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The SuperKEKB accelerator complexes are now being upgraded to bring the world highest luminosity (L=8x1035/cm2/s). Hence, the KEKB Injector Linac is required to produce: electron: 20 mm mrad (7GeV, 5nC), positron: 10 mm mrad (4GeV, 4nC). To achieve this, the accelerator components have to be aligned within ± 0.1mm (1 σ). BPM is essential instrumentation for Beam Based Alignment, and is required one magnitude better position resolution to get better alignment results. Since current BPM readout system using oscilloscopes has ~50um position resolution, we decided to develop high precision BPM readout system with narrow bandpass filters. It handles two bunches with 96ns interval and has a dynamic range between 0.1nC (for photon factory) to 10nC (positron primary). To achieve these criteria, we adopt semiconductor attenuators and optimized two-stage Bessel filters at 300MHz center frequency to meet both time and frequency domain constraints. To correct position drift due to gain imbalance during operation, calibration pulses are output to the BPM between beam cycles (20ms). The beam position and charge calculations are performed by onboard FPGA to achieve fast readout cycle.
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