Multi-frequency Point Supported LLRF Front-end for CiADS Wide-bandwidth Application

The China initiative Accelerator Driven System, CiADS, physics design adopts $162.5 \,\mathrm{MHz}$, $325 \,\mathrm{MHz}$, and $650 \,\mathrm{MHz}$ cavities, which are driven by the corresponding radio frequency (RF) power system, requiring frequency translation front-end for the RF station. For that application, a general-purpose design front-end prototype has been developed to evaluate the multi-frequency point supported design feasibility. The difficult parts to achieve the requirements of the general-purpose design are reasonable device selection and balanced design. With a carefully selected low-noise wide-band RF mixer and amplifier to balance the performance of multi-frequency supported down-conversion, specially designed local oscillator (LO) distribution net to increase isolation between adjacent channels, and external band-pass filter to realize expected up-conversion frequencies, high maintenance and modular front-end general-purpose design has been implemented. Results of standard parameters show an $R^2$ value of at least $99.991\%$ in the range of $-60 \sim 10\,\mathrm{dBm}$ for linearity, up to $18\,\mathrm{dBm}$ for P1dB, and up to $89\,\mathrm{dBc}$ for crosstalk between adjacent channels. The phase noise spectrum is lower than $80\,\mathrm{dBc}$ in the range of $0 \sim 1\,\mathrm{MHz}$, and cumulative phase noise is $0.006^\circ$; amplitude and phase stability are $0.022\%$ and $0.034^\circ$, respectively.