PWM for motor control waveform


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I've built a PWM circuit for controlling a DC motor. The PWM circuit consist of a triangular wave generator compared with a variable voltage. When the triangular wave is more than the variable voltage, there will be output waveform. Therefore I can vary the duty cycle. However, the output waveform does not coincide with the triangular wave. The output waveform lags the triangular wave when i measured it on the oscillosope. Anyone know what is the reason? How can I rectify the problem? The lag is undesirable. Thank you.