Example embodiments are directed to an apparatus and method for measuring a pulse width. A side-band signal generator may be configured to receive a given data pattern and output a side-band signal by modulating a pulse width of the received data pattern in a test mode. A phase detector may be configured to receive the side-band signal and a reference clock signal, and output a pulse signal corresponding to a phase difference between the received side-band signal and the reference clock signal. A charge pump may be configured to receive the pulse signal and output an output voltage by increasing or decreasing the output voltage based on the pulse signal. A pulse width measurer may be configured to receive the output voltage of the charge pump and determine whether pulses forming the side-band signal have appropriate widths based on whether the output voltage is included in a desired reference voltage range.