A new IQ detection method for LLRF

Digital LLRF technology has been widely used in new generation particle accelerators. IF quadrature sampling is a common method for amplitude and phase detection. Many strategies, which obey the same rule of fsample=(M/N)fIF (M/N is a rational number), have been proposed to reduce the effects of spectrum aliasing. However, we found that M/N does not need to be a rational number according to Shannon's theorem. Therefore, we propose a new IQ detection method in this paper. This method is based on a special IIR filter which is derived from an RLC circuit. The unique characteristic of the method is that the value of fIF is independent of the value of fsample. We have set up an experimental platform to verify our method. A 122.88 MHz sampling clock is used to sample a 3 MHz IF signal. The DDS and PI control techniques are used to realize the closed-loop control. Results show that the stability of the system is within ± 0.05% (peak to peak) for the amplitude, and with ±0.03° (peak to peak) for the phase in 5 h.