Lab 8 - Measuring Time‐of‐Flight (ToF) and Time‐over‐Threshold (ToT)

In this lab, we extend our FPGA design to measure:

• ToF (Time of Flight): The time difference between an external reference signal (T0) and the main signal on the analog input.
• ToT (Time over Threshold, sometimes referred to as Time to Amplitude): The width of the analog signal above a certain threshold.

1. Schematic Overview

Below is the simplified block diagram:

1. LEMO_0 (I/O1)

   • Receives the T0 (trigger reference) signal.
   • Routed through an Edge Detector (U4) that generates a clean rising‐edge pulse (t0).

2. Analog Input (A0, CH0)

   • Carries the main analog signal for which we want to measure ToT or amplitude.
   • Passes into a Trigger LE Hyst block (U2), which compares the signal to a user‐configurable threshold.
     • THRESHOLD: 16‐bit register controlling the comparator level.
     • POLARITY: 1‐bit register specifying rising/falling or positive/negative logic.
     • DELTA: Additional hysteresis or window parameter, if needed.
     • INHIBIT and RESET for gating or clearing.
   • The Trigger LE Hyst block outputs:
     • TRIGGER: A digital pulse marking the time the analog crosses threshold.
     • TOT: (Time over Threshold) signal indicating when the input stays above threshold.

3. Chrono (Enable) Block (U7)

   • Counts clock cycles from CLK while its ENABLE input is high.
   • This is used to measure the time the ToT signal is high (the pulse width).

4. Edge Detector FALL (U5)

   • Detects the falling edge of the Trigger or TOT signals, generating an “end” interrupt for the ToT measurement.

5. TOF Spectrum (U1)

   • Collects the time difference between the T0 reference and the main signal (the moment the main signal crosses threshold).
   • In many designs, the Chrono or a TDC approach is used to measure this difference. The results can be histogrammed in a Spectrum block.

6. Spectrum (U6)

   • A histogram component that accumulates counts of measured values (e.g., TOT or amplitude/time data).
   • DATA = 16‐bit value representing the measured time or amplitude.
   • STROBE triggers an entry into the histogram.
   • ACCEPT indicates the data is latched.

7. Oscilloscope (U18)

   • Monitors multiple signals in real time:
     • analog_in (the analog input A0),
     • trigger (the digital trigger from Trigger LE Hyst),
     • tot (the time‐over‐threshold digital representation),
     • end_int (the falling‐edge signal),
     • t0 (the reference from LEMO_0),
     • etc.
   • Useful for verifying the timing relationships and amplitude crossing.

2. Physical Setup

• T0 signal → Connect your external reference or trigger out from a pulse generator to LEMO_0 (I/O1) input on the DT1260.
• Analog signal → Connect to CH0 (A0).
  • We want this signal to occur some time (delay) after T0.
  • We also want to be able to vary its amplitude to see changes in ToT or threshold crossing.

Example Test Device

We’re using a CAEN DT5810 ( link here ) as a pulser:

• It can output a Trigger Out (for T0).
• It can output another analog channel for the main signal, with adjustable delay and amplitude.

3. Test Procedure

1. Set Threshold:

   • Open the Resource Explorer in Sci-Compiler.
   • Find the register THRESHOLD and set it (e.g., 2000–3000) so the analog signal crosses above it.

2. Adjust Delay:

   • On the DT5810 (or your pulse generator), set the main analog output to appear ~1 µs after T0.
   • Observation: In the Oscilloscope, you should see T0 on t0 first, then the analog pulse crossing threshold later. The difference in time is the ToF.

3. Oscilloscope:

   • In Resource Explorer → “Oscilloscope_0” → View.
   • Check “Channel 1” for your analog input, and enable “Digital lines” for trigger, tot, etc.
   • You’ll see:
     • The analog pulse shape crossing threshold.
     • The trigger line going high at the rising edge.
     • The tot line staying high until the pulse falls back under threshold.
     • t0 line showing the reference pulse.

4. TOF Spectrum:

   • Also in the Resource Explorer, look for “TOF_0” or “Spectrum_0” (depending on how your design is named).
   • When the main signal arrives ~1 µs after T0, the ToF histogram should show a peak around that time bin.
   • If you increase the delay, the histogram peak shifts accordingly.

5. Energy / ToT Spectrum:

   • The design can also measure Time over Threshold, which correlates to pulse amplitude for certain signal shapes.
   • In the “Spectrum” block that captures TOT (or amplitude/time), you should see a distribution (e.g., a Gaussian if your signal is stable).
   • Varying the pulse amplitude on the DT5810 will move the peak in the TOT histogram.

4. Observing Results

• Changing the Delay:
  • The ToF spectrum peak moves to the right or left, indicating a longer or shorter flight time between T0 and the main signal.
• Changing the Amplitude:
  • The ToT spectrum or “energy” measurement changes because the signal stays above threshold for a longer or shorter duration.
  • In a histogram, you’ll see the main peak shift in amplitude channels.