NIM Model 621L Quad Discriminator

LRS Model 621 L retains the format and operating features of the world's most widely used discriminator, the LRS Model 321BL. A new hybrid input stage provides substantial improvement in input characteristics: an almost perfect impedance match to eliminate reflections and consequent multiple-pulsing; a drift-free -30 mV threshold; overload protection to withstand outputs from even the most serious phototube malfunctions; virtually no input dc offset; and a new standard of compactness and reliability. A threshold monitor test point is provided on each channel to permit accurate and reproducible threshold settings using an external DC voltmeter.

Output durations are continuously adjustable from 5 ns to us and are highly stable and independent of input amplitude, duration, and rate. Their long-term stability is excellent, permitting their direct use in critical coincidence applica- tions without any need for external clipping cables. Each channel provides five standard amplitude negative N I M current source outputs and one complementary output. The flexibility resulting from this doubling of the output fan-out capability over previous circuits permits simpler and more efficient logic design. This greatly increased fan-out is achieved by means of a new output circuit design that utilizes very little quiescent power.

The -30 mV threshold offered by the Model 621 L is almost a factor of two lower than that of the most sensitive previous circuits. It will permit experimenters to routinely reverse-terminate photomultiplier anodes. This procedure, coupled with the greatly improved input termination characteristics of the 621 L, greatly reduces the possibility of multiple-pulsing due to reflections in the input system.

The pulse-forming circuit in the Model 621 L is deadtimeless (updating), and the unit may be retriggered during the time an output from a previous input signal is being produced.

     SPECIFICATIONS
      HIM Model 621L
     QUAD DISCRIMINATORS

 INPUT CHARACTERISTICS

 Signal Input:   Threshold, -30 mV to -1.0 volt continuously variable (front-panel
     screwdriver adjustment: screwdriver included); 50 Q protected to +/-5 A
     for 0.5us clamping at +.5 V, -6.5 V; direct-coupled; reflections <2% for
     input pulses of 2 ns risetime; stability <0.2%/'C over 20' C to 60' C
     operating range; offset 0 +/-2 mV; threshold monitor 1 0: 1 ratio of monitor
     voltage to actual voltage.

 Gate:    Slow gate via rear connector and real panel ON-OFF switch; rise and fall
     times, approximately 50 ns; clamp to ground from +5 inhibits;
     direct-coupled.

 OUTPUT CHARACTERISTICS

 Outputs:    Six, NIM; one positive (quiescently -16 mA, 0 mA during output), two
     bridged negative (0 mA quiescently, -32 mA during output); one negative
     (0 mA quiescently, -16 mA during output).

 Duration:    Continuously adjustable via front panel screwdriver control from 5 ns to
     1 us.

 Rise and Fall Times:  Less than 2.0 ns, all outputs, 1 0% to 90%. Output falltimes slightly longer
     on wide output durations.

 GENERAL

 Maximum Rate:   1 20 MHz typical, input and output.

 Double Pulse Resolution:  8 ns.

 Time Slewing:   1 ns for input amplitudes 1 10% of threshold and above.

 Input-Output Delay:  9.0 ns typical.

 Multiple Pulsing:   None; one and only one output pulse of preset duration is produced for
     each input pulse, regardless of input pulse amplitude or duration.

 Packaging:    In RF shielded AEC/NIM #1 module.

 Power Requirements:  -6 V at 340 mA; +6 V at 130 mA; - 1 2 V at 30 mA; + 1 2 V at 40 mA.
     Optional, at slight additional cost: internal power supply operating from
     120 VAC to eliminate +/-6 V requirement.