SR400 gated photon counter

Yes, provided the detector pulses are compatible with the electrical input limits of the SR400 gated photon counter. Its two 50 Ω inputs have DC to 300 MHz bandwidth, accept pulses within a ±300 mV linear range and can detect pulses as small as 10 mV. Each discriminator can be set between −300 mV and +300 mV in 0.2 mV steps, with rising- or falling-edge selection for either pulse polarity. A suitable fast preamplifier may be required when the detector signal is below 10 mV or cable losses reduce the available pulse amplitude. For PMT installations, a leakage path of approximately 100 kΩ to ground should be provided within the PMT base to prevent cable charging that could damage the input.

The gate width should be matched to the part of the detector response that contains the required signal while excluding as much unrelated background as practical. Each channel provides a gate width from 5 ns to 999.2 ms, with a 25 ns insertion delay and a programmable delay extending to 999.2 ms. Timing resolution is 0.1% or 1 ns minimum, while specified gate accuracy is 2 ns + 1% and jitter is 200 ps RMS + 100 ppm. For lifetime measurements, the gate can be scanned across as many as 2,000 count periods to reconstruct the temporal response. The 1 MHz maximum trigger rate must also be considered when setting the repetition rate of the experiment.

Yes, the independent A and B channels can be arranged to count signal and background windows within the same experiment. In a chopped optical measurement, one gate can be positioned during the illuminated interval while the other measures the closed or background interval. Selecting A − B then produces a background-corrected count result, while A + B is available when the two channels need to be combined. Equal gate widths should normally be used for synchronous subtraction so that both channels observe equivalent time intervals. Because the gate delays and operating modes are independently adjustable, the same approach can also support a scanned signal window with a fixed background reference.

The counter electronics accept rates up to 200 MHz and specify 5 ns pulse-pair resolution, but the complete detector chain may impose a lower practical limit. The signal inputs provide DC to 300 MHz bandwidth and require a minimum pulse amplitude of 10 mV, so pulse shape, cable bandwidth and impedance matching remain important. The separate 1 MHz specification applies to the gate trigger rate rather than the individual pulses counted inside a gate. At high PMT rates, detector recovery time, base design, gain variation and pulse pile-up can affect counting linearity before the electronic counter limit is reached. The full installation should therefore be validated over the intended count-rate range using the selected detector, base, bias voltage and preamplifier.

The instrument provides GPIB and RS-232 interfaces for remote setup, control and data transfer. A scan may contain between 1 and 2,000 count periods, with the A and B values stored in separate 2,000-point internal buffers. Dwell intervals can be programmed from 2 ms to 60 s or controlled through external start and stop inputs, allowing other instruments to be adjusted between measurements. RS-232 communication operates at up to 19.2 kbaud, while GPIB can be used where an IEEE-488 laboratory bus is already installed. Generic *IDN? queries should be avoided during National Instruments Measurement & Automation setup because they can lock the instrument; the dedicated SR400 command set should be used instead.

Continuous operation is suitable when the photon stream is not tied to a short timing event and counting should remain enabled throughout each count period. Fixed-gate operation is intended for pulsed or synchronised experiments where only a defined time window after the trigger contains useful information. The minimum 5 ns gate width can isolate short events, while widths up to 999.2 ms accommodate slower processes. Scanned-gate operation moves the timing window between successive count periods, enabling lifetime curves and other time-varying responses to be reconstructed. Chopped measurements can use separate A and B gates for illuminated and background intervals, with the resulting counts combined through A − B processing.

The instrument requires bench or rack space for an enclosure measuring 16 × 13 × 3.5 inches and weighing 10 lb. It consumes 35 W and can be configured for 100, 120, 220 or 240 VAC operation at 50/60 Hz. The voltage selector must match the available supply before power is connected, and a 1 A slow-blow fuse is specified for each supported voltage setting. A protective earth connection is required because exposed metalwork and the front-panel BNC shields are connected to chassis and mains earth. No voltage should be applied to the BNC shields or instrument outputs. PMT systems also require appropriate high-voltage procedures and a correctly designed base that prevents stored cable charge from reaching the signal input.