Multichannel downconverter for the next generation RF field control for VUV- and X-ray free electron lasers

For pump-probe experiments at VUV- and X-ray free- electron lasers, in the injector and bunch compression section the electron beam and timing reference must be in phase within 0.01 degree (rms) and have an amplitude stability within 10^-4 (rms). The performance of the field detection and regulation of the acceleration RF critically influences the phase and amplitude stability. For the RF field control, a multichannel RF downconverter is used to detect the field vectors and control the vector sum of 32 cavities. In this paper a new design of an 8 channel downconverter is presented. The downconverter front-end consists of a passive RF double balanced mixer input stage, intermediate filters, and an integrated 16 Bit analog-to-digital converter (ADC). The design includes a digital motherboard for data pre-processing and communication with the controller. In addition, we characterize the downconverter performance in amplitude and phase jitter, temperature drifts, and channel cross-talk in a laboratory environment.