Readout and trigger electronics

The trigger and readout electronics are integrated in a complex system, sharing wire chamber signals, Timing and Trigger Control (TTC), power supplies, cooling and mechanics. The block structure of the chamber electronics is shown in Figure 3.4.1.

Fig. 3.4.1 : Block scheme of the DT chambers electronics.

Readout electronics

Overview

The system is basically composed of readout boards (ROB) and readout servers (ROS and ROS Master). All these units are housed in crates which are placed in racks on the balconies. A block diagram of the system is shown in Fig. 3.4.2.

Front-end signals and cables

The front-end electronics, placed inside the gas volume, will provide discriminated differential signals at a feed-through connector grouping, 16 channels together. These signals will be taken to the Read Out Boards (ROBs) by a 25 m twisted-pair cable.

Fig. 3.4.2 : Block diagram of the readout electronics.

ROB

Each ROB will hold four 32-channel TDC circuits and will receive up to 128 differential signals from the FE. (Fig. 3.4.3). These signals, presumably LVDS, will be converted by 32 4-channel differential line receivers and sent to the TDC's.

Fig.3.4.3 : Block diagram of the TDC readout board.

In order to simplify the overall electronics design and minimize the number of components, converted signals are also sent to the trigger logic which is placed nearby.

At present, our interest is focused on the “32-channel general purpose TDC” developed by the CERN-Microelectronics Group. It is a good candidate to populate the 1680 ROBs required for the readout system.

This a highly programmable 32 channels TDC based on the Delay Locked Loop (DLL) principle. It has a time bin of 0.78 ns at 40 MHz, with a dynamic range of 21 bits. Each channel consists of two time registers where measurements are stored until they can be written into a common on-chip 256-word deep event buffer. With this mechanism, the two-pulse resolution is 15 ns. A trigger-matching function selects hits related to a given trigger, i.e. hits located within a programmable time window to accommodate the maximum drift time). Overlapping trigger are also supported. As a hit may belong to several closely spaced triggers (it falls inside several trigger windows), a fast and efficient search mechanism, which takes this fact into consideration, has been implemented as two search pointers and two programmable pointer windows. After trigger matching, data are passed to a 32 words deep readout FIFO. In this way one event can be readout while another is being processed by the trigger matching. Readout of data is performed via a synchronous bus, which can be shared by several TDC's [12].

In addition, ROB boards will receive signals from the TTC (clock, trigger, BC counter reset, etc.), and the from control (TDC configuration, channel masks, status query, etc.).

ROS

After digitization, data are collected by one dedicated ROS per chamber, and sent to its corresponding ROS Master in one data block per trigger.

ROS Master

Each ROS master receives data from the ROS servers corresponding to one sector (four chambers). Data belonging to one event will be packed in one data block and sent to a FED. An optical data link connects each ROS master to its corresponding FED. The required bandwidth of this link is 80 Mbytes/s.

TDC measurements

Several tests have been performed on the 32 channels TDC, namely:

• time resolution
• linearity
• cross talk
• double hit resolution
• radiation tolerance
• Also a set of necessary capabilities have been checked:
• overlapping triggers
• programmable trigger window
• programmable trigger latency
• raw data readout
• multi chip readout
• JTAG programmability
• channel masking.

For all time measurements a pattern generator has been used (Tektronix DG2020, clock jitter 50 ps). An example of this test is presented in Fig. 3.4.4, in which a hit is delayed in steps of 100 ps. The time measured, the time error and the corresponding histogram are shown.

Fig. 3.4.4 : Test of the TDC chip: linearity and resolution measurements in small dynamic range. Left hand side picture shows a 100 ps time sweep over 25 ns (top graph) and the corresponding deviations from the expected values (bottom graph). On the right it is shown the histograms of the residuals.

The expected distribution is flat, with limits at ± 0.39 ns. The table 3.4.1 summarizes the main test results:

Table 3.4.1

TDC prototype test results

Operation

The design parameters of the readout system have been:

• trigger rate 100 KHz
• track rate 1 track / cm² / s.

Since each track can produce up to 48 hits (i.e. one per layer), and each hit is 3 bytes big, an estimated event size of the order of 4 Kbytes is expected, corresponding to 30 tracks per event with a sector cross section of 5 m².

A TDC channel will provide a datum whenever there is a hit on its corresponding wire and within the trigger window. At the reception of a trigger, all data available from the four TDC devices placed in every ROB will be sent to the corresponding chamber server (ROS). At each ROS, data will be packed in one data block and sent to the corresponding ROS Master (one per sector).

In the ROS Master, data blocks corresponding to the four chambers of one sector will be merged into one single block and sent to the corresponding FED. Summing the whole detector, the DAQ will receive 60 blocks of data for every trigger, one from each sector.

The following table shows the sequence of events from the arrival of a trigger at TDC to the arrival of data to the FED. The estimated delays are minimum values. As a time-scale reference, the mean time between triggers is 400 bx.

Table 3.4.2.

Readout delay estimates (units are in bunch-crossings).

Notes:

1. Mean time required for the trigger matching mechanism;
2. Time required to fetch data from TDC's common bus and serialize data into the transmitter;
3. transmission time for the last serialized byte;
4. time required to fetch data from each ROB of a chamber and serialize data into the transmitter;
5. transmission time of the last serialized byte over 25 meters of cable;
6. time required to fetch data from four chambers and serialize data into the transmitter;
7. transmission time of the last serialized byte over 120 m of optical fiber.

On-chamber electronics

The placement of the trigger and readout electronics on the periphery of the iron yoke, in a position accessible without opening the detector, has clear advantages for the commissioning and the long-term maintenance of the detector. This has been therefore chosen as the base line solution since the Technical Proposal.

However, space is available around the honeycomb plate to house large parts of the electronics if sufficient miniaturization can be reached. Recent studies on the reliability of the electronics do not appear to exclude its use in positions of difficult access. This option has a big advantage in terms of the overall cost. To allow a complete study of important aspects of this solution, such as the reliability, effectiveness of cooling and integration in the chamber, it has been agreed that in the realization of the electronics prototypes, “the prototypes will be compatible with a chamber on-board installation”.