This project is currently not being worked on.
Introduction
The main idea here is to measure the charge of the signal with the help of the charge-to-time converter, Figure 1.
The duration of a pulse on a digital output of the charge-to-time converter represents, with some accuracy, charge of the signal on the analog input. For calculation of the time of arrival, the leading edge of the signal is taken. Instead of collecting information of many data samples, as a flash-ADC does, in the TDC based readout electronics one needs to record only two values: the arrival time of the signal and the output pulse duration of the charge-to-time converter.
TPC readout electronics based on TDC
For the test of the TDC-based readout electronics,
a small data acquisition system has been built.
As a charge-to-time converter, a compact and low power ASDQ circuit [1] was chosen. The ASDQ chip provides eight channels of full analog signal processing between the chamber and the TDC. It contains a low noise preamplifier, ion tail compensation and multipole shaper, baseline restorer and dE/dx discriminator. Specifications of the ASDQ chip can be found in [1].
Many parameters of the ASDQ, like threshold, can be varied by applying external signals. Charge-to-time conversion can be tuned with an external voltage on the QDR-pin of the ASDQ ASIC, Figure 2.
As a first step, a single channel of the ASDQ chip was connected to the Large TPC [2] readout pad. In this test signals from the ASDQ preamplifier monitor output and digital output of the dE/dx discriminator were recorded with the help of a digital oscilloscope, Figure 3. This showed the feasibility of TPC readout with the ASDQ circuit.
To install the ASDQ ASIC onto the TPC, a 16 channel Front-End electronics board has been built, Figure 4. Each board contains two ASDQ chips.
The main component of the data acquisition system is a commercial 128 channel multi-hit VME TDC (CAEN v767, 1-unit wide 6U module), based on four 32-channel TDC chips [3]. This TDC chip has an 0.8ns time bin size and an 800 microsecond time measurement range. An event buffer of 256 steps allows multiple hits to be recorded for every channel. Both the front and the rear edge of the signal on the input of the TDC are recorded, thus making possible to reconstruct signals encoded by the charge-to-time converter.
The measurement of the Z-coordinate is performed by the measurement of the drift time of the electrons from primary ionization in the gas as given by:
Z =Vdrift * Tdrift ,
where Vdrift is the drift velocity of the electrons from primary ionization in the gas, Tdrift is the drift time of the primary ionization. The drift time is defined as the period of time between the external trigger signal and the arrival time (the leading edge of the signal on the output of ASDQ) of the electrons of the primary ionization to the pad plane. The time difference between the arrival time of the rear edge and front edge of the signal is a pulse duration from the ASDQ and is a measure of the charge on the input of the ASDQ.
Tests of the readout electronics have been performed with several TPC prototypes: the MediTPC in the previous DESY test beam setup [4], with the Big TPC using the laser setup [5] and with the Large Prototype in the current test beam setup.
References
- [1] W. Bokhari et al., CDF Note 4515, March 1999
- [2] T. Behnke, M. Hamann and M. Schumacher, "Development of a TPC with GEM readout", LC-DET-2001-006
- [3] J. Christiansen, "32-channel TDC with on-chip buffering and trigger matching", Given at 3rd Workshop on Electronics for LHC Experiments, London, England, 22-26 Sep 1997
- [4] A. Kaukher, "A GEM TPC with readout electronics based on TDC: first results", Given at ECFA Linear Collider Workshop, Durham, England, 31 August - 04 September 2004
- [5] A. Kaukher, "A Study of Readout Electronics Based on TDC for the International Linear Collider TPC Detector", Given at 14th IEEE-NPSS Real Time Conference, Stockholm, Sweden, 5-10 June 2005
Additional information:
Eudet-Memo-2009-08: Status of TPC-electronics with Time-to-Digit Converters, by A.Kaukher, O.Schaefer, H.Schroeder, R.Wurth
