Speech Filter

Posted on Feb 7, 2014

A simple circuit which is switchable between a sharp cut-off above 2. 5 kHz and a narrow bandpass around 800 Hz. In the low pass (LP) state it can be used on speech as the filter section of a direct conversion receiver. For CW operation, the cut-off frequency is lowered to 800 Hz and another high pass (HP) section with cut-off below 800 Hz

Speech Filter
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is added to form a narrow bandpass around 800 Hz. To use this. the CW Morse beat note is adjusted to the bandpass centre frequency. In the following paragraphs operation of the circuit is described. If filter terminology used appears unfamiliar, the reader is referred to the opening paragraphs of the writer`s previous article (reference 1). The article also discusses a little more about the types of filter employed here. The circuit makes use of the National Semiconductor switched capacitor LP filter package type MF6. which includes two operational amplifiers with the sixth order Butterworth filter. The filter is also operated from its own internal clock operating at 50 times the cut-off frequency. To achieve the 2. 5 kHz and 800 Hz LP cut-off, the clock frequency is switched between 125 kHz and 40 kHz respectively. As a sixth order Butterworth, the switched capacitor filter has an attenuation slope beyond cut-off of 36 dB per octave. To produce the 800 Hz high pass, one of the operational amplifiers is connected as a second order Chebychev filter. This type filter gives a theoretical attenuation at the first octave of 17 dB, not as great as the LP filter but quite effective in reducing audio noise or interference below the 800 Hz centre frequency. Figure 1 illustrates the filter system in block form. For speech, the LP filter is operated on its own. For CW its cut-off frequency is lowered and its output fed via the HP...

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