Digitally programmable active RC bandpass filter with constant absolute bandwidth

摘要

A narrow-band RC active bandpass filter, in which the center frequency may be changed by changing the resistance values of certain resistive elements, has the certain resistive elements switchably connectable into and out of the RC network so that the ratio of time during which the resistors are connected to the network to the time during which resistors are not connected to the network determines the effective resistance thereof. A programmable timing circuit has a plurality of outputs with each output supplying a different number of equal width pulses during a predetermined like time period whereby each output corresponds to a different ratio. The timing circuit actuates and deactuates the switchable connection from a selected output thus deriving the required effective resistance for a predetermined center frequency. The bandwidth of the passband is set by a different resistive element and remains substantially unchanged as the center frequency is changed by changing the effective resistance of the certain resistive elements.