Home Photon Counters SRS SR400 Dual Channel Gated Photon Counter

SRS SR400 Dual Channel Gated Photon Counter

Product Code:
Manufacturer: Stanford Research Systems
  • Two independent counting channels
  • Count rates to 200 MHz
  • 5 ns pulse-pair resolution
  • Gated and continuous modes
  • Gate scanning for time-resolved counting
  • Built-in discriminators
  • Gate and discriminator outputs
  • GPIB and RS-232 interfaces

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The Stanford Research Systems SR400 Dual-Channel Gated Photon Counter offers a convenient, integrated approach to photon counting that avoids the complexity and expense of older counting systems.

No longer is it necessary to mix and match amplifiers, discriminators, gate generators and counters. The SRS SR400 combines all these modules into a single, integrated, microprocessor controlled instrument. Complex measurement tasks such as background subtraction, synchronous detection, source compensation and pile-up correction can all be performed easily with the SR400.

The SR400 has two independent channels that can count at rates up to 200 MHz. Different counting modes allow you to count for a fixed amount of time, until a certain number of counts have been received, or for a fixed number of triggers. Each counting channel has its own gate generator providing counting gates as short as 5 ns or as long as 1 s. The gates can be set in a fixed position relative to the trigger signal, or scanned to measure lifetimes or recover time-varying waveforms.

The actual inputs to the counters can be viewed as NIM level pulses from the discriminator outputs on the front panel. The discriminated pulses are negative going from 0 to -0.7 V. The DISC outputs are very useful when adjusting discriminator thresholds or gate timing.

Signal Inputs and Discriminators
Both analog signal inputs (A and B) are internally terminated into 50 Ω. The inputs accept signals between ±300 mV and are protected to ±5 VDC. Each input is followed by a DC to 300 MHz amplifier, allowing detection of pulses as small as 10 mV. If additional sensitivity is required, a remote preamplifier (like the SR445A) can be used.

Discriminators are provided for each channel with a selectable threshold from -300 mV to +300 mV in 0.2 mV steps. Pulse-pair resolution is 5 ns, and pulses of either polarity may be detected. Each threshold may be programmed to scan in either direction with selectable step size. This can provide a pulse height analysis (PHA) output and is useful for choosing photomultiplier tube bias or discriminator levels.

Count Periods
The SR400 may be programmed to cycle from 1 to 2000 count periods in a single scan. At the end of the programmed scan, the counters may be stopped or the scan may be restarted. Consecutive count periods are separated by a "Dwell Time" which can be set from 2 ms to 60 s. During the Dwell Time, counting is disabled and data may be transferred or external parameters may be changed. The dwell output provides a TTL signal which is high during the dwell time. This can be convenient for interfacing other instruments used in the experiment.

The front panel can display counts up to 109. Results from both counters can be displayed individually or combined as A-B or A+B. A front-panel D/A output provides an analog signal proportional to A, B, A-B, or A+B, depending on the counting mode. The scale may be logarithmic (1V/decade) or linear.

Built-in RS-232 and GPIB interfaces provide a convenient means of controlling the instrument and retrieving data. While the SR400 is scanning, each of the count values for the A and B counters are stored in a 2000 point internal buffer. This buffer can be transferred on a point-by-point basis, or dumped all at once through either interface.

Signal Inputs  
Bandwidth DC to 300 MHz
Input impedance 50 Ω
Linear range ±300 mV (at input)
Input protection ±5 VDC, 50 V for 1 µs
Overload recovery 5 ns
Discriminator range -300 mV to +300 mV
Rising or falling
Resolution 0.2 mV
Input offset voltage  
Minimum pulse input 10 mV
Pulse-pair resolution 5 ns
DISC outputs NIM levels into 50 Ω
Inhibit input TTL high stops count
Trigger Input  
Impedance 10 kΩ
Threshold ±2.000 VDC in 1 mV steps
Slope Rising or falling
Protection 15 VDC, 100 V for 1 µs
Gate Generators  
Insertion delay 25 ns
Maximum delay 999.2 ms
Minimum gate width 5 ns
Maximum gate width 999 ms or CW
Resolution 0.1 %, 1 ns minimum
Accuracy 2 ns + 1 %
Jitter 200 ps rms +100 ppm
Maximum trigger rate 1 MHz
GATE view output NIM levels into 50 Ω
GATE view error  
Scan and Dwell  
Display Mode  
Continuous Displays current counter value
Hold Displays final count value
D/A Output  
Full scale ±10 VDC
Resolution 12 bits (5 mV)
Current rating 10 mA
Output impedance  
Accuracy 0.1 % + 5 mV
Interfaces GPIB and RS-232
Dimensions 16" × 3.5" ×13" (WHL)
Weight 10 lbs
Power 35 W, 100/120/220/240 VAC, 50/60 Hz
Warranty One year parts and labor on defects in materials and workmanship


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