Integrated Electrical Properties of the Frequency Multiplexed Cryogenic Readout System for Polarbear/Simons Array

POLARBEAR/Simons Array is a cosmic microwave background (CMB) polarization experiment, with science goals including a deep search for CMB B-mode polarization created by inflationary gravitational waves, and characterization of the CMB B-mode signal from gravitational lensing. When completed, Simons Array will have over 20,000 superconducting transition-edge sensor bolometers, cooled to 300 mK with closed-cycle cryogenics. This significant increase in detectors and sensitivity is enabled by advancements in frequency multiplexed readout. The detectors are read out with DfMux digital frequency-domain multiplexing, with 40 channels read out by each SQUID array, with frequency channels between 1 and 5 MHz defined by an inductor and capacitor in series with each detector. After the system was completed and characterized for POLARBEAR-2a, modifications were made that further improved performance for POLARBEAR-2b. We present the characterization of the integrated cryogenic readout system for POLARBEAR-2a and POLARBEAR-2b, and comparison of properties including stray impedance and frequency scatter.