The low level radio frequency (LLRF) system for booster accelerator at Shanghai Synchrotron Radiation Fa-cility (SSRF) was upgraded by a digital controller based on field programmable gate array (FPGA) technology. Parameters of voltage, frequency and field flatness in the two 5-cell cavities are controlled to meet the require-ments of booster. In this article, the ramping curve of cavity voltage, amplitude and phase control loop with vector sum of the two 5-cell cavities, tuning loop and field flatness loop are analyzed and discussed in detail. A different method in tuning loop is adopted due to the limitations of ADC channels. The function realizes energy ramping of electron beam from 150 MeV to 3.5 GeV with a repetition rate of 2 Hz. With the new LLRF controller, the phase stability at ramping mode in 10 hours long operation is improved from ±1.5◦(RMS) with open loop to ±0.15◦ (RMS) with close loop, while the detuning phase and field flatness are maintained to within ±2◦and ±1%, respectively.
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