Filter arrangement including a non-reversible circuit element, a band-pass filter, and an active circuit

摘要

A filter apparatus in accordance with the present invention comprises, for example, a band-pass filter and a plurality of band-stop filters connected as the post-stage to the band-pass filter. The band-pass filter transmits a band containing a desired frequency band, and the band-stop filters attenuate undesirable frequency bands. Another filter apparatus in accordance with the present invention comprises, for example, a first band-pass filter, a circulator connected to the first band-pass filter, and second band-pass filters connected to the circulator. The first band-pass filter transmits a band containing a desired frequency band to the circulator, and the second band-pass filters transmit undesirable frequency bands and reflect the desired frequency band back to the circulator. The desired frequency band is outputted from this circulator. Still another filter apparatus in accordance with the present invention comprises, for example, a circulator and a band-pass filter connected to the circulator. The band-pass filter transmits an undesirable frequency band and reflects a desired frequency band to the circulator. The desired frequency band is outputted from this circulator.