In many radio voice receivers or transmitters, a voice band pass filter is often needed to keep the voice signal inside the 300-3000Hz vocal band. The filter is used at the transmitter side in order to keep the transmitted spectrum as narrow as possible or at the receiver side in order to suppress any unwanted noise. An ideal voice filter would have an ideal flat response and infinite attenuation for frequencies inside and outside of the pass-band, respectively.
An ideal voice filter does not exist. However, a 5th order band – pass Butterworth filter for the 300-3000Hz band is considered to be a good solution for radio-telephony applications. Such a band-pass filter is presented below.
The circuit is actually a cascading topology of a high pass and a low pass filter. Both, the high and the low pass filters are of 5th order Butterworth type. U1A,B and U2A form a 5th order high pass filter and the U2B, U3A and B form the 5th order low pass filter. Minus 3db, low and high pass cut off frequencies, are set to 300 and 3000Hz, respectively.
The circuit is based on the TL082 operational amplifier and can be powered from a symmetrical power supply unit of about ±9 to ±15V. For best results, the circuit must be assembled by using high quality and low tolerance components. Quit often, at the receiver side, the high pass section of the filter can be omitted.
The filter is based on Sallen–Key topology and has been designed for unity gain (0db) at pass-band using the Microchip FilterLab filter design software.