Yukio Yamamoto’s research while affiliated with Osaka Sangyo University and other places

A review of a tunable active bandpass filter developed in our research group is given here. Presented first is the basic structure of an end-coupled microstrip-line bandpass filter. Next discussed is the concept of using coupled negative resistance to compensate the loss of tank circuit. Then, the method of using varactor diode or mesfet varactor to tune the center frequency of the passband is discussed. The capability of optical-control of mesfet varactor and the concept of using mesfets as three-terminal varactors are also discussed. Ľarticle présente ľévolution de la conception du filtre au sein d’un groupe de recherche: structure de base d’un filtre à lignes microrubans couplées par les extrémités; emploi de résistances négatives couplées pour compenser les affaiblissements des circuits; emploi d’une diode varactor; emploi d’un transistor mesfet ayant une fonction de capacité variable (accord électrique ou optique de la fréquence centrale, varactor mesfet à trois bornes et son application à un filtre accordable en mode passif ou en mode actif).

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.

A MESFET-controlled active bandpass filter that was developed for use with X-band frequencies is reported. This filter loss is compensated for by the negative resistance generated by a MESFET. The series tank circuit of the filter includes another MESFET that is controlled either by a gate-to-source bias or by a semiconductor laser illumination. The center frequency of the passband can be shifted by 75 MHz with an accompanying bandwidth change from 6 MHz to 17 MHz when the gate-to-source voltage of the tuning MESFET is varied. Laser illumination of this MESFET shifts the center frequency of the filter by 57 MHz with a bandwidth change from 6.5 MHz to 10 MHz.< >