A testing system includes a signal generator circuit, a jitter modulation circuit, and an oscilloscope circuit. The signal generator circuit is configured to generate a clock pattern signal with a single clock pattern frequency. The jitter modulation circuit is configured to generate a jitter signal. A device-under-test is configured to receive an input signal. The input signal is a combination signal of the clock pattern signal and the jitter signal. The device-under-test includes a clock data recovery circuit and is further configured to generate an output signal according to the input signal. The oscilloscope circuit is configured to receive the output signal for determining performance of the clock data recovery circuit.