The NA48 LKr Calorimeter Readout Electronics

The electronics to read out the LKr Calorimeter was developed and produced during 1990 to 1996. An overall description of calorimeter is available in the detector paper "The Beam and Detector for the NA48 Neutral Kaon CP Violation Experiment at CERN, CERN-PH-EP/2007-06, p20-28 http://na48.web.cern.ch/NA48/NA48-3/groups/antiphoton/LKr/LKR/doc/phep-2007-006.pdf. The various parts of the electronics have been described in many technical notes and papers, but a technical description with references available in one place (TWiki or WWW) is useful when analyzing the signal processing chain in detail especially for future applications. The different parts of the present electronics are described in separate chapters below.

LKr Structure

Pictures of the "naked" calorimeter and when it was cabled and slides from talks given by NA48 collaborators. For the noise analysis it is necessary to know the capacitance at the input of the preamplifier. The capacitance of the readout strip is found to be 131 pF with the MAXWELL program. To this there is a calibration capacitor of 22 pF and of course the input capacitance of the preamplifier (modelled with Spice parameters) and some unknown stray capacitance. The external capacitance value (153 pF) is within +-10%?

Details are at: LKr Calorimeter Description

Cold Preamplifier

The noise of the whole readout electronics is determined to about 90% by the input JFET transistor of the cold preamplifier from INTERFET . The transistor IF4500 from the process P_NJ450L has minimum noise at 160K and still a very good noise performance at the Krypton operating temperature 120K. The preamplifier is analysed with help of the LTSpice program with parameters obtained 1991 from S.Rescia of BNL. In order put some limit on the noise of the LKr electronics the transconductance (gm) of the IF4500 is varied from 25, 55 and 105 mA/V. This give a variation of the noise voltage of of about 25%. The amplitude and frequency response of the electronics including the preamplifier is given by passive external components. Therefore the spread in gain and delay between channels are very small (+-1%).

Details are at: http://na48.web.cern.ch/NA48/NA48-3/groups/antiphoton/LKr/LKR/PREAMP/preamp.html

Transceiver

The transceiver has as task to receive and amplify to the signal from the preamplifier and send it the readout module the Calorimeter PipeLine Digitizer (CPD). The tranceiver reduces the coherent noise while having a neglible effect on the thermal noise of the electronics. It amplifies the noise spectrum with a peak at about 20 MHz. The noise bandwidth limited by the combined effect of the preamplifer and transceiver but also the cables to about 25 MHz. The attachement shows the Signal-to-Noise Ratio at the input of the shaper after 20ns differentiation.

Details are at: http://na48.web.cern.ch/NA48/NA48-3/groups/antiphoton/LKr/LKR/TRANSCEIVER/transceiver.html

Shaper

The analog subcard of the CPD CPDAS contains the shaper, ADC, pipeline memory and trigger circuits. The shaper consists of a 20ns differentiation, 9-pol Bessel filter and 2-pol High Frequency filter. Prototypes with descrete components were tested and finally an ASIC KRYPTON designed and produced by the compay CISS and Austrian Miktosysteme', Graz, Austria. The performance of the LKr electronics chain is detemined by the shaper because it limits the bandwidth to about 8 MHz.

Details are at: http://na48.web.cern.ch/NA48/NA48-3/groups/antiphoton/LKr/LKR/SHAPER/shaper.html

Neutral Trigger

To be done

Summary

The total noise as function of frequency is shown in the attachment. The contribution of the different noise sources assuming that the JFET IF4500 has a gm of 55 mA/V is the following:

Function	Noise (mVrms)
Preamplifier	7.06
Shaper	3.48
Transceiver	2.51
Sum of above	8.26
ADC 8.7b noise	1.04
Coherent noise	0.5
Total	8.3

The conversion factor from mVrms to MeV is about 1.0, (1 GeV shower produces 2.5 uA peak current in the central hit cell, while in the simulation 1 uA gives 410 mV).

The noise contributions of the different components of the NA48 LKr calorimeter are shown below integrated over frequency:

The noise contributions of the preamplifier and transceiver at the input of the shaper

-- BjornHallgren - 29 Jan 2008

• LKr_structure: