The RF-to-IF converter module of IP-BPM needs an external local oscillator (L.O.) to switch the mixer inside. The needed L.O. frequencies are 5712MHz for Y-converter and 6426MHz for X-converter. We have developed a system to generate the L.O. starting from 714MHz reference signal of DR-RF. Since the detection system turns to be a beam synchronized one with beam-locked L.O., it becomes possible to determine the absolute phase without the phase information of the reference cavity.