Nuclear Instruments and Methods in Physics Research A 386 (1997) 138-142 NUCLEAR

ELSFVIER

INSTRUMENTS & METHODS IN PHVSICS

RY%YW

Analogue optical data transfer for the CMS tracker

G. Hall*

Imperial College, London SW7 2A.Z UK

Abstract A description is given of the electronic readout system proposed by the CMS experiment for the microstrip tracker

emphasising the technologies which could be employed for transferring the large volume of analogue data from the front end electronics. The system contains both gas and silicon microstrips read out by similar CMOS front end chips, modified to

accommodate the different detector characteristics, with pulse height information transferred as analogue values by modulation of the light output of an optical transmitter.

1. Introduction

The CMS central tracker [ 1,2] is based on silicon and gas microstrips with an inner pixel detector system. The detectors will be operated in a 4T solenoidal magnetic

field contained in a cylindrical volume of 1.3 m radius and length 7 m. The total number of channels to be read out is

11 X lo6 MSGC and 3 X lo6 silicon. A single electronic readout system is envisaged for both [3] based on a front end chip originally developed for silicon microstrip read- out but suitably modified for the MSGCs.

2. Overview of readout system

Each microstrip is read out by a charge sensitive amplifier with a time constant 50 ns whose output voltage is sampled at the beam crossing rate of 40 MHz. Samples are stored in an analogue pipeline for up to 128 crossings (3.2 ps) and, following a level 1 trigger, are processed by an analogue circuit forming a weighted sum [4,5]. This confines the silicon signal to a single beam crossing interval and enables measurement of signal amplitude and bunch crossing associated with the hit. For the MSGCs the

optimal signal processing is still under investigation but a variant of the filter can meet the CMS requirements of high efficiency and timing precision of two bunch crossings

[6,71. The external data acquisition for the tracker is based on

a VMEbus system housed in the barracks outside the experimental area after cable paths of up to 100 m. The

* E-mail g.hall@ic.ac.uk.

pulse height data from each channel of the front end chips, with no zero suppression, will be serially transferred at

40Ms/s by an optical link to a receiver module using a multiplexing level of 256 detector channels per fibre. Since

each analogue value corresponds to 6-8 bits of infor- mation, the effective data transmission rate is -300 Mbit/s.

In the counting room, the Front End Driver module digitises the analogue data, performs zero suppression and

simple cluster finding and stores the results in a local memory until required by the higher level data acquisition. A separate module, the Front End Controller, will be responsible for control of the front end electronics and

distributing the LHC machine master clock and first level triggers to the front end electronics via separate fibre

ribbons. A schematic diagram of the proposed system is shown in Fig. 1.

One of the arguments for an analogue system is related to achievable position resolution. Charge sharing between detector strips will be frequent as a consequence of non-

normal incidence, magnetic field effects and energy loss fluctuations. In the CMS 4 T field the Lorentz effect will distribute charge over 200 pm on the p-side and 37 pm on the n-side of silicon detectors. Analogue readout improves

the MSGC resolution at small angles. Another important reason for retaining analogue data is

immunity to unexpected noise, which is a major concern in a large system. Excess noise from external sources, such as ground loops or transient pickup, can be expected, as well as common mode effects. A digital system may lead to a slightly smaller volume of data to be transferred, but there are potential difficulties such as extra custom radiation hard front end electronics, power consumption, control of thresholds and address errors during data transfer. The material budget of the tracker is strongly influenced by front end functions since the topology of the hybrid, which makes a significant material contribution [8,9], is de-

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G. Halt I Nucf. Instr. and Meth. in Phys. Res. A 386 (1997) 138-142 139

Fig. 1. Schematic of the CMS tracker readout electronics.

termined not only by the chip area but by the number of

chips and their distribution. Analogue data will give greater ability to identify

problems and allow correlations to be applied in the FED. Some degradation due to radiation damage is expected even with hardened technologies as well as “minor”

damage, such as connector failures. Pulse shapes, am- plitudes, occupancies and unexpected effects of radiation damage can be frequently monitored. Access to external

electronics means that greater reliability can be ensured than if it were within the tracker volume, which has an impact on the cost of replacement should be a failure

occur. Most of the circuits required, such as 40 MHz FADCs, can already be obtained from commercial sources

and there is little to be gained by implementing custom, radiation hard versions, since the cost and performance of

commercial components are also expected to evolve

favourably without effort on our part.

2.1. Front End electronic chain 2.3. Front End Driver

The front end chip is based on a design originally developed by the RD20 collaboration [11,12]. Its novel feature is the use of analogue deconvolution [4,5] of the shaped and amplified signal pulse to give the precise timing information required at LHC for fast signals from silicon detectors. Because of similarities in signal sizes and detector capacitance, the chip is also suitable for MSGC readout. The time development of signals in the silicon and gaseous microstrips is different and the same signal processing will not be optimal for the MSGCs. This has been examined in beam tests and simulations of MSGC

current pulses from minimum ionising particles [7,13] and

changes to the system are anticipated to be minor.

