Hardware

Overall configuration

Light from the pulsed laser is guided into the dark room through a polarization-maintaining fiber and focused onto the sensor surface by an objective. The readout board carrying the sensor sits on a 3-axis stage, and the stage moves to scan the illumination point. Four strips, C1–C4, are read out through the on-board amplifiers into the DC 50 Ω inputs of the oscilloscope. The time reference is the laser TRIG OUT, fed into the external trigger (EXT) of the oscilloscope. All devices are operated from the GUI on the control PC.

flowchart LR
    subgraph DARK["Dark room (whole room)"]
        OBJ["Objective<br/>PAL-50-NIR-L (50×)"]
        DUT["AC-LGAD strip<br/>on FNAL 16ch board"]
        STG["3-axis stage<br/>SHRC-203"]
        CAM["Focus camera (USB)"]
    end
    LAS["PiLAS DX<br/>1064 nm / 50 ps"] -->|PM fiber FC/APC| OBJ
    OBJ -->|IR spot| DUT
    STG -.- DUT
    DUT -->|"C1–C4 (GALI-S66 ×2 stages)"| OSC["LeCroy WavePro 804HD<br/>DC 50 Ω"]
    LAS -->|TRIG OUT → EXT| OSC
    HV["HV −190 to −195 V<br/>Matsusada P4L650-0.1"] --> DUT
    LV["LV 6.0 V<br/>R&S NGP800"] --> DUT
    PC["Control PC (Windows)<br/>laser_gui"]
    PC ---|LAN| OSC
    PC ---|LAN| STG
    PC ---|"USB virtual COM (COM5)"| LAS
    PC ---|USB| CAM

Photos

The measurement table inside the dark room (the whole room). Oscilloscope on the left, fiber and optics at the back center, stage on the right

Close-up of the measurement table. From left: oscilloscope, fiber and optics, objective and stage. The joystick JS-301 is in front

50× objective (PAL-50-NIR-L) and the readout board on the stage

Readout board inserted in the fixing jig on the stage. The MCX–BNC cables in front carry CH labels (ch5, ch8, etc.)

Individual devices

PiLAS laser head with the "CLASS 1 LASER PRODUCT" label

PiLAS controller (back left of the dark room). Switch the power ON with the key at the bottom right

Stage controller SHRC-203 (under the workbench). The power switch is at the bottom right

Joystick JS-301. M-ORG can return it to the origin, but use [Mechanical Home (H:W)] in the GUI instead (why)

Oscilloscope LeCroy WavePro 804HD (left side of the dark room). The power button is at the bottom left

Visible light source for the camera. ON/OFF with the red switch at the bottom right. Always OFF during measurement

Equipment

Device	Model / specification	Setting / measured value in this measurement
Laser	NKT Photonics PiLAS DX (head NPIL1-106-40FC, controller EGI-D2-40). Wavelength 1064 nm (1063.4 nm in the test report), pulse width 50 ps, repetition rate adjustable up to 40 MHz, trigger jitter < 3 ps rms	Intensity set with TUNE [%] (0 % strongest, 100 % weakest)
Fiber	Polarization-maintaining single mode (PANDA type), FC/APC	—
Objective	OptoSigma PAL-50-NIR-L: 50×, NA 0.45, working distance 15.1 mm, focal length 4 mm (a 10× PAL-10-NIR-A also exists, but it is not swapped in for use)	Spot σ ≲ 1.2 µm (in focus)
Stage	OptoSigma SHRC-203 (3 axes, 5-phase stepper motors), joystick JS-301	2 µm/pulse (X, Y and Z, confirmed on the hardware)
Oscilloscope	Teledyne LeCroy WavePro 804HD (8 GHz, 12 bit)	10 GS/s, sequence mode 2000 segments/point, 50 mV/div, DC 50 Ω
Sensor	HPK strip AC-LGAD: pitch 500 µm, metal electrode width 50 µm, exposed length 9318 µm, thickness 50 µm, n+ sheet resistance 1600 Ω/□, coupling capacitance 600 pF/mm². C1–C4 of the 6 strips (C0–C5) are read out	HV −190 to −195 V (at a sensor temperature of 33 °C). No cooling
Readout board	FNAL 16-channel board. Two stages of Mini-Circuits GALI-S66 per strip. Mounted by inserting it into a fixing jig	LV 6.0 V
Signal cables	MCX–BNC, 75 cm long	Change the connection on the board side to the channels you want to see
LV supply	Rohde & Schwarz NGP800 series	6.0 V (manual operation)
HV supply	Matsusada P4L650-0.1	−190 to −195 V (manual operation, changed with temperature; limit −220 V)
Focus camera	USB camera (manual-focus lens, no autofocus)	Read with OpenCV, device index 0
Control PC	Windows, Python 3.10 or later. GUI laid out for a 16:9 wide monitor (EX-LDHU431DB)	Fixed IP on the instrument LAN

Power and switches

The devices can be powered on in any order. However, bias the sensor last, in the order LV → HV.

