Quad Notch Filter Module, X41667
The Raven 41667 Quad Notch Filter Module provides four signal paths, each with flat pass band response except near the notch frequency. Notch frequencies range from 60 Hz to 5 KHz, customer specified. The 41667 filter can be installed inside the same shelf as the bridge modules eliminating the need for external notch filters. Amplifiers are incorporated to provide adjustable gain input to output, with a +30 dB to -18 dB adjustment range. Notch rejection greater than 60 dB.
A typical application is for 2175 Hz filtering to eliminate the otherwise audible control tones used in radio applications.  The filter is active so there are no losses as with passive filters.  In addition, up to four filters can be installed on the same PCB saving space and money.
THEORY OF OPERATION

Four independent signal paths exist on the 41667 Quad Notch Filter. The first signal path (IN 1) on pins 21 and 22 is a balanced 600Ω input and is routed to amplifier U1A, where RC components make this a notch amplifier. The output of U1A is routed to amplifier U1B, where, again, RC components make this a notch amplifier. The filtered output is then presented to amplifier U1C. Typically gain is set for unity, but potentiometer R15 provides a range adjustment of −18 to +30 dB. Transformer T1 provides a balanced 600Ω output for the first signal path (OUT 1) on pins D and E.

The second signal path (IN 2) on pins 19 and 20 is a balanced 600Ω input and is routed to amplifier U2A, where RC components make this a notch amplifier. The output of U2A is routed to amplifier U2B, where, again, RC components make this a notch amplifier. The filtered output is then presented to amplifier U2C. Typically gain is set for unity, but potentiometer R22 provides a range adjustment of −18 to +30 dB. Transformer T2 provides a balanced 600Ω output for the second signal path (OUT 2) on pins F and H.
The third signal path (IN 3) on pins 16 and 17 is a balanced 600Ω input and is routed to amplifier U3A, where RC components make this a notch amplifier. The output of U3A is routed to amplifier U3B, where, again, RC components make this a notch amplifier. The filtered output is then presented to amplifier U3C. Typically gain is set for unity, but potentiometer R29 provides a range adjustment of −18 to +30 dB. Transformer T3 provides a balanced 600Ω output for the second signal path (OUT 3) on pins K and L.

The fourth signal path (IN 4) on pins 14 and 15 is a balanced 600Ω input and is routed to amplifier U4A, where RC components make this a notch amplifier. The output of U4A is routed to amplifier U4B, where, again, RC components make this a notch amplifier. The filtered output is then presented to amplifier U4C. Typically gain is set for unity, but potentiometer R36 provides a range adjustment of −18 to +30 dB. Transformer T4 provides a balanced 600Ω output for the second signal path (OUT 4) on pins M and N.