A radiation hardened version of the RD20 front end chip

has been developed at Rutherford Appleton Laboratory using the Harris 1.2 p.m AVLSIRA bulk CMOS process [ 141. Prototyping of an equivalent chip is also under way in DMILL process [ 151 where individual transistors show noise and radiation hardness very similar to the Harris devices [16-181. A 32 channel version of the chip, the APV3, comprising amplifier-shaper, full length pipeline and analogue signal processor was tested in 1994 [ 191. A

128 channel chip, the APVS which also contains a multiplexer, was delivered and tested in 1995 [20].

2.2. Control and monitoring

It is inevitable in a large distributed system such as the

CMS tracker, with strong constraints on material, space and power dissipation, that the internal volume of the tracker should be monitored electronically. Past experi- ments have frequently delayed the definition of the “Slow

Control” system until a late stage; this is virtually im- possible for the CMS tracker because many functions will

require the use of ASICs either to transmit or measure quantities of interest, such as temperature and voltage.

Since the minimum time to develop even a simple ASIC in a radiation hard technology is not less than two years, design must commence early.

A dedicated module, the Front End Controller, will distribute the LHC clock and trigger signals which will be

passed to it from the TIC system developed at CERN [21]. In addition commands will be passed to the front end readout chips and responses from control signals will be

returned. Data monitoring the system environment will also be transmitted from the interior of the tracker. An optical fibre link, acting effectively as a bi-directional bus,

will transmit all these signals to an internal control module which will distribute them locally electrically. It is envis-

aged to make use of the same optical technology used for analogue data transfer for transmission of all the digital signals.

The Front End Driver module [22] receives 64 analogue signals, each of which is digitised by a fast ADC, before

being passed to the synchronisation circuitry. Each ADC on the FED requires a clock signal, to sample the incoming analogue signals at the correct point, which is derived from the 40 MHz clock from the Global Timing System. This must be distributed to all the ADCs on the FED, and it is also necessary to adjust the clock phase relative to the incoming analogue signals. The synchronisation circuit tests in real time for the pulses which identify the start of each of the data frames. Once the start of each frame has

III. HADRON COLLIDERS VERTEX DETECTORS

140 G. Hall / Nucl. Instr. and Meth. in Phys. Res. A 386 (1997) 138-142

been correctly identified, the data frame can be separated into its constituent parts, analogue and digital information.

In the digital processing stage the data are formatted in a suitable way for read out. Part of this process includes the addition of headers to the data to identify its origin. The data will also be manipulated to correct for errors that are

introduced during measurement, such as baseline shift and suppress below threshold data. The processed data from each of the 64 channels is passed to the Local Event

Builder where it is collected together into a single packet and sent to the global data acquisition system.

3. Optical link

Optical data transmission is essential at LHC, especially in tracking systems, to minimise the material budget and

transfer data at high rates. In most cases the bandwidth available from fibre optic systems is well above what is

required for LHC applications, although high power is required to drive a link at full speed. There are other

advantages; the system should be immune to electrical interference. The linearity (<2%) and dynamic range (7-

8 bits) for tracking requirements are not especially large and a 40Ms/s transfer rate is well below the full band- width of the fibre link. In the CMS tracker the deadtime

caused by transferring 256 values (in 6.4 ps) can be tolerated provided buffer depths are adequate.

An analogue data link is attractive to avoid on-detector digitisation and reduce power consumption. The original proposed system has been developed by the RD-23 col-

laboration [23] and employs passive reflective modulator technology with CW lasers providing the optical power. The transmitters are III-V semiconductor multi-quantum-

well (MQW) electro-absorptive structures, tuned around 1.55 pm wavelength. They translate the front end elec-

tronics output into an optical signal by modulating incident optical power from lasers located outside the detector. Power consumption is restricted within the tracker volume to the very low photocurrent in the modulator. Radiation hardness at levels required for LHC is not expected to be a problem; modulators have been irradiated without bias to 1.3 X lOI fast neutrons cm -’ and gammas, under bias, to 20Mrad with no significant change in properties. More

recently neutron irradiations have been carried out under bias to even higher levels with similar results. The effect of magnetic field has been estimated from a device model and is sufficiently small to ensure the required performance in the 4 T CMS field and this has been confirmed by direct measurements of the absorption spectra in MQW material samples (241.

The transceiver unit contains laser die, serving multiple modulators, an optical waveguide and splitter and photo- diodes in a stable temperature environment, although the laser light output will be controlled. Connectors should be kept to a minimum in the system to avoid reflections at

interfaces which attenuate signals and act as sources of additional noise. Noise also arises from the laser itself [24,25] and from the receiver amplifier so the dynamic

range is finite and care must be taken to minimise the additional noise contribution.

