Two Beetle chips are connected to a PR01 VELO Phi-detector, which enables us to collect signals from both the inner and outer ..... chi-square. 2. 4. 6. 8. 10. 12.
LHCb 2002-053 VELO
Investigation of the Beetle.1.1 chip in the X7 testbeam N. van Bakela , M. van Beuzekoma , H.J. Bultenb , E. Jansa , T. Ketelb ,S. Klousa,b , H. Snoekb , H. Verkooijena a
b
NIKHEF, P.O. Box 41882, 1009 DB Amsterdam, The Netherlands Vrije Universiteit Amsterdam, de Boelelaan 1081, 1081 HV Amsterdam, The Netherlands
Abstract Two Beetle1.1 chips, bonded to a Hamamatsu PR01 VELO Phi-detector, have been tested for the first time in a testbeam. The main goal was to measure the signal to noise ratio of the Beetle1.1 connected to a prototype VELO Phi-detector. Furthermore we investigated the general behaviour of the Beetle1.1 to adapt the design of the chip if desirable. This note presents the measured S/N numbers as well as some features and characteristics (e.g. rise time, spillover) of the Beetle1.1 chip.
12th February 2003
1 Introduction A collaboration between the ASIC-lab in Heidelberg, Oxford University and NIKHEF is developing a radiation hard frontend chip for the silicon detectors of the LHCb vertex detector. The second prototype chip, the Beetle1.1 [1][2], has been submitted in a 0.25 µm CMOS technology in 2001. Two of these 128 channels Beetle1.1 chips are bonded to a 300 µm thick Hamamatsu PR01 VELO Phi-detector (BeetlePhi detector). This configuration was used for performance tests in the X7 testbeam at CERN and are a follow up of earlier lab-tests [3]. The presented results contribute to the further development of the Beetle chip and are necessary to make a well-based choice for the appropriate frontend chip for LHCb. In section 2 the experimental setup, DAQ-system and software used are described. The Beetle chip settings and the acquired statistics are presented in section 3. The analysis of the data in stand-alone mode and in combination with the VELO telescope are discussed in section 4, where also the alignment of the Beetle station is considered. Conclusions are presented in section 5.
2 Experimental setup In Fig.1 the testbeam setup is shown. The VELO telescope consists of three stations with an R- and Phi-detector each and a fourth station with three tilted detectors. The Beetle test box contains the BeetlePhi-detector. Two scintillators (Sc) cover the area of the detector that is read out and are used in the trigger for the data-acquisition.
2.1 The Beetle1.1 in the testbeam The layout of the Beetle1.1 chip [4] is presented in Fig. 2. The characteristics of the shaper and preamp circuit of the chip are determined by 5 parameters (V f p , V f s , I pre , Isha and Ibu f ), which are programmed via an I2 C interface. Two Beetle chips are connected to a PR01 VELO Phi-detector, which enables us to collect signals from both the inner and outer region. These chips are mounted on a standard Heidelberg test-PCB and connected to the detector
1
Beam Sc1 R R Phi Phi
R Phi
R
Telescope
R Phi Sc2
Sc3 Phi
4th station Beetle−test
Figure 1: The Beetle1.1 testbeam setup with VELO telescope, fourth station, and Beetle testbox in the X7 testbeam. by a specially designed two-layer pitch adapter. The physical distance between the detector and the VME-DAQ is about 8 meters. Therefore the analog signals coming from the Beetle chip are amplified by a factor 25 before sending them over shielded twisted pair cables. The amplifiers of type max435 are located on a separate PCB at a distance of 25 cm from the Beetles. This PCB also contains the electronics needed for the buffering of the I 2 C signals that are used for down loading the settings in the Beetle chip. The analog signal of the Beetle chips is sent out via a single analog output per chip instead of the 4 analog output as foreseen in the LHCb experiment. This was done to limit the space required on the amplifier PCB.
2.2 Detector geometry The detector used is a first prototype Silicon Hamamatsu n-on-n Phi-detector [5] of 72 ◦ , see Fig. 3. For this 300 µm thick detector, one minimum ionising particle (MIP) corresponds to 22000 electrons. The inter-strip pitch at the inner region of the detector varies from 45 − 126 µm along the radius and at the outer region from 44 − 79 µm. The detector is divided into 6 routing line areas; routing line area 2 and half of routing line area 3 are bonded in the present setup. This corresponds to routing line number 130 to 256 and 257 to 385 for area 2 and 3, respectively. Beetle chip 0 is bonded to the inner region (area 2) and Beetle chip 1 is bonded to the outer region. A few routing lines are not bonded to study the noise contribution. For the complete bond scheme see Table 1. The strip length in the inner and outer region is 17.93 and 21.86 mm, respectively. Reconstructed tracks in the VELO telescope are extrapolated to the BeetlePhi-detector in the Beetle testbox (Fig. 4), for details see section 4.4.1. The VELO tracks that also gave a hit in the BeetlePhi-detector are marked.
