Integrator calibration

Integrator calibration

The integrators include three 32-channel modules (D-LTX 133s), and several single-channel crate-mounted modules (E1939s). This document describe the calibration of the D-LTX 133s.

Hardware Constraints

Integrator Circuit

The design of the LTX integrator circuit inputs and outputs has the following constraints that must be observed:

1. Gate: The external integrator control signal is a gate, not a trigger, so it must be kept high throughout the integration period.
2. Input: Since the inverting-leg RC capacitor of the amplifier ties to ground, an unbalanced input will tie straight to ground and not include the inverting leg in the calibration. A balanced (ungrounded) signal source must be used for a faithful calibration.
3. Output: The female LEMO port contact is tied to ground in the integrator chassis. The output monitor (e.g. scope) must either be balanced, or must have cabling which ties the same conductor to ground at the receiving end (e.g. BNC inverter).

Cal Lab Calibration Box

Warning:The integrator calibration box borrowed from the Cal Lab contains an amplifier chip suspected to be vulnerable to damage if powered on while cables are connected. The power cable should be connected and the power switched on before cables are connected, and all cables should be disconnected before the power is turned off and the power cord unplugged.

DC Optical Isolator

Fig 1: DC Optical Isolator

To avoid the sagging flat-top signal associated with transformer outputs, a DC optical isolator circuit was assembled. Although the signal still sagged somewhat over longer pulse lengths (>1ms), possibly due to limitations of the optocoupler output architecture as a voltage supply, it was quite stiff for shorter pulses. As of July 2018, the isolation board does not yet have an enclosure and must be handled with care.

In the DC optical isolator diagram (right), the DC power supplies (J1, J2) provide power to the two mutually isolated sides of the optocoupler (U1), with power supply filtering (R1-2, C1-4) and voltage regulators (U2-3) to reduce noise. The 0-5V input signal (J3), grounded by the signal generator, is modulated by a voltage divider (R3+R4) and transferred to the ground-isolated output (J4) where it is measured as the difference of two 10x scope probe signals. NB: the schematic for the optocoupler (U1) is only approximate; the actual schematic can be found here.

Calibration Setup

Components

• 4-channel Oscilloscope
• Signal generator
• Pulse generator
• DC optical isolator board
• 9 BNC cables
• 4 BNC splitters
• 1 BNC inverter (e.g. male+female BNC-Banana adapters)
• 2 x10 scope probes
• 2 BNC-LEMO(2) adapters
• 1 BNC-clip lead adapter

Connections

• The pulse generator's Gate Output to (2 splitters):
  • The integrator's Integrate Gate input
  • The signal generator's Ext Trig input
  • The oscilloscope's Channel 1 input
• The signal generator's Channel 1 to (1 splitter):
  • The oscilloscope's Channel 2 input
  • The isolator board's VIN
• The isolator board's VOUT to (1 splitter):
  • The integrator's Input, through BNC-LEMO(2) adapter
  • The BNC-clip lead adapter
• The oscilloscope's Channel 2 input w/ x10 scope probe to:
  • Signal (red) side of BNC-clip lead adapter
• The oscilloscope's Channel 3 input w/ x10 scope probe to:
  • Return (black) side of BNC-clip lead adapter
• The integrator's Output through LEMO(2) adapter to the oscilloscope's Channel 4 input
  • May require BNC inverter, if LEMO(2) male pin ties to BNC center pin

Calibration Procedure (July 13-17 2018)

In this configuration, the pulse generator provides the integrate gate, as well as a rising-edge external trigger for a single square wave pulse from the signal generator which drives a DC optical isolator board (replacing the transformer-based calibration box used previously). The signal generator provides a delay between the integrate gate and calibration box signal to avoid transients from the integrate gate switching on. The isolation board provides the integrator input with a half-square/tophat signal which can be set for up to 5V, or trimmed down with the on-board potentiometer. The integrator output connects (with a BNC inverter due to the polarity of the BNC-LEMO adapters) to the oscilloscope.

Hardware Settings

• Pulse generator:
  • Delay/Period set between 70μs and 4ms depending on channel gain
    • e.g. 150.1μs: Knob set to minimum (left-most μs position), thumbwheels set to 01501
  • Delay/Period Switch set to Delay
  • Gate Output should ignore amplitude knobs, but both are centered
• Signal generator (Ch1 only)
  • Square wave
    • Freq between 15kHz and 300Hz (typically set for half-period ~80% of gate length)
    • Amplitude 5V
    • Offset 0V
  • Burst mode on
    • Ncycle = 1
    • Trigger = external
    • Delay between 10μs and 500μs (typically ~10% of square wave half-period)
• Oscilloscope
  • Set to plot Math as Ch2-Ch3

Results

Only about 53% of channels have gains within 5% of the values reported in the 2012 calibration.