Audio Bandpass Filter
There are two different approaches to bandpass-filter design. The first involves use of a high-Q resonant network. Youíll find this type of device sold as a CW filter, an application in which it excels. However, the selectivity of a resonant bandpass filter is such as to favor a very few frequencies to the exclusion of all others, and this makes it useless in voice reception. To filter the garbage out of an SSB transmission, you need a filter that freely passes the band of frequencies between about 300 and 2500 Hz but drastically attenuates frequencies outside the passband. An audio filter of this type is constructed by cascading (i.e., hooking in series) very sharp high - and low - pass filters.
Thatís what weíve done here U1a and U1b comprise a sharp, 4-pole Butterworth high-pass filter with a 300-Hz cut-off. The two remaining stages function as a low-pass 4-pole Butterworth filter having a 2500-Hz cut-off frequency. Overall circuit gain is 16. Insert the filter into your receiverís audio chain at a point where the input signal level will be less than lOOm V peak-to-peak. If the filterís extra gain causes problems, chop its output down with a resistive divider. A dual supply furnishing, anywhere between 2.5V and 15V can be used to power the circuit.
Copyright by Bill Bytheway, K7TTY February 2012