Gated Integrators and Boxcar Averagers Model SR255 — 100 ps gated integrator · Gates of 100 ps, 200 ps, 500 ps & 1 ns · Jitter <2 ps rms · Droopless analog output · Linear over full range · 20 ns insertion delay · Trigger rates to 50 kHz SR255 Fast Sampler The 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 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 shortedcable 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. Stanford Research Systems 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. In this example, the SR255 is used with the SR200 Gate Scanner and an X-Y oscilloscope to provide sampling oscilloscope operation. A narrow gate is scanned over a repetitive waveform in order to recover its shape. A sampling “scope” made from the SR255 and SR200 can achieve 100 ps resolution at rates up to 50 ksamples/s. Signal Input The SR255 is designed to be used with RG58A cable. To achieve an optimally flat response, two meters of RG58A cable...

Gate Generator Trigger in Termination Thresholds Trigger rate Gate delay Delay range Gate jitter Gate width Gate view Gate view accuracy Gate view rise time 50 Ω –0.5, +0.1 or +1 VDC DC to 50 kHz 20 ns + delay range × control voltage (Control input on rear) 1, 10, 100 or 1000 ns/V <2 ps rms 100, 200, 500, 1000 ps Leading edge indicates when gate opens. (N-type connector) ±50 ps with respect to signal out <50 ps SR255 rear panel Signal Channel Signal In Shot noise (typ.) Sensitivity Signal Out Characteristic impedance is 50 Ω. Full-scale input level equals the sensitivity setting. Protected to...