Digitally programmable active RC bandpass filter with constant absolute bandwidth

A tunable narrow-band bandpass filter in which the absolute bandwidth and the center frequency can be controlled independently comprises:

• a 3-terminal RC active network in which a ground terminal provides one connection to both the input and the output ports, a first terminal provides the ungrounded connection to the input port, and a second terminal provides the ungrounded connection to the output port, said RC active network comprising:
• a first resistor having one terminal end thereof connected to said first terminal of the input port, said first resistor establishing the bandwidth in accordance with the relationship .delta.=.sqroot.3/2RO, where .delta. is the relative bandwidth, and RO is the value of the first resistor;
• a first operational amplifier having the non-inverting input terminal connected to the other terminal end of said first resistor, having an inverting input terminal and having an output terminal connected to the second terminal of said active network;
• a first capacitor having one terminal end connected to the other terminal end of said first resistor and having the other terminal end connected to said ground terminal:
• a feedback network including certain resistive elements the resistance value of which set the center frequency of the filter in accordance with the relationship Wo=1/RC, where Wo=2.pi.fo, and fo is the center frequency, R is the value of the resistance of the resistor and C is the value of the capacitor in the network;
• said feedback network having an input terminal connected to the output terminal of said first operational amplifier, having a first output terminal connected to the inverting input terminal of said first operational amplifier and having a second output terminal connected to the non-inverting input of said first operational amplifier;
• and
• switch means interposed in said feedback network to effect a connection between said resistive elements and said first and second output terminals of the feedback network when said switch means is actuated and opening said connection otherwise;
• and
• programmable timing means providing a plurality of timed outputs for selective connection to said switch means, each timed output setting a predetermined time period during which said switch means is actuated and thereby selectively varying the effective value of the certain resistive elements so that the center frequency of the filter may be programmably changed.