CCAT: a two-octave 1.024 GHz KID readout featuring an overlap-channel polyphase synthesis filter bank on RFSoC

Next-generation submillimeter instruments need gigahertz-scale readout bandwidths to handle the growing detector counts of large microwave kinetic inductance detector (KID) arrays. The CCAT Prime-Cam 850 GHz and 410 GHz instrument modules required a readout system with sufficient bandwidth and tone capacity, motivating the development of the second-generation (Gen2) KID readout system on a Xilinx ZCU111 radio frequency system-on-chip (RFSoC).

The Gen2 system uses a parallelized overlap-channel polyphase synthesis filter bank paired with a wideband receiving channelizer. This two-octave architecture reads out four independent RF networks, each with 1.024 GHz instantaneous bandwidth and support for up to 2048 detectors. The polyphase synthesis doubles the baseline channel count and bandwidth, and enables on-the-fly control of individual tone frequency, amplitude, and phase, which facilitates optimized KID biasing and provides a gateway to tone tracking. These capabilities could also benefit other frequency-division multiplexed (FDM) readout systems beyond CCAT.

The design was validated in DSP simulation, RF loopback tests, and with two KID test chips, one spanning the full two-octave band. Loopback measurements show noise performance comparable to the first-generation (Gen1) baseline, while preliminary on/off-resonance measurements indicate detector-noise-limited operation for the majority of channels. Digital channel crosstalk was measured and discussed. The paper also reports FPGA resource and power utilization and assesses scalability toward future large-format KID instruments.