The ORTEC Model 420 Timing Single Channel Analyzer is a modular electronic instrument designed for precise determination of the time of occurrence of nuclear events. It functions as both a single channel analyzer and a timing unit, providing outputs that are precisely timed relative to specific features of the input signal.
Function Description:
The Model 420 performs two primary functions:
- Single Channel Analyzer: It allows for the selection of a specific pulse height range (window) from an input signal. The lower level of this window is controlled by the "E" discriminator, and the window width by the "ΔE" discriminator. When the DIFF./INT. switch is in the INT. position, the "ΔE" discriminator is disabled, and the unit acts as an integral discriminator, triggering on all signals exceeding the "E" threshold. In DIFF. mode, an anticoincidence function ensures that an output is generated only for input pulses within the selected window (between "E" and "E" + "ΔE").
- Timing Unit: For doubly differentiated input signals (bipolar), the Model 420 generates an output pulse precisely at the time these signals cross zero volts. This crossover time is a known, fixed reference point, making the output timing highly accurate and independent of pulse height. For unipolar input signals, the timing output is generated when the signal recrosses the "E" discriminator setting. However, precise timing is only available with bipolar signals, as unipolar timing depends on the signal's fall time and amplitude, leading to time shifts.
The unit includes a variable delay, adjustable from 0 to 1000 nanoseconds, to allow for time balance in coincidence circuitry and analyzer gating. It provides two standard logic outputs: a negative fast spike (less than 20 nsec wide) for very fast timing applications like time-to-pulse-height converters, and a positive 5-volt, 500-nanosecond-wide pulse for slower applications such as coincidence circuits and analyzer gating.
Important Technical Specifications:
- Input Signal Requirements:
- Amplitude Range: 100 mV to 10 V
- Pulse Width Range: 200 nsec to 20 µsec
- Polarity: Positive (Unipolar) or Positive Portion leading (Bipolar)
- Input Impedance: 2000 ohms, AC-coupled
- Maximum Input: 10 volts
- Controls:
- "E" (Lower Level): 100 mV to 10 V, 10-turn control
- "ΔE" (Window Width): 100 mV to 10 V, 10-turn control
- Delay: 0 to 1000 nsec, 10-turn control
- Differential-Integral Mode: Switch (Integral disables "ΔE" discriminator)
- Unipolar-Bipolar Mode: Switch
- WALK Adj.: Screwdriver adjustment for optimizing crossover timing.
- Linearity (as percent of full span):
- "E": < ± 0.25%
- "ΔE": < ± 0.25%
- Delay: < ± 2%
- Temperature Stability (as percent of full span):
- "E": < ± 0.015%/C°
- "ΔE": < ± 0.015%/C°
- Delay: < ± 0.4 nsec/C°
- Time Shift vs. Pulse Height: < ± 2 nsec for a factor of (X10) or ± 10 nsec for (X50) when specified with ORTEC Model 410 Amplifier, Double-Delay-Line Mode, Integrate 0.2 µsec.
- Output Signals:
- Fast (Negative): 0.6 V (min), Tr < 5 nsec, Width < 20 nsec. Source impedance < 10 ohms, DC-coupled from zero volts.
- Slow (Positive): 5 V, Tr < 20 nsec, Width ~ 500 nsec. Source impedance < 10 ohms, DC-coupled from zero volts.
- Power Required:
- +24 V: 30 mA
- -24 V: 30 mA
- +12 V: 50 mA
- -12 V: 40 mA
- Physical:
- Weight: ~ 2.5 pounds net
- Size: Standard double width module (2.70 inches wide) per TID-20893
Usage Features:
- Compatibility: Designed for use with Nuclear Standard Bins and Power Supplies (e.g., ORTEC Model 401/402). Compatible with linear amplifiers producing 10-volt unipolar or bipolar output signals. Vacuum tube amplifiers with 100-volt outputs can be used if attenuated to 10 volts.
- Timing Optimization: The unit is optimized for timing with bipolar signals, particularly from double delay line clipped amplifiers, where the zero-crossing time is independent of pulse height.
- Delay Control: A 10-turn potentiometer provides a continuously variable delay for output logic signals, useful for synchronizing with other circuitry.
- Mode Selection: Switches for Differential/Integral and Unipolar/Bipolar operation allow adaptation to different signal types and analysis requirements.
- Output Signals: Provides both fast negative spikes for high-resolution timing and positive 5-volt pulses for general logic and gating, compatible with ORTEC 400 Series modular instruments.
- Input Termination: For optimal performance and to minimize reflections, the input (CN1) should be externally terminated in the characteristic impedance of the connecting coaxial cable, especially for low impedance voltage sources.
Maintenance Features:
- Quality Control: Each instrument undergoes stringent quality control tests before shipment, with records maintained for warranty repair and design improvements.
- Repair Service: ORTEC offers repair and checkout services for instruments, even out of warranty, ensuring they pass the same quality control tests as new units.
- Calibration Adjustment: Includes a "WALK Adj." (R69) screwdriver adjustment on the front panel to trim time shift over dynamic ranges, and a zero potentiometer (R14) on the printed circuit board for "E" discriminator calibration.
- Troubleshooting Aids: The manual provides detailed testing procedures, waveform observations, and tabulated test point voltages on etched boards to assist in diagnosing malfunctions.
- Accessibility: The guide plate and shield cover can be removed to allow oscilloscope and voltmeter observation of the etched board.
- Component Replacement: Transistor types may vary from schematics; replacements can be made with either the schematic-specified type or the type actually used in the instrument.
- Power Supply Monitoring: Test points on the ORTEC Model 401/402 power supply control panel allow monitoring of DC voltages.
- Environmental Considerations: Designed for rack mounting, but requires adequate cooling to prevent temperatures exceeding 120°F (50°C) for optimal transistor circuit operation.
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