TRIUMF's ARIEL pre-buncher is powered by a sawtooth waveform which is the combination of 11.79 MHz, 23.57 MHz and 35.36 MHz harmonics. To upgrade the existing analog and digital hybrid LLRF control system, reduce the hardware cost, and achieve more flexible control of the system, an FPGA based digital daughter board is designed. The generation, control, and regulation of these three harmonics are all incorporated digitally inside a single FPGA. To reduce the number of RF interfaces on the motherboard, the new daughter board is designed to control the amplitude and phase of the sawtooth waveform with one channel ADC and DAC. To lock the output phase of the sawtooth waveform to the reference signal, a digital phase-locked loop is designed to lock all three harmonics to the external reference signal. The related work, including the system design of the LLRF control system and the development of the digital phase-locked loop, are presented in this paper.