Introduction

pysmurf is the Python control software for the SLAC Microresonator RF (SMuRF) cryogenic detector readout system. It reads out frequency-multiplexed superconducting microresonator arrays (umux TES bolometers, MKIDs) via closed-loop tone tracking.

For the full instrument description, see Yu et al. 2022.

Typical SMuRF integration. RF probe tones interrogate cryogenic resonators; DC lines supply TES bias, flux ramp, and amplifier bias.

Hardware Overview

Block diagram of a full SMuRF system. Green: FPGA carrier. Blue: AMCs and RF daughter cards. Purple: RTM and cryostat card.

A single SMuRF system occupies one slot in an ATCA crate and consists of:

FPGA carrier (Xilinx KU15P) – performs all real-time DSP: tone synthesis/analysis via polyphase filter banks, per-channel tone tracking, flux ramp demodulation, and data streaming.

Advanced Mezzanine Cards (AMCs) – up to 2 per carrier, each providing 2 GHz of RF bandwidth. Each AMC has 4 ADC/DAC pairs (defining 4 bands), clock generation, and an LO that up/downmixes between the ~614 MHz digital baseband and the 4–8 GHz RF frequencies. Interchangeable RF daughter cards select low-band (4–6 GHz) or high-band (6–8 GHz) operation.

Rear Transition Module (RTM) – generates low-frequency signals:

• TES bias: bipolar DAC pairs providing DC voltage bias to detector bias groups

• Flux ramp: sawtooth waveform that drives all SQUIDs on a line through multiple flux quanta, linearizing the SQUID response

• Amplifier bias: gate/drain voltages for cryogenic 4K and 50K LNAs

Cryostat card – mounts at the cryostat vacuum feedthrough; conditions and filters the RTM signals before they enter the cryostat.

The system reads out up to 3328 channels across 4 GHz, consuming ~210 W per slot.

Bands, Sub-bands, and Channels

Construct	Width	Count	Hardware mapping
Band	500 MHz	0–7	One ADC/DAC pair on an AMC; 4 bands per AMC, 2 AMCs per carrier
Sub-band	2.4 MHz	512 per band (416 processed)	One bin of the 2x-oversampled polyphase filter bank; one channel processor slot
Channel	2.4 MHz	0–511 per band (416 active)	1:1 with sub-band; one tracked resonator

A band corresponds to the 500 MHz analog bandwidth of one ADC/DAC pair. Each AMC has 4 bands; the LO on each AMC up/downmixes between baseband and the target RF frequency, and the bands are stitched together via a quadruplexer to cover up to 4 GHz total (4–8 GHz with two AMCs).

The polyphase filter bank channelizes each band into 512 sub-bands, each 2.4 MHz wide (2x-oversampled 64-point FFT, yielding overlapping bins). Each sub-band holds exactly one channel – a single tracked resonator. The outer sub-bands are dropped to stay within the analog filter passband, leaving 416 active channels per band.

In pysmurf, you address a resonator by its (band, channel) pair. Most single-bay setups use bands 0–3.