The basic operation of an MKID, republished from Day et al. 4 (a) Photons with energy hν are absorbed in a superconducting film, producing a number of excitations, called quasiparticles. (b) To sensitively measure these quasiparticles, the film is placed in a high frequency planar resonant circuit. The change in the surface impedance of the film following a photon absorption event pushes the resonance to lower frequency and changes the amplitude of the transmission through the circuit. The dotted line in (c) shows the change of the transmitted microwave signal due to an incident photon. To monitor the resonant circuit, it is continuously excited with a microwave signal. (d) The phase of the transmitted microwave signal depends on the proximity of the continuous microwave signal frequency and the variable resonant circuit frequency. The energy of an absorbed photon can be determined by measuring the degree of phase shift of the transmitted microwave signal.
The block diagram of the SDR readout for ARCONS.
The required noise floor density in dBc/Hz of an ADC as a function of the number of resonators being read out and the readout power per resonator for an amplifier noise temperature of 6 K. With a 512 MSample/s sample rate, 10.6 ENOB, we expect about −147 dBc/Hz for the ADC’s phase noise. Even at the relatively high readout power −85 dBm, this would allow for reading out over 1000 channels. Alternatively, 256 channels could be read out with power as high as −70 dBm.
Custom crate to house up to eight ROACH assemblies. The SMA connectors to each IF board are visible on the front panel.
One of four ROACH assemblies including the ADC, DAC, and IF boards.
Schematic of channelizer firmware.
(a) Three sample frequency bins output from the 1st stage of channelization with the PFB. Each bin is 2 MHz wide, with ∼1 MHz overlap with adjacent bins. (b) The irregularly spaced, narrow frequency bins output from the 2nd stage. The bin width is 500 kHz wide and centered on the resonant frequency for a single pixel.
(a) Phase noise for the readout under three conditions. The red line is a loop-back test connecting the IF outputs and inputs, characterizing the noise contributed by the readout electronics. Aside from the spurious signals, the noise floor is close to the expected −147 dBc/Hz contributed by the ADC. The blue and green lines show the phase noise of a single channel through the array with one and 256 probe signals, respectively. Again, the noise floor is close to the expected −106 dBc/Hz contributed by the HEMT amplifier. (b) Typical phase response of a single channel in time due to 254 nm incident photons for both a low-pass filter and a matched filter.
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