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This audio noise filter circuit is a bandpass filter for audio frequency band. It filters unwanted signals that are lower or higher than the audio frequencies.
It has 2 filters: a low pass filter and a high pass filter in a cascade configuration.
Both filters are second-order filters with a 24 dB/octave fiter capability. The 3 dB cut-off freq. are 11.8 Hz and 10.7 kHz.
This fm stereo noise suppressor circuit is to be used when the reception of far FM stations is bad but improves when you switch the radio receiver to mono. The cause of this effect is that in stereo reception the biggest part of the noise in the right channel is in opposite phase with the noise in the left channel.
FSK or Frequency-Shift Keying is a form of frequency modulation in which the modulating signal shifts the output freq. between predetermined values. If problems arise in demodulating FSK signals, it is helpful to use the FSK filter circuit presented here.
Filters with high orders are designed usually with 2 or more cascaded sections. This order 4 filter need only one OA IC , so we can achieve lower distorsions, lower intermodulation …
Resistors values represent the load on the OA output, the maximum TL081 load is 2kΩ . R1-R4 build a 2.5kΩ impedance and so the external load cannot be lower than 10kΩ .
