Name of the exercise

Trigger Exercise

Responsible for the exercise

F.Pastore and W.Vandelli

Description of the exercise

With this exercise the students will get in touch with the main principles of a trigger system: how to setup a trigger signal, how to control a trigger rate, how to minimize the jitter, how to get rid of noise and background, how to adapt a trigger system with the DAQ requirements. The main purpose of the exercise is to prepare the students to the use of NIM modules and digital/analogical signals.

What will the students learn

  • Use of an oscilloscope and a signal generator
  • Use of discriminators to select a trigger signal
  • Use of coincidence units and scalers


The students should be able to complete an exercise in 2 hours. Indicate if the exercise takes much more or less time than this default duration.

List of material

  • 1 oscilloscope
  • 1 signal generator
  • 1 Voltmeter
  • NIM modules: time unit + coincidence unit + delay unit + scaler + NIM-TTL
  • 2 types of discriminators: threshold and constant fraction (4737)
  • Little screw driver (the CFD requires a particularly small ones)

Relevant information

This is a basic exercise, does not depend on other exercise. The introduction of the trigger lecture is essential. Indeed it can be adapted as an introduction to exercise number 3 and 4. Look at the schema of the exercise and follow the instructions

  • Part 1a: Threshold Discriminator
    • The signal generator will be pre-configured to provide a triangular pulse (T=300us, leading=100ns, traiing=200ns, width=200ns, offset=0, amplitude=-100mV)
    • First the students should look the signal (MAIN OUT) in the oscilloscope (CH1), using the SYNC OUT of the generator as a oscilloscope trigger (EXT)
      • The SYNCOUT is TTL signal. Transform it into a NIM signal using the dedicated level-adapter module
    • Split the generator output signal and connect one branch to the input of the threshold discriminator. The other branch should be properly terminated on the oscilloscope side (1MΩ)
    • Connect one output signal of the discriminator to the scaler module and a second output to the oscilloscope (CH2)
    • Check the threshold on the discriminator with a Voltmeter (x10 output)
    • Change the threshold with a screw driver and
      • observe the behaviour of the output signal on the scope
      • observe the rate on the scaler * Can you relate them to the threshold values?

  • Part 1b: Threshold Discriminator - Jitter
    • Using the above setup, set the threshold to 30mV and change the amplitude of the input signal. Which is the effect on the discriminated signal? How does it affect a timing measurement?
    • Measure the discriminated signal delay with respect to the reference as a function of the amplitude of the input signal (-50, -100, -150, -200 mV)

  • Part 2: Constant Fraction Discriminator
    • Use the previous signal as input of the constant fraction discriminator.
    • Connect to the oscilloscope the input signal (CH1)
    • Setup the CFD parameters:
      • threshold (T) --> 27 mV - Measure with Voltmeter (x10 output)
      • walk (Z) --> 2mV - Measure with Voltmeter
      • delay (D) --> 80 ns - Set with delay module + 2x10ns cables
    • Connect the monitor output (M) of the CFD to the oscilloscope (CH2). Can you recognize the CFD technique? Which is the effect of varying D?
    • Connect to the oscilloscope the discriminated output of the CFD (CH2)
    • Change the amplitude of the input signal. What happen to the output of the discriminator?
    • Measure the discriminated signal delay with respect to the reference as a function of the amplitude of the input signal (-50, -100, -150, -200 mV). Compare the results with the previous measurements.

  • Part 3: Trigger veto or dead-time
    • Configure one stage of a dual timer module to generate signals with 10ms width
    • Connect the output of discriminator and the negated output (OUTbar) of the timer (the "veto") to a coincidence unit.
    • Connect another output of the discriminator and one output of the coincidence to two scaler ports
    • The output of the coincidence has to drive the timer module (START)
    • Compare the counting rates of the scalers. How do they relate with the timer setting?

Input Signal Amplitude (mV) Trailing edge D (ns) Constant Fraction D (ns)
50 128 156
100 72 148
150 56 140
200 44 140


You have to provide a possible solution for the exercise witch the students can consult after the exercise or if they get stuck. The solution should not be in this TWiki (we don't want the students to see it)

-- GokhanUnel - 2009-09-28

Topic attachments
I Attachment History Action Size Date Who Comment
JPEGjpg IMG_0009.jpg r1 manage 502.4 K 2009-11-23 - 18:54 FrancescaPastore Trailing Edge Discriminator output
JPEGjpg IMG_0010.jpg r1 manage 474.6 K 2009-11-23 - 18:55 FrancescaPastore CFD Monitor output
JPEGjpg IMG_0011.jpg r1 manage 476.9 K 2009-11-23 - 18:55 FrancescaPastore CFD output
JPEGjpg IMG_0013.jpg r1 manage 577.2 K 2009-11-23 - 18:55 FrancescaPastore crate photo
PDFpdf appendices_CFD.pdf r1 manage 260.2 K 2010-07-22 - 15:19 WainerVandelli  
PDFpdf exercise-2-modules.pdf r1 manage 668.6 K 2009-12-02 - 15:32 WainerVandelli Picture of NIM modules
PDFpdf exercise.pdf r2 r1 manage 12.7 K 2009-12-18 - 15:56 WainerVandelli Electrical scheme
Microsoft Word filedoc exercise2.doc r4 r3 r2 r1 manage 2730.0 K 2010-10-29 - 16:04 UnknownUser Instructions
Microsoft Word filedoc exercise2_2013.doc r3 r2 r1 manage 2724.5 K 2013-12-05 - 18:35 WainerVandelli  
PDFpdf trigger-veto-timing.pdf r1 manage 7.5 K 2009-12-18 - 17:04 WainerVandelli Drawing of trigger veto timing
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Topic revision: r16 - 2013-12-05 - WainerVandelli
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