A Direct-Sampling RF Receiver for MOLLER Using the PYNQ RFSoC 4x2 Board

We describe our progress on realizing a direct-sampling RF receiver using the PYNQ RFSoC 4x2 board. For the precision measurement of the weak mixing angle, MOLLER requires a readout resolution of 10 ppm for the beam charge monitors at Jefferson Lab's Continuous Electron Beam Accelerator Facility (CEBAF). Historically, the readout has been done using superheterodyne receivers, but not to the required resolution for MOLLER. One possible reason for the noise floor in such receivers is the intrinsic amplitude and phase noise. Instead, by directly sampling the RF cavity signal at high frequency and then demodulating digitally, the amplitude noise is small and one can use the excellent phase noise of the numerically controlled oscillator to reach the required 10 ppm resolution. This has already been achieved on the bench using off-the-shelf ADCs and FPGA, but the PYNQ RFSoC 4x2 board offers an all-in-one package of 14-bit 5 GSPS ADCs and a Xilinx Zynq Ultrascale+ FPGA. We present the board's resolution and quantify its nonlinearities and other systematic uncertainties.