QuantumBOX Firmware

FPGA firmware for the Nuclear Instruments QuantumBOX (NIDAQ121), a digitizer for quantum optics experiments with SiPM and PMT detectors. All the front-end processing runs on the board in real time: signal conditioning, triggering, energy measurement, timestamping, coincidences and event readout. The host only sets parameters and reads data over Ethernet.

Overview

• 8 analog processing channels with 4 ns sampling
• Two operating modes: photon counting and photon timing
• Leading edge and CFD triggers on every channel
• QDC charge integration or peak holder for energy
• Timestamps with 4 ns resolution, or about 2 ps with CFD interpolation
• Hardware energy spectra, counters and coincidence units
• Built-in oscilloscopes and a test pulse generator
• Full control over Ethernet through SciSDK

Signal processing chain

Input routing. The 8 physical inputs (two RJ45 connectors with 4 differential pairs each) go through an input router (INSEL1…INSEL8). Any input can be mapped to any processing channel, which helps with non-standard wiring or when several channels should process the same signal.

Conditioning. Each channel has:

• a programmable input delay
• baseline restoration with selectable mode, length, hold and constant
• pole-zero compensation, which can be bypassed
• an adjustable baseline zero offset

Triggering. Each channel has two discriminators:

• Leading edge: threshold, low threshold (hysteresis), inhibit time and polarity.
• CFD (constant fraction): delay, fraction and threshold. Gives sub-sample timing that does not depend on pulse amplitude.

Energy. Pulse charge is integrated by a QDC with programmable pre-integration, integration time (QDC_TQLONG) and gain. A peak-holder mode can be selected instead. Calibration energy spectra are built in hardware for all channels.

Operating modes

Photon counting (ACQ_MODE = 0)

A common trigger samples all 8 channels at the same time. The trigger can be validated by AND/OR logic over a selectable channel mask, with a programmable coincidence window and delay, and an optional acquisition gate.

Each event goes to the List_0 FIFO as a 32-byte record:

Host software uses these events for photoelectron classification, photon statistics (mean, variance, Fano, g²(0)) and channel-pair correlations (g₁₁, NRF, γ).

Photon timing (ACQ_MODE = 1)

Each channel triggers on its own. Events from all channels are merged by a round-robin arbiter into the List_1 FIFO. Each event is a 64-bit word:

• Leading edge: the timestamp is linear with a 4 ns LSB. It wraps after about 6.5 days.
• CFD: bits 46–16 are the coarse time (4 ns LSB, wraps after about 8.59 s). Bits 15–0 are the signed fine time (1 sign + 4 integer + 11 fraction bits, in samples), for about 1.95 ps resolution.

Event order is guaranteed within a channel but not between channels. Cross-channel measurements must therefore match events by time on the host side. Typical uses are inter-arrival histograms and channel-to-channel time differences.

Monitoring

• HW counters: per-channel event counts in both modes (EVCOUNTS, page_chcounts_X_COUNTS_TOT), lost events, and trigger-input and lost counters in counting mode.
• Coincidence units: independent blocks, each with an 8-bit channel mask (bit = 1 means the channel is excluded), its own counter, and per-channel trigger pulse width and delay (TRG_C_WIDTH, TRG_C_DELAY).

Diagnostics

• Trigger oscilloscope (Oscilloscope_CFD): input and CFD signals.
• Channel oscilloscopes (page_ch_core_X_Oscilloscope_1): input plus a selectable second trace (QDC input or CFD), with digital traces for trigger and gate signals.
• Pulse generator: programmable period and width, for tests without detectors.

Interface

The board is reached over Ethernet (default 192.168.102.199:8888) through the SciSDK library. Registers and components are described by the register map daq1x1.json. List_0 and List_1 can be read on separate connections, so streaming data never blocks register access or oscilloscope readout.

Supported software:

• QUANTA: desktop application for configuration, calibration, acquisition and analysis
• quantumbox-sdk: Python package (pip install quantumbox-sdk)

License

© Nuclear Instruments. All rights reserved.