A tester for testing high-speed serial transceiver circuitry. The tester includes a jitter generator that uses a rapidly varying phase-selecting signal to select between two or more differently phased clock signals to generate a phase-modulated signal. The phase-selecting signal is designed to contain low- and high-frequency components. The phase-modulated signal is input into a phase filter to filter unwanted high-frequency components. The filtered output of the phase filter is input into a data-transmit serializer to serialize a low-speed parallel word into a high-speed jittered test pattern for input into the transceiver circuitry.