Whilst evaluating receivers for professional NDB monitoring it became apparent that the strong-signal performance of some left much to be desired. Presenting the receiver with a wide-band aerial such as the Wellbrook ALA1530 at my location caused problems with intermod from strong long-wave signals particularly Droitwich on 198kHz which causes a second-order response at 396kHz - A real problem when monitoring EMW on 393kHz!
The following circuits where designed and constructed using standard filter tables. They are both 7th Order 0.1dB Chebyshev responses. The High-Pass design cured all my LW problems and the Low-Pass design should help reduce problems caused by strong local MW transmitters.
The filter responses are those measured from the prototype units, using a Marconi 2030 signal generator and Racal 9303 RF level meter.
Created 20 Aug, 2000