Device	Location	Operation
Laser (PiLAS controller EGI-D2-40)	Back left	Turn the key to the right to switch ON. Normally control emission from the GUI with [Emission ON (ld=1)] / [Emission OFF (ld=0)]. When working inside the dark room, use the red button on the unit
Stage controller SHRC-203	Under the workbench	Switch at the bottom right of the unit
Visible light for the camera (illumination)	Upper right side	Switch at the bottom right of the unit. Always OFF during measurement
Oscilloscope	Left side of the dark room	Power button at the bottom left
LV supply (R&S NGP800)	Left side of the shelf below the oscilloscope	Button at the bottom left to switch ON → select CH1 → OUTPUT button to turn the output ON (lights blue). Fixed at 6 V, current limit 1.0 A
HV supply (Matsusada P4L650-0.1)	Right side of the shelf below the oscilloscope	Switch for power ON/OFF → black button at the top to SET the HV → red button to turn the bias ON. The supply ramps up automatically. Limit −220 V, current limit 1.0 mA

LV supply (R&S NGP800) with the output OFF

LV output ON (CH1: 6.000 V, about 0.47 A). The OUTPUT button lights blue

HV supply (Matsusada P4L650-0.1) with the bias OFF

Bias ON (display 189.8 V; a current display of 0.0 mA is normal). Turn the bias ON with the red button at the bottom left

Typical currents: the current limits are set to 1.0 mA (HV) and 1.0 A (LV). In normal operation the HV current is about 0.4–5 µA (depends on the bias voltage; the supply displays 0.0 mA) and the LV current is about 0.450 A. If the readings are far from these, turn the bias OFF and find the cause.

At the end, turn off everything except the control PC (all devices in the dark room). Leave the control PC on for remote control.

Swapping the readout board

The readout board is mounted by inserting it into a fixing jig. For the first time we recommend having an experienced person mount it, but it is not a particularly dangerous task.

1. Set the laser to [Emission OFF (ld=0)] and turn the HV bias OFF. The supply ramps down automatically, so wait until the HV display reads 0 V before turning the LV OUTPUT OFF.
2. Disconnect the MCX–BNC cables.
3. The red fixing rods (the red pins at both ends of the board in the photo below) are hard to remove by hand, so pull it out with thin pliers. Take care not to damage the sensor or the objective.
4. Pull the board out of the fixing jig and insert the new board. Tape or the like stuck on the board makes it hard to insert, so peel it off.
5. Connect the MCX–BNC cables to the channels you want to see.
6. Select that sensor in the GUI (register it if it is new). The position changes after a swap, so run a Full Calibration.

Board inserted in the fixing jig. The red pins at both ends are the fixing rods. The objective is directly above, so do not hit it when pulling them out

MCX connectors on the board and the MCX–BNC cables. Check the CH by the cable labels

The readout board on its own (with a ruler for scale; graduations in cm)

Connections and addresses

The GUI controls 4 devices, and each panel on the screen corresponds to one device. The defaults are in config.py; per-PC differences are overridden in config_local.json (Control PC and software).

Panel	Device	Connection	Default address
Oscilloscope	LeCroy WavePro	LAN. VICP (default) or VXI-11. On connect, the *IDN? response is checked; if it fails, the other protocol is tried automatically	VICP::<scope IP>::INSTR
Stage	SHRC-203 (3 axes)	LAN (TCP) or RS-232C. SHOT-compatible commands, CRLF delimiter	<stage IP>:<port>
Laser	PiLAS DX (EGI-D2-40)	USB virtual COM port (19200 baud, 8N1, CR terminated). Requires the FTDI VCP driver	COM5
Camera	Focus camera	USB. OpenCV (DirectShow preferred on Windows)	Device index 0

• The IP addresses, ports and user names of the devices are in Internal information (password protected).
• The control PC and the devices are on the same LAN segment. Set the control PC's subnet mask to 255.255.255.0 (with 255.255.255.255, even devices on the same segment are not reached directly and the stage stops responding; see Troubleshooting).
• The oscilloscope is synchronized by feeding the laser TRIG OUT into its external trigger (EXT).
• Scope data are retrieved to the PC over SSH/SFTP (user name: Internal information; ask the contact person for the password).

Readout channel map (standard sensor)

Scope CH	Board pad	Strip	Electrode center Y (logical coordinates)
C1	J8	C1 (reference CH)	0 µm
C2	J7	C2	−500 µm
C3	J6	C3	−1000 µm
C4	J5	C4	−1500 µm

C0 (+500 µm) and C5 (−2000 µm) are outer strips that are not read out. The signals go to the scope through MCX–BNC cables (75 cm); change the connection on the board side to the channels you want to see. This map can be set per sensor profile and is shown in the GUI status bar and in the CH labels of the oscilloscope panel.

Time reference

Arrival times are measured from the trigger time of each scope segment (LeCroy TRIGTIME). For the time resolution relative to TRIG OUT, laser synchronization and digitizer jitter add a floor common to all channels, \(\sigma_\text{ref} \approx 12.5\) ps. This floor cancels in the time difference between two strips.

Do not use horiz_offset as the time origin

horiz_offset in the .trc file header is a random number within one sampling period; using it as the time origin adds about 29 ps of fake jitter. In ROOT-converted files, use the trig_offset branch.

Setup performance (reference)

Measured values from the analysis note matome (all numbers are preliminary).

Item	Value
Spot size	σ ≲ 1.2 µm (in focus, error-function fit to the metal edge). 1.3–1.5 µm when 6–10 µm out of focus
Laser intensity stability	Drift of about 1.3 %/h, ±2–3 % over the whole period
Baseline noise	RMS about 1.7 mV (S/N about 40)
Stage reproducibility	Residual RMS of straight-line fits to strip centers 1.8–2.6 µm
Common time floor	\(\sigma_\text{ref} = 12.5 \pm 0.5\) ps