Bipolar Multiplexed Pulse Width Modulation

Abstract

This paper presents generation and detection of two-channel bipolar multiplexed pulse width modulated signal. The positive half cycles of a polar triangular wave of frequency fc are modulated in amplitude by one message signal and the negative half cycles are modulated in amplitude by another message signal. The two modulated polar triangular waves, each carrying a distinct message signal, are added to generate a carrier whose positive envelope resembles one message signal while negative envelope resembles second message signal. This is a bipolar division multiplexed (BDM) signal in which the initial carrier considered is polar triangular wave. The BDM signal is passed simultaneously through positive and negative clipper circuits. The clippers output unipolar trapezoidal waveforms of opposite polarity. Each polar trapezoidal waveform is differentiated to generate a sequence of positive and negative pulses. Positive pulses alone are considered from the pulse train obtained by differentiating positive trapezoidal signal while negative pulses alone are considered from the pulse train obtained by differentiating the negative trapezoidal signal. The two sets of pulses are added to form Bipolar multiplexed pulse width modulated signal. The message signal recovery involves demultiplexing the pulse trains, clipping their amplitude levels to avoid the effect of additive noise followed by low pass filtering. The proposed scheme offers multiplexing feature with inherent synchronizing gap resulting in no requirement for synchronizing pulses. The proposed scheme requires more bandwidth and hence better noise performance when compared to conventional multiplexed PWM scheme. The proposed scheme is better suited when the dynamic range of the message signal is in the order of a few tens of millivolts.