LOW_PASS_AF

Published:2009/7/2 20:36:00 Author:May | From:SeekIC

Can be used to attenuate un desired high-frequency audio response in superhet or direct-conversion receivers having inadequate IF selectivity, to improve CW or SSB reception. Resistor values determine cutoff frequency; 700 and 900 Hz are for CW and 2500 Hz for SSB. Insert filter at point having low audio level. Filter has input buffer, three cascaded active low-pass filter stages, and IN/OUT switch.Overall gain is unity. U1 is Fairchild μA4136, Raytheon RC4136, or equivalent quad opamp.Overall response is 1.5 dB down at cutoff frequency, about 36 dB down at twice cutoff, and about 60 dB down at three times cutoff. R7 and R8 provide pseudoground of half supply voltage, to eliminate need for negative supply. Will operate with supply from 6 to 36 V, drawing about 7 mA.-T, Berg, Active Low-Pass Filters for CW or SSB, QST, Aug. 1977, p 40-41.



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