SRS SR255 Gated Integrator

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The Stanford Research Systems SR255 Fast Sampler module is a gated integrator with four discrete, user-selected gate widths from 100 ps to 1 ns. All of the necessary electronics are built into this high-speed module including an A/D, D/A and a PROM correction circuit to eliminate the inherent non-linearities in the sampling bridge.

The SRS SR255 is perfect for fast pulsed experiments where the 2 ns minimum gate width of the SR250 is insufficient. The gate delay is controlled by a rear-panel voltage input which can be supplied by the SR200 Gate Scanner or the SR245 Computer Interface. Convenient gate view and signal outputs allow precise positioning of the gate—particularly important in applications such as time domain reflectometry or shorted-cable baseline subtraction. The SR255 can be used alone, combined with the SR245 Computer Interface module for automated data acquisition, or operated with the SR200 Gate Scanner for scanning gate waveform recovery.

Triggering
The SR255 has a DC coupled trigger input (50 Ω) that supports trigger rates up to 50 kHz. The trigger threshold can be set to -0.5 V, +0.1 V or +1 V on the front panel. For reliable triggering, the trigger must remain over threshold for at least 5 ns and not exceed 5 V.

Gate Delay
The delay from trigger to sample is controlled by an analog voltage applied at the rear of the unit. You can select delay ranges of 1, 10, 100 or 1000 ns/V with four switches accessed through the instrument's side panel. In addition to the adjustable delay, there is a fixed insertion delay of about 20 ns. With only 2 ps rms gate jitter, the SR255 makes it easy to set up and maintain precise gate timing.

Gate Width
Four fixed gate widths of 100 ps, 200 ps, 500 ps and 1000 ps can be selected. A front-panel LED indicates which gate width is being used.

Gate View and Fast Timing
You can use the gate view output to time the sample gate with respect to the signal. The leading edge of the gate view output (50 ps rise time) indicates when the sample gate is being opened. The output is a pulse of approximately 3 V with an exponential decay of about 4 ns.

Signal Input
The SR255 is designed to be used with RG58A cable. To achieve an optimally flat response, two meters of RG58A cable should be used with a BNC to N-Type converter (provided with each SR255). The frequency response of the SR255's front end has been peaked above 2 GHz to compensate for the losses in 2 meters of cable, and so this length of cable is recommended for the signal input line.

The signal input is passed to the signal out BNC via an internal 300 ps delay line. The signal output should be terminated in 50 Ω with a high quality terminator to minimize reflections and pulse distortion. The signal output aids in synchronizing the sample with the gate. The input signal should not exceed the sensitivity selection on the front panel, as the input is only protected to +5 VDC.

The sensitivity of the module (Vin/Vout) can be set to 1 V/V, 0.25 V/V or 0.1 V/V. For example, when 0.1 V/V sensitivity is selected, a 100 mV input will produce a 1 V output. The red overload LED will come on when the output exceeds 1 V.

Analog and Digital Outputs
The SR255 has both analog and digital outputs making it easy to interface in a wide range of experiments. The sample output provides a ±1 VDC full-scale analog voltage proportional to the value of the signal during the gate interval. Resolution is 0.5% of full scale, and the output can drive up to 10 mA. This output is available 20 µs after the signal is sampled. Each unit is custom linearized with a PROM to ensure excellent linearity and full dynamic range.

Interface
The Fast Sampler also provides a digital interface through a 15-pin connector on the rear panel of the module. This interface is a parallel, binary interface which may be connected to either the 20-pin connector on the circuit board of the SR245 Computer Interface module or to any 8-bit digital I/O port.