Abstract: Input tone signals are supplied to a square wave generator for producing square wave output signals corresponding to the tone signals. The square wave generator includes an operational amplifier having a tone signal input and a control signal input for receiving a first control voltage for activating the amplifier and a second control voltage for squelching the amplifier. The control voltages are produced by squelch control voltage generating means for receiving the input tone signals and producing the first control voltage when the tone signals exceed a predetermined level, while producing the second control voltage when the input tone signals are less than such predetermined level.