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CW_FILTER_FOR_INTERFERENCE

2017-02-19 08:56  
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CW_FILTER_FOR_INTERFERENCE
Audio bandpass filter, designed for connection between loudspeaker jack of receiver and external loudspeaker or phone, has half-power bandwidth of about 70 Hz but rolls off gradually without causing ringing, Series LC combination, connected in hot line to loudspeaker,looks like 5-ohm re sistance at resonance, cutting signal amplitude about in half. At Iower frequencies filter looks like large capacitive reactance and at higher frequencies it resembles large inductive reactance, both causing high attenuation. Filter thus discriminates against all except switch-selected resonant frequency, either 760 or 1070 Hz.Choose best frequency for particular receiving situation by trial.-F. Noble, A Passive CW Filter to improve Selectivity, QST, Nov. 1977, p 34-35.