The reflective modulator solution offers significant advantages in terms of minima1 power dissipation in the

detector volume. Since there is almost no power dissipa- tion in the transmitter element it is also expected to offer long component lifetime. However the system is quite

complex and the devices required, both modulator and transceiver, are not commercial developments but custom-

ised for this application. At present, optical link technology is still developing

rapidly and relatively few components are readily available which could meet the CMS requirements. However this

situation could change in the next few years and there is a clear benefit from taking advantage of commercial de- velopments if this is possible. The most promising alter-

natives which have been considered are actively modulated laser diodes, which appeared until recently to have the

disadvantages of higher power dissipation, lower reliability and poor radiation hardness [IO]. However a number of

semiconductor lasers have been evaluated recently [25], for some of which extremely long mean time to failure rates have been measured by the manufacturers. In general

it appears that the trend towards more reliable, lower power and, ultimately, low cost solid state lasers will continue as they have many applications for both analogue

and digital data transmission. The semiconductor laser [26] is based on a p-n diode in

a direct band-gap material in which forward biasing creates

the population inversion necessary for laser light emission.

The operating wavelength depends on the band-gap and can be tailored by utilising compound semiconductors, e.g. InGaAs, to match the low attenuation transmission win-

dows in optical fibres. The other essential condition for laser operation is an optical cavity to generate oscillations

and stimulate emission. It is constructed in the Fabry- Perot laser by cleaving the crystal wafer to create optical facets which act as partially reflective mirrors. Several other types of edge emitting laser exist, including the Distributed Feedback type where a Bragg grating is created in the cavity region structure to perturb periodically the

refractive index in the material and generate interference between backward and forward wavetrains. Vertical cavity lasers, as the name suggests, are devices which are transverse to the wafer surface.

A minimum current, the threshold current, is required before laser action commences since a certain fraction of photons are lost from the cavity via external emission and internal absorption. However, improved designs have significantly reduced thresholds since the operation of the first semiconductor lasers and values as low as a few mA and forward voltage drops of l-2 V are now achievable with output power of many mW. Above threshold the light

G. Hall I Nucl. Instr. and Meth in Phvs. Res. A 386 (1997) 138-142

output power is usually highly linear with current over a wide range.

Low cost devices are required for the large scale of the

CMS tracker. This is usually achieved in semiconductor manufacture by high volume production with huge num- bers of devices on each wafer. Automatic testing before dicing then maximises yield at the packaging stage. This is

not so easy if cleaving is required before operation so one reason for the relatively high cost of lasers is the testing requirement. Packaging, which involves accurately cou-

pling the emitter to the core of an optical fibre, is also an

important cost driver. These problems seem to be gradual- ly being overcome and prices are expected to drop considerably in the future to as little as a few tens of dollars per transmitter in large volumes.

laser fluctuations in intensity, phase and frequency may

give rise to noise. None of these issues are expected to present insuperable problems but many will require atten-

tion in building the CMS system. As usual, it may be the apparently trivial problems which will generate the most work.

4. Conclusions

A major concern is radiation damage, given the well known problems of semiconductor devices in the LHC environment. Traps created in the active volume of the laser are likely to act as non-radiative recombination centres which reduce the optical gain. However there are

reasons to be optimistic. Most lasers are constructed using III-V materials which are known to be relatively radiation hard. In the case of gallium arsenide, for example, the raw

material has a relatively high defect density prior to irradiation, compared to crystalline silicon, and thus addi-

tional radiation created traps may cause little degradation. In addition modern lasers are deliberately constructed to minimise the active volume using heterostructure quantum well designs to increase the laser gain by optimising the overlap of carriers with the optical field in the cavity. The

lateral dimensions of the laser are also designed to be small to minimise electrical power requirements and maximise efficiency. Since the magnitude of radiation

damage effects is related to volume, it may be that the drive for lower threshold lasers coincides with the require- ments for radiation tolerant transmitters. Irradiation tests have been carried out with neutrons and gammas. Initial

results appear to be very promising.

The analogue optical link is a critical part of the CMS tracker readout system where significant progress has been

made in the development of suitable transmitters. The implementation of the system is expected to present many

further challenges but we can be optimistic about the outcome.

Acknowledgements

I would like to acknowledge the important contributions to the optical link developments by G. Stefanini and F. Vasey of CERN. I would also like to thank my colleagues

at Imperial College for their efforts in beginning to implement them for CMS. Many other people have

contributed to my education in this new area.

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cheaper than mono-mode connectors. Fibres are not in- dividually as robust as electrical cables and can be damaged, or light transmission altered, by incorrect hand- ling. For many applications they are made up into cables with protective sheaths, often with heavy reinforcements.

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