SRS Photon Counter

The SR400 gated photon counter is a Stanford Research Systems instrument that can receive, and then count, electronic pulses from a single-photon detector. This page will describe the basic operations of the SR400 and give some examples of how to use the unit with signals sent from a function generator. Additional details about the instrument, and more complicated examples, can be found in the manual for the instrument.

The SR400 is equipped with three counters, labeled A, B, and T, which can count rates up to 200 MHz. The input to counter A can be either the 10 MHz internal timebase or the signal at INPUT 1. The input to counter B can be either the signal at INPUT 1 or INPUT 2. The input to counter T can be the 10 MHz internal timebase, INPUT 2, or an external TRIGGER source. Each of these inputs can be selected using the MODE menu on the front panel. To move between different settings, use the up, down, left, and right arrows. Once on a setting, the knob on the front panel can be used to change the settings.

In order to count any signals, the COUNT PERIOD needs to be set. The simplest way to set the COUNT PERIOD is to select the source to counter T as the 10 MHz timebase. This allows the user to select a fixed interval, ranging from 100 ns to 25 hours, over which to count. This interval may be selected by changing the TSET parameter, which is on the same setting page as the one used to select the source to input T. More than one COUNT PERIOD can be implemented by changing N PERIODS to a number other than 1. If multiple count periods are being used, then there will be a DWELL TIME between COUNT PERIODS. The DWELL TIME can be changed under the DWELL setting in the MODE menu. On the same setting page there is an option to set N=STOP or N=START. If N=STOP is selected, then once the last COUNT PERIOD is reached all counting is paused. If N=START is selected, then once the last COUNT PERIOD is reach, all counting is repeated. This setting is used for continuous data acquisition. To count during the entire COUNT PERIOD, the GATE settings on A GATE and B GATE should be set to CW. Gate settings can be adjusted using the GATE menus. Lastly, the discriminator level should be set under the LEVEL menu. The discriminator level determines what threshold is used to count signals. The DISC level may be FIXED or in a SCAN mode (in which case a SCAN step size must be chosen). The FIXED or starting SCAN level can be adjusted under DISC LVL. Once the COUNT PERIOD, DWELL TIME, and GATE menus are set, one can connect signals to INPUT 1 and INPUT 2. Both inputs are internally terminated into 50 ohms and can accept signals up to ±300 mV (but are protected to ±5 Vdc). Pulses as small as 10 mV can be detected. To display the counts for A and B select A,B FOR T PRESET on the MODE menu. Before moving onto the example experiments, it is worth noting that SR400 has more features that can be taken advantage of. For example, one can scan a gate across the COUNT PERIOD, or use and external TRIGGER to start the COUNT PERIOD, instead of the internal timebase.

Example 1: Counting the Timebase

This example is taken from page 59 of the manual and involves no external inputs to the counters. Before turning on the SR400 press down the STOP key on the front panel and hold it down. Then push in the power button. This resets all settings to their defaults and will display a message RECALL DEFAULT. In the MODE menu select the A input to be the 10 MHz timebase, and then press the START key once to start a count period. The A counter should display A=10000000, as the default count period is 1 second. Press STOP to reset the counter. Now set the N PERIODS to N PERIODS=10 and press START again. The counter should cycle through 10 COUNT PERIODS with a DWELL TIME of 1 second.

Example 2: Counting from an External Source

Reset the settings to their defaults by pressing down the STOP key while powering on the SR400. This resets all settings to their defaults and will display a message RECALL DEFAULT. Connect a BNC splitter to a function generator to allow the signal from the function generator to be viewed on an oscilloscope and sent to the SR400. Use the function generator to send a 1 kHz square wave with a trough to peak amplitude of 100 mV. Verify this is the output on an oscilloscope. Now connect the function generator to the SR400, with a 50 Ohm termination, and press START. The count on A should read 1000, because the COUNT PERIOD is 1 second by default. One can extend this example by scanning the discriminator threshold. To do this set the A DISK LVL to -15 mV and A DISC SCAN to -10 mV. Next set N PERIODS to 5. Now press START. There are five count periods, but the counter should only make it to 1000 counts during the first three count periods as the signal will be less than the discriminator threshold level for the last two count periods. The discriminator threshold level can be viewed on the DISC LVL settings page. To add another signal, one can use INPUT 2 with another function generator.