Laser Tuning of a Planar Active Band-Pass Filter using MESFETs

The MESFET controlled active band-pass filter is examined using a semiconductor laser for tuning. The filter is basically end coupled microstrip band-pass filter. The series tank circuit of the filter includes a tuning MESFET. The cover of the MESFET element is removed and the capacitance between the gate and source of the MESFET is controlled either by a gate-to-source bias or by a semiconductor laser illumination. Another MESFET is used in the negative resistance circuit to compensate for the filter loss. The shift of the center frequency is found proportional to the square root of the incident laser power. The saturation of the frequency shift is observed. The saturation is accompanied by an increase of passband-loss. The maximum tuning range of 77 MHz is observed, which is 35% more than the previously reported tuning range.