The first step in building an RF speech clipper is modulating a carrier oscillator with the microphone audio into single sideband. Two popular techniques for doing this are the filter method and the phasing method. The filter method produces suppressed carrier double sideband signals using a balanced modulator, one of which is then filtered to leave USB or LSB. Performance is dependent on carrier rejection using sharp cut-off filters. The phasing method generates two different double sideband signals and adds or subtracts them. The output of one balanced modulator is USB + LSB. The product of the second balanced modulator using phase shifted audio and carrier signals is USB - LSB. By simply adding or subtracting these sets of sidebands, the upper or lower sideband can be extracted. Performance is dependent on the audio phase shift (quadrature) network to maintain a constant 90° phase shift over the entire 300-3 KHz speech frequency spectrum.
