Stanford Research Systems SR540 Optical Chopper Controller SR541 Motor & Blades

Stanford Research Systems SR540 Optical Chopper Controller SR541 Motor & Blades

The Stanford Research Systems SR540 is a precision optical chopper controller designed to drive the SR541 motor head at stable, selectable frequencies for use in phase-sensitive detection and lock-in amplifier experiments. The SR540 generates a reference frequency output synchronized to the spinning chopper blade, allowing a lock-in amplifier to extract weak signals buried in noise. The controller's LED display shows the chopper frequency in real time, and the frequency can be set by the front-panel dial or controlled externally via a 0–10 VDC analog voltage input.

Specifications & What's Included

• Controller model: Stanford Research Systems SR540 Chopper Controller
• Motor head: SR541 (connected via rear-panel Chopper Head Cable RJ port)
• Chopper frequency display: 4-digit red LED, reads in Hz
• Frequency adjust: front-panel dial; external control via 0–10 VDC BNC input
• Max frequency by blade configuration: 4 kHz (30-slot blade), 400 Hz (6-slot blade), 40 Hz (6-slot low-speed mode)
• Reference mode outputs (BNC): f_INNER, f, sf (sub-frequency), f_SLIM, f_DIFF
• Rear panel: Motor Cal trimmer, Chopper Head Cable RJ port, IEC AC inlet, rocker power switch, fuse holder
• Line fuse: 100/120V ≈ ¼A slow-blo; 220/240V ≈ ⅛A
• Power: 100/120V or 220/240V AC, 50/60Hz
• Included chopper blade 1: 6/5-slot, 4" diameter (large black blade on motor stand)
• Included chopper blade 2: 30/25-slot, 4" diameter (smaller blade)
• SR541 motor head mounts on adjustable black anodized aluminum stand with slotted base for optical rail mounting
• Country of manufacture: United States

Applications

Optical choppers are a standard tool in optical spectroscopy, photoacoustic spectroscopy, and laser measurement labs. By periodically interrupting a light beam at a known frequency, the SR540/SR541 system creates a reference signal that a lock-in amplifier (such as the SRS SR830 or SR810) uses to detect only the signal at the chopping frequency — rejecting broadband noise and drift. This technique is widely used in absorption spectroscopy, fluorescence measurement, laser power characterization, and any experiment requiring phase-sensitive signal recovery. The multiple reference outputs (f_INNER, sf, f_DIFF) make the SR540 particularly useful in dual-beam and differential chopping configurations, where two light paths are chopped at related or different frequencies. The SR540 was introduced by Stanford Research Systems in the late 1980s and remains in active use in research labs and university physics and chemistry departments today.