The Stanford Research Systems SR400 Dual-Channel Gated Photon Counter is a precision instrument that integrates amplification, discrimination, gating, and counting functionality into a single unit for low-light-level photon detection. It delivers independent counting on two primary channels with sophisticated gate control, variable discriminator thresholds, and flexible measurement modes—enabling time-resolved spectroscopy, waveform recovery, and photon counting across physics, chemistry, and optical research.
Technical Specifications
Counting Channels & Rates
• Two independent counting channels (A and B)
• Third counter (T) for timing, presets, or trigger input
• Count rate: up to 200 MHz per channel
• Pulse-pair resolution: 5 ns
Signal Inputs
• Two analog signal inputs (A and B)
• Input impedance: 50 Ω (internally terminated)
• Input range: ±300 mV
• Bandwidth: DC to 300 MHz
• Minimum detectable pulse: 10 mV
• Overload recovery: 5 ns
• Input protection: ±5 VDC, 50 V for 1 µs
Discriminators
• Independent threshold control per channel (A, B, T)
• Threshold range: −300 mV to +300 mV in 0.2 mV steps
• Detects either polarity
• Outputs: NIM level, negative-going 0 to −0.7 V
Gating
• Two independent gate generators (channels A and B)
• Gate duration: 5 ns to 1 second
• Gate delay from external trigger: 25 ns to 1 second
• Fixed or scanned gate positions for time-resolved measurements
Counting Modes
• Gated and continuous operation
• Count for fixed time, fixed count, or fixed trigger count
• Programmable scan cycles: 1 to 2000 periods
• Dwell time: 2 ms to 60 s between periods
Counter T
• Counts internal 10 MHz clock, Input 2, or external trigger
• Preset range: up to 25 hours or 9 × 10¹¹ counts
• Timer accuracy: 25 ppm from 0 to 50 °C
Outputs
• Gate and discriminator outputs
• Front-panel D/A: ±10 VDC, proportional to A, B, A−B, or A+B
• Rear-panel D/A (ports 1 and 2): ±10 VDC full scale, 12-bit resolution (5 mV)
• Dwell output: TTL level during dwell periods
– Key Features
• Remote preamplifier option (SR445A) for extended sensitivity
• Threshold scanning for automated discriminator optimization
• D/A output scaling (linear or logarithmic)
• External TTL control of count period initiation and termination
– Typical Applications
Time-resolved fluorescence spectroscopy, photon statistics measurement, waveform recovery from weak optical signals, and gated detection in pulsed optical experiments.
– Compatibility & Integration
Computerized instrument control via rear-panel interface; compatible with external trigger sources and TTL logic signals.
