The STT100 is a new approach to a classic problem, time-of-flight without extra delay cables. The STT100 is a clocked Time Stretcher, described in LeCroy publication P-43. The STT100 is implemented as a small printed circuit board (1.5" x 3") containing one complete channel. This component is designed to be mounted on a detector or on a standard rack mounted board.

The time stretcher ratio is 20:1 and an external clock in the range of 60-100 MHz is required. Both the signal input and the clock input are differential ECL terminated in 100 Ohm.



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, laser ranging instrument (LIDAR) may require a resolution of less than 100 ps (corresponding to a distance of 15 mm) over a range of several microseconds.



The Model STT100 Single Channel Time Stretcher is the key element of the new LeCroy Time-of-Flight Readout System. It is used in the Model 1275 VME Time Stretcher. 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, 96-Input FASTBUS 1877 TDC, VIPA 96-Channel VT960 TDC or the 8-Input MTD133B TDC component), the time-of-flight for a detector can be measured with a resolution of < 25 ps over 32 µs in Common Stop and Common Start modes.

As described in LeCroy Publication P-43 and elsewhere, the STT100 "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 STT100 measures the time difference between two events when they are separated by up to 32 µs.

A separate clock oscillator is required to provide the long range time base. This clock must not be harmonically related to the 250 MHz clock, the multi-hit TDC. For example, a 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 for a single measurement. This effect is significantly reduced by taking the difference between two measurements.


Figure 1: STT100 Timing Diagram

Figure 2: STT100 Block Diagram



Inputs and Outputs: 1 each, differential ECL.

External Clock Required: 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 61.5 MHz clock.

Packaging: Surface mount board, 1.5" x 3".

Power: +9 V, +5 V, -5 V, -2 V, -9 V; 2 W total.


Copyright© April 1998. 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.