2.3 DAQ-system Central component in the DAQ-system is a PC running Windows/NT equipped with a VME-MXI-2 PCI-card of National Instruments. The trigger logic is setup such that a choice can be made between data taking in either standalone mode or coincidence mode with the VELO beam telescope [6], in which case the trigger is determined by a coincidence between a selection of scintillators. The VELO beam telescope consists of 3 R and 3 Φ PR-01 300 µm thick silicon detectors. The time of passage of a particle with respect to the 40 MHz clock is measured with a TDC. The DAQ software consists of a Labview program reading the VME-modules i.e., 2 channels of a 40 MHz Joerger VTR812 12 bits ADC (1 ADC count corresponds to 1 mV), scalers and TDC’s. Each trigger leads to the readout of eight consecutive clock samples, thus permitting to scan the complete pulse shape over 200 ns. During the readout of 8 consecutive pipeline columns we do not accept new triggers. The data from the VELO beam telescope are collected by the standard VELO-X7 DAQ-package. The data is first stored on a local disk before being copied to
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Vdcl
Or
Vfp Vfs Ipre Isha Ibuf Icomp Ithmain Ithdelta Itp
3
Figure 2: Layout of the Beetle1.1.
pipeline readout−amplifier
Table 1: The bonded routing lines on the BeetlePhi detector and the corresponding channels on the two Beetle chips. “Ground bias” means that the routing line is bonded to the ground bias of the front side of the BeetlePhi detector. Beetle chip Routing line Beetle channel Status 0 130-138 0-8 Bonded 0 139,141,143,145 10,11,14,15 Floating 0 140 9 Bonded 0 142,144 12,13 Ground bias 0 146-257 16-127 Bonded 1 258-266 0-8 Bonded 1 267,269,271,273 10,11,14,15 Floating 1 268 9 Bonded 1 270,272 12,13 Ground bias 1 274-385 16-127 Bonded CASTOR. Each event in both data streams contains counter and time stamp information such that in the off-line analysis the data of matching events can be merged.
2.4 Software; raw data correction The software for the Beetle analysis of the 2001 test beam data is based on Veloroot. Veloroot is an analysis tool developed by the LHCb VELO group [7]. Amongst other things it contains several libraries for detector and hybrid definition, hit extraction, clustering, tracking and alignment. The Beetle analysis software can either process data coming from the setup in stand-alone mode or in combination with the tracks from the telescope. Telescope part The combined analyses with the telescope uses Veloroot to process the telescope data and find the tracks. The program determines the positions of the clusters for an event in the telescope detectors according to the procedure described in [8]. With those clusters it makes the best possible combinations of tracks, using each cluster only once. Each of the resulting tracks is stored in a cluster set with the label of the event and a time stamp. The cluster set thus contains all clusters that belong to the track it describes. These sets are stored in a file and used by the Beetle analyses software when the data from the Beetle is analysed in combination with the telescope. Synchronisation The time stamp of a telescope event is matched with the Beetle events to synchronise the data. The test beam has a beam structure of a 4.8 s spill and a no-beam period of 14 s. Due to this structure the time stamp is a very reliable way to synchronise events. If the number of events in the VELO telescope and BeetlePhidetector during one spill does not match, all the events of the spill are rejected. About 60% of all events can be correctly synchronised with this method. Pedestals & Common mode For each trigger all channels are read out during 8 clock samples. The first step in the data analysis is the determination of the pedestals. To that end, for each clock sample and each channel the raw data (summed over all events in one run) have been fitted with a Gaussian function. The pedestals are stored and can later be read in from a configuration file. Hence this procedure is executed once for every hardware setting of the Beetle chip.
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Figure 3: Routing line read-out scheme of the 1998 Hamamatsu PR01 Phi-detector. The numbers indicate different routing line areas. In this setup the left half of area 3 and entire area 2 are read out; a total of 129 + 127 strips are read out. A typical pedestal file used in the analysis is shown in Fig. 5. The fitted pedestal distributions show a sine trend comparable with the pedestals of the bare chips. An absolute threshold of 40 ADC counts and a sigma threshold of 1 σ are used. The pedestals are subtracted from the raw data of the corresponding chip, channel and clock sample. In the second analysis step, an ADC spectrum (AdcCor) is produced, in which pedestals and common-mode noise are subtracted. Common-mode noise was calculated for each chip and each clock sample separately. We used two slightly different procedures. In the first procedure we the common mode noise c i of the pedestal determine corrected ADC values below a certain signal threshold ( ri, j < threshold; typically this threshold is 4 times the RMS of the pedestal peak)
ci =
1
channel
Nno hit
j=1
∑
(ri, j − p j )
where i is the event number, Nno hit the number of channels with no hit, ri, j the raw ADC value and p j the pedestal for channel j. The summation runs over the channels with no hit. In the second method, which is the same as applied in some analyses of the SCTA chip, we fit the common mode as a function of read-out channel with a first-order polynomial (instead of with a constant). In this case, a channeldependent common mode subtraction can be made. No strong channel-dependent effects in the common mode have been observed. The common-mode correction reduces the width of the pedestal distributions by about 10-20 percent, fitting with a first-order polynomial gives an additional 1 - 2 percent reduction. Hence the last method is not used in our analysis. Clustering After common-mode correction the noise for each channel and clock sample is determined by fitting a Gaussian to the AdcCor spectrum. These average noise values are stored for each channel and used in the cluster 5
local y (cm)
Extrapolated tracks to