584 Constant-Fraction Discriminator

ORTEC 584 Constant Fraction Discriminator• Good time resolution over a wide range of pulse amplitudes with scintillation and semiconductor detectors
• 50-MHz count-rate capability
• 5 mV minimum threshold
• Time walk ≤100 ps for 100:1 dynamic range
• Constant-fraction, leading-edge, and slow-rise-time reject modes

The ORTEC Model 584 Constant-Fraction Discriminator allows good time resolution to be obtained from all commonly used detectors such as HPGe, silicon charged-particle, fast plastic, NaI(Tl), and photomultiplier tubes. Three timing modes are provided in the Model 584: constant-fraction, constant-fraction with slow-rise-time reject, and leading-edge. This economical unit has a minimum threshold of –5 mV, allowing good timing measurements to very low energies. The maximum input signal acceptable without saturation is –5 V,which provides a 1000:1 input dynamic range. The Model 584 is useful in high-count-rate applications to 50 MHz with ≤20 ns pulse-pair resolving time. The time walk of the Model 584 is ≤±100 ps for a 100:1 input dynamic range.
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    A variety of controls is provided, allowing optimization of the Model 584 in various applications. A precision 10-turn potentiometer sets the threshold from –5 mV to –1 V. The blocking time set by the Blocking Output Width is continuously adjustable from ≤10 to ≥1000 ns. This feature is useful for preventing multiple triggering on pulses from scintillators having long decay time, e.g., NaI(Tl). A front-panel LED indicates that the discriminator has been triggered and can therefore be used to set the threshold just above the noise. Walk is adjusted by a front-panel 20-turn potentiometer. The Constant-Fraction Monitor on the front panel can be used to optimize walk adjustment. Since the constant-fraction shaping delay is selected by external cable, the optimum delay for a specific detector application is easily selected.

    Four NIM-standard output signals are available from the Model 584. The positive output signal is continuously variable from ≤0.5 to ≥2.5 µs by means of a printed wiring board (PWB) potentiometer. The polarity of the positive output is PWB selectable to be either a NIM-standard positive output signal or the complement signal. The two timing output signals are NIM-standard fast negative logic signals, each having a 2-ns rise time and a 5-ns width FWHM. The blocking output signal is a NIM-standard fast negative logic signal whose width is adjustable from ≤10 to ≥1000 ns.

    The Model 584 can be gated externally. A rear-panel locking toggle switch selects either Gated or Ungated operation. In the Gated Mode, a printed wiring board jumper selects the Bin Gate line in the NIM bin, a NIM-standard positive signal via the rear-panel BNC connector, or a NIM-standard negative signal via the rear-panel BNC connector.

    Logic current for the Model 584 is selected from either the –6 V or –12 V NIM supply by means of a rear-panel locking toggle switch. The Model 584 is within the allotment of current for a single-width NIM module for a NIM Class V power supply when the logic current is obtained from –6 V.
  • Specifications +


    Performance
    Input
    Accepts negative input signals from 0 V to –5 V without saturation; dc-coupled; Zin = 50 Ω; reflections ≤±5% for tr ≤2 ns.
    Threshold Range –5 mV to –1 V.
    Threshold Integral Nonlinearity ≤±0.25% of full scale.
    Threshold Instability ≤±100 µV/°C, 0 to 50°C.
    Propagation Delay Nominally 25 ns, with external CF Delay ≈2 ns.
    Minimum Pulse Pair Resolution ≤20 ns.
    Dead Time Nominally 20 ns or Blocking Output Width, whichever is greater.
    Blocking Output Width Adjustable from ≤10 to ≥1000 ns.
    Time Walk ≤±100 ps for the 100:1 input range from –20 mV to –2 V. Conditions: External CF Delay = 2 ns; input rise time ≤1 ns; input pulse width = 10 ns.

