible for MMICs. Thus, there is a strong incentive to exploit the power of RF MEMS to implement classic impedance matching schemes in an automated, reconfigurable fashion.