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A diagnostic instrument being designed to measure the beam phase and energy of the drift tube linac in the proton accelerator of the China Spallation Neutron Source is described, and the characterization of the system is presented. The signals received from fast current transformers are radio-frequency (RF) signals with a frequency of 352.2 MHz and a dynamic range from -30 to 3.5 dBm. The RF signals are converted to orthogonal streams directly with the in- and quadrature-phase undersampling technique based on high-speed high-resolution analog-to-digital (A/D) conversion. To guarantee a high-quality A/D conversion, two different sampling clock circuits are designed with corresponding simulations and tests conducted for comparison. To build a stand-alone instrument with compact architecture, all digital signal processing algorithms are implemented within one single field-programmable gate array; meanwhile, a Nios II embedded system is also integrated in it for data transfer through the Ethernet. This system achieves a phase resolution better than 0.09° over the input signal amplitude range from -41 to 7 dBm, well beyond the 0.5° required.