     Controls
    Threshold Front-panel 10-turn precision locking potentiometer determines the discriminator threshold setting in the range from –5 mV to –1 V.
    Timing Mode Switch Front-panel 3-position locking toggle switch selects one of the three timing modes:
    • CF (Constant-Fraction)  Attenuation factor is internally set at f = 0.2 (can be changed upon request). An external 50-Ω coaxial cable must be provided for the constant-fraction shaping delay (CF Delay).
    • SRT (Slow-Rise-Time) Reject  Provides constant-fraction timing and inhibits output signals that would be produced by leading-edge timing from the leading-edge arming discriminator. An input signal that does not cross the discriminator threshold before the constant-fraction zero-crossing time does not produce an output pulse.
    LE (Leading Edge) Inhibits timing from the constant-fraction circuitry. The timing is derived as the leading edge of the input signal crosses the discriminator threshold level.
    CF Delay Two front-panel BNC connectors accept 50-Ω coaxial cable to set the required constant-fraction shaping delay for the CF and SRT Modes: total delay is ≈0.8 ns plus the delay of the external cable. In the LE Mode, the user may either connect a piece of 50-Ω coaxial cable between these two connectors or connect a 50-Ω termination to each of the two connectors.
    Walk Front-panel 20-turn screwdriver adjustment sets the walk compensation for each application.
    CF Mon Front-panel BNC connector permits observation of the constant-fraction bipolar timing signal; Zo = 50 Ω. 50-Ω coaxial cable required; 50-Ω termination suggested.
    Width Front-panel 20-turn screwdriver adjustment sets the width of the Blocking Output pulse. Variable from ≤10 to ≥1000 ns. Sets the instrument dead time for widths greater than nominally 20 ns.
    Gating Mode Switch Rear-panel 2-position locking toggle switch controls the use of the Gate Inputs. (One of three Gate Input signal paths is selected by a PWB jumper.)
    • Gated  A "true" logic level from the selected Gate Input permits output signals to be generated by the discriminator. A "false" logic level from the selected Gate Input inhibits output signals from being generated by the discriminator. A set of Output signals already in progress is not terminated prematurely by a logic "false" signal from the selected Gate Input.
    • Ungated  The signal level of the selected Gate Input does not inhibit normal generation of output signals from the discriminator (i.e., the discriminator is always enabled).
    Logic Current Switch Rear-panel 2-position locking toggle switch selects either the –6 V or the –12 V NIM supply line for providing current for the high-speed ECL logic used in the discriminator.
    NOTES:
    (1) The module is within the current allotment for a single NIM width when using the –6 V position with a NIM Class V power supply or equivalent.
    (2) The module exceeds the current allotment for a single NIM width on the –12 V supply when using the –12 V position. However, this position permits using the discriminator in bins with power supplies not providing –6 V.
    Gate Input Jumper (G+, G-, or BG) PWB jumper selects one of three Gate Input signal paths:
    • G+  Selects the rear-panel BNC Gate Input connector to accept slow positive NIM input signal levels for gating; dc-coupled; Zin >1 kΩ.
    • G–  Selects the rear-panel BNC Gate Input connector to accept fast negative NIM input signal levels for gating; dc-coupled; Zin = 50 Ω. 
    • BG  Selects the Bin Gate line (pin 36 of the NIM power connector block) to accept slow positive NIM input signal levels >+2 V for gating; dc-coupled; Zin >1 kΩ.
    Positive Output Width (+ Width) PWB 4-turn potentiometer sets the width of the slow positive NIM output signal in the range from ≤0.5 to ≥2.5 µs.
    Positive Output Signal Polarity (PO or PO) PWB jumper selects the slow positive NIM output signal (PO) or the complement output signal (PO).

    Inputs
    Input Front-panel BNC connector accepts fast negative input signals from 0 V to –5 V without saturation; dc-coupled; Zin = 50 Ω; reflections ≤±5% for tr ≥2 ns.
    Gate Input Rear-panel BNC connector; input signals accepted according to PWB Gate Input Jumper.
    • G+ Jumper Position  Accepts slow positive NIM input signal levels for gating; dc-coupled; Zin >1 kΩ.
    • G– Jumper Position  Accepts fast negative NIM input signal levels for gating; dc-coupled; Zin = 50 Ω.

    Outputs
    Timing
    Two front-panel BNC connectors provide simultaneous NIM-standard fast negative logic signals; tr ≈2 ns; tf ≈3 ns; tw ≈5 ns.
    BK Out Front-panel BNC connector provides a NIM-standard fast negative logic pulse that occurs simultaneously with the Timing Outputs; width variable by front-panel adjustment from ≤10 to ≥1000 ns; tr ≅2 ns.
    Positive Front-panel BNC connector provides NIM-standard slow positive logic pulse simultaneously with Timing Outputs; Zo <10 Ω; width variable by PWB width adjustment from ≤0.5 to ≥2.5 µs. The associated LED is triggered for approximately 3 ms (updating) by each positive output pulse.

    Electrical and Mechanical
    Power Required
    Logic Current Switch*
       Position
      -6 V (mA)    -12 V (mA) 
    +12 V
    -12 V
    +6 V
    -6 V
    +24 V
    -24 V
    117 V ac 
    100
    100
    0
    400
    0
    80
    0
    100
    500
    0
    0
    0
    80
    0

    *See "NOTES" on Logic Current Switch, "Controls" Section of Specifications.
    Weight Net  1.2 kg (2.6 lb).   Shipping  2.25 kg (5.0 lb)
    Dimensions NIM-standard single-width module 3.43 X 22.13 cm (1.35 X 8.714 in.) per DOE/ER-0457T. 
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    Model Description
    584 Constant-Fraction Discriminator