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 f sample = ( M / N ) f IF ( 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 Shannonʹs 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 f IF is independent of the value of f sample . 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.