The present invention relates to a band-pass filter and more particularly to a band-pass filter for band-pass filtering frequency modulated signals with good frequency response characteristic and good dely time characteristic.
In general, in a frequency modulated signal transmitted in a transmission system, when a variation occurs in a side band as a result of effects such as that of the amplitude characteristic and the group delay characteristic, distortion occurs in the demodulated output when the signal is demodulated. A distortion arising from such a cause becomes a very great problem in the case where the transmission system is a narrow band circuit having selectivity as in a band-pass filter and the like.
In a band-pass filter known heretofore, however, the amplitude characteristic cannot become symmetrical with respect to the center frequency of the pass band, and, moreover, the group delay characteristic does not become flat and has a great deviation. Consequently, in a frequency modulated signal which has passed through a band-pass filter of this known character, a large distortion is generated.
Accordingly, with the aim of minimizing the occurrence of distortion, various kinds of forms of band-pass filters have heretofore been proposed. Each of these filters, however, has required a large number of passive circuit elements and much time for adjustment and has been accompanied by other problems such as inability to attain sufficient reduction of distortion. Accordingly, with the aim of leveling the delay characteristic of the filter as much as possible, the selectivity of the filter has been lowered in some cases.
However, it is necessary to increase the selectivity in a band-pass filter wherein it is necessary to extract one signal of a band close to that of another signal as, for example, a band-pass filter used in a discrete 4-channel record disc reproducing system to extract an angle-modulated wave signal from a multiplexed signal of a direct wave signal and an angle-modulated wave signal reproduced from a record disc. Furthermore, if the delay time characteristic is not flat, the stererophonic sensation imparted by the reproduced sound formed by the loudspeakers will be unsatisfactory.