Dielectric receiving filter with sharp stopband using an active feedback resonator method for cellular base stations

The development of an 800-MHz-band dielectric receiving filter with a sharp stopband is presented. The filter is composed of a conventional dielectric antenna filter and thee active band-stop filters, each sharply eliminating one band in the passband. In the active band-stop filters, small dielectric resonators, in which the unloaded Q is raised to about 50000 by means of an active feedback resonator method, are used. These filters are designed to obtain optimum stability and an optimum noise figure. One of them has a center frequency of 845.75 MHz, a stopband width of 1.0 MHz, and an attenuation of 30 dB. Deviation of the resonant frequency is held without ±30 kHz, and the noise figure at passband is adequately small. The size of the dielectric receiving filter is 480×250×44 mm³, and the volume is less than 1/20 that of a conventional filter using cavity resonators