SPDT switch and Quasi-optical frequency multipliers

SPDT switch:

A millimeter-wave "Single Pole, Double Throw" (SPDT) switch, realized with a Si-MMIC microstrip technology has been studied. The analysis has been performed with both commercial, equivalent circuit based simulator and mixed electromagnetic-device simulator. The sinusoidal RF signal (ampl. 0.2 V, freq. 76 GHz) is injected at one end of the structure, and then forwarded along either output branches, depending on the bias supplied by the square-wave generator (ampl. 2.5 V, freq. 214 MHz). Microstrip stubs and matched loads complete the structure. To isolate the inactive

branch, {p-i-n} diodes have been used, due to their high impedance in the off-state, and to the compatibility with the high-resistivity silicon, planar process. Feasibility of such Si-MMICs, suitable for operating frequencies up to 100 GHz, has been demonstrated.

Quasi-optical frequency multipliers:

The basic quasi-optical frequency doubler is shown in Fig.6a. In this device the frequency multiplication is achieved by a diode bridge connected to the center of two /2-dipoles. These dipoles are placed in a cross configuration (referred to as crossed dipole): the longest one receives the incoming power at the fundamental frequency (3.5 GHz in our prototype), while the shortest one transmits the generated power at the doubled frequency (7.0 GHz in our prototype). The advantages of such structure are the following. First, a good isolation between input and output signals is achieved since incoming and outgoing waves are orthogonally polarized. Second, the symmetry of the structure has been exploited to obtain a balanced topology which is characterized by a good conversion efficiency (only even harmonics are generated within the multiplier). However, the conversion efficiency of the above structure is intrinsically limited by the omnidirectional nature of both receiving and transmitting antennas. To overcome the above mentioned problem, the front-to-back ratio (and thus the directivity) of both receiving and transmitting dipoles has been increased by additional layers with parasitic elements. The resulting structure is essentially constituted by two Yagi-Uda antennas working at orthogonal polarizations. The new, multi-layer structure can be better understood with the example of Fig. 6b. In this case only two parasitic elements are adopted, one for each antenna of the frequency doubler. The longest element (left of the photograph) is used as a reflector for the fundamental frequency dipole; the shortest element (right of the photograph) is used as reflector for the doubled frequency dipole.