Study of Time Walk Behaviour of CMS Pixel Modules

Abstract

The CMS pixel barrel is built and tested at the Paul Scherrer Institute (PSI) in Switzerland. In previous beam tests it had been seen that the 16 readout chips of a module showed different time walk behaviour. The time walk behaviour of a chip is the fact that signals with different pulse heights activate the digital hit discriminator at slightly different times and this results in an incorrect bunch crossing identification. In this work systematic studies of the time walk behaviour of pixel chips were done and it was investigated whether the behaviour can be improved by suitable changes of the programmable chip DACs. It has been shown that the time walk behaviour of a pixel readout chip can be studied by activating internal calibration signals alone. It was found that the variation between chips in a module is the dominant factor over the variation of pixels within a chip. The analog current of the pixel amplifiers (Iana) is the essential parameter to change time walk behaviour. However, the comparator threshold must be adjusted accordingly. In this work an algorithm was written to do this whole procedure for a pixel module. If the analog current Iana is modified from its default value of 24mA by ±6mA a corresponding time walk change of ±5 ns can be observed. The algorithm finds new DAC values for each of the 16 readout chips and as a result the time variation between the chips could be reduced from 8.0 ns to 3.7 ns. The trim settings (threshold adaptations per pixel) are found to be adequate after changing the currents and therefore do not have to be redetermined. In order to verify the time shift resulting from the chip internal calibration process, a fast pulsed X-ray source was used as an external reference signal. The source could be turned on (off) using the focussing cup within less than 15 ns (75 ns). Because of the capability to switch on the tube very fast it could be shown that changing Iana shifts the time walk behaviour by the expected amount.