The Model 1275 is a new approach to the classic time-of-flight measurement problem without using expensive delay cables. It is the key element in a multiple input, multihit time-of-flight TDC system that operates in Common Stop mode.

The 1275 is a 16-input clocked time stretcher, described in LeCroy publication P-43. It includes 16 channels of the LeCroy STT100 time stretcher implemented on a 9 U VME board, conforming to VME In Physics Application (VIPA) recommendations. When combined with a LeCroy multihit, multiedge time digitizer (500 ps LSB), the resulting TDC system has exceptional performance characteristics. The TDC system can measure a time interval up to 32 µsec with a 25 psec LSB.




High resolution time measurements are used in particle physics experiments to measure particle velocity. If the velocity and momentum (measured by bending in a magnetic field) are known, the mass of the particle can be calculated. The velocity is calculated from a time-of-flight measurement over a known flight path.

High resolution measurements are required in many other fields of instrumentation. For example, a laser ranging instrument (LIDAR) may require a resolution of less than 100 ps (corresponding to a distance of 16 mm) over a range of several microseconds.



The Model 1275 16-Channel Time Stretcher is the key element of the new LeCroy Time-of-Flight Readout System. When combined with a high speed, pipeline, multiple hit, time-to-digital converter (multi-hit TDC like the LeCroy 32-input CAMAC Model 3377 TDC, the 96-input FASTBUS 1877 TDC, the VIPA 96-channel VT960 TDC or the 8-input MTD133B TDC component), the time-of-flight for a multiple sensor detector can be measured with a resolution of < 25 ps over 32 µs in both Common Stop and Common Start modes.

As described in LeCroy publication P-43 and elsewhere, the 1275 "stretches" a part of the time interval under measurement, performing the same function as a Wilkinson ADC, but with the equivalent of a 2 GHz clock. The multi-hit TDC measures both the stretched time and the initial (before stretching) time difference. A comparison of the two times permits self-calibration.

The combination of the stretcher with a multi-hit TDC has many advantages over the traditional high resolution timing methods.

  1. Both Common Start and Common Stop operation are permitted. For Common Stop, this means no long expensive delay cables are necessary.
  2. Multiple measurements per channel are possible with modest dead time.
  3. Calibration of the time scale is possible using the normal data only. No special calibration signals are required.
  4. A high stability design is not absolutely necessary. The system can be completely calibrated in situ. Continuous calibration using the data itself can track slow changes in the scale factors.



The LeCroy 1275 measures the time difference between two events when they are separated by up to 32 µs. It has 16 separate timing channels based on the LeCroy Model STT100 component.

A separate clock oscillator is required to provide the long range time base. An external clock or the internal 61.5 MHz clock may be used. This clock must not be harmonically related to the 250 MHz clock the multi-hit TDC. For example, the internal 61.5 MHz clock is a good choice with a period of 16.261 ns. This clock will have a phase slip of 261 ps every cycle with respect to the multi-hit TDC's 250 MHz clock.

Note that the LSB of each stretcher circuit will be different and none will be exactly 25 ps. Also, at the end points of the time stretcher range, there is usually a time bin with a smaller (possibly much smaller) width causing a local differential non-linearity to a single measurement. This effect is significantly reduced by taking the difference between 2 measurements.


Figure 1: STT100 Timing Diagram

Figure 2: STT100 Block Diagram



Inputs and Outputs: 16 inputs, differential ECL.

Clock: Jumper-selectable for either internal 61.5 MHz or external 60 - 100 MHz.

Normal Stretch Ratio: 20:1.

LSB: 25 ps when used with a 500 ps LSB multi-hit TDC. Calibration method employs LeCroy propriety technique; patent applied for.

Operation Modes: Common Stop or Common Start.

Double pulse Resolution: < 800 ns with internal clock.

Packaging: 9U VME, compatible with VIPA specifications.

Power: +12 V, +5 V, -5 V, 2 V, -12 V; 40 W total.


Copyright© May 1997. LeCroy is a registered trademark of LeCroy Corporation. All rights reserved. Information in this publication supersedes all earlier versions. Specifications and prices subject to change without notice.