Morse Filters

Morse, or CW (continuous wave), is still widely used thanks to the fact that the necessary equipment can be kept relatively simple, and therefore inexpensive, if the operator is sufficiently trained in selective listening. A morse decoding computer, however, requires an adequately filtered 1 input signal, because it lacks the noise discriminating capa- bility of the human ear. Some receivers can be upgraded with a 250 Hz IF filter for this purpose, but such an extension is usually well beyond the financial reach of most radio amateurs. The filters discussed here operate in the audible fre- quency range, and compare favourably with far more ex- pensive types for 455 kHz. Figures 1 and 2 show the circuit diagram and the typical re- sponse of an eighth-order in- verse Chebishev filter which has been optimized for non- computer using listeners. The filter of Fig. 3 is less complex, and intended for driving a com- puter. The associated fre- quency response is shown in Fig. 4. Both filt...