The readout system is another enabling technology, developed over many years specifically for CMB experiments. The polarbear readout allows the simultaneous recording of data from all pixels through a minimum number of wires. The system uses SQUIDS for their wide bandwidth and current sensitivity, paired with high speed digital signal processing.The reduction in wiring is accomplished by multiplexing many bolometer signals into a signal wire. The multiplexing comes in the form of placing the individual signals at different frequencies, much like different frequencies on the radio.
Large arrays of bolometric detectors require sophisticated readout schemes. To reduce thermal loading onto the coldest stages of the experiment and to reduce the complexity of instrumenting large arrays, the readout of these arrays is multiplexed. The readout multiplexing scheme that has been developed and demonstrated at Berkeley is frequency-domain multiplexing.Each sensor is biased with a sinusoidal voltage at a unique frequency. The sensor signals are thus separated in frequency space and can by summed before being readout by SQUID electronics (superconducting quantum interference device). Each sensor is placed in series with a tuned filter consisting of an inductor and a capacitor with values chosen to give center frequencies from 300 kHz to 1 MHz.
Pictured above is a schematic outlining the demonstration of an eight-channel readout multiplexer. Also shown is a picture of the inductor chip. The chips were manufactured at TRW-Northrop Grumman and each chip contains eight inductors.
Pictured above is a spectrum across the readout multiplexer bandwidth showing Nyquist noise from the multiplexed sensors well above the expected readout noise. The sensor biased at 396 kHz sees an optical signal from a cooled LED and the adjacent sensor is monitored for crosstalk. The crosstalk is well below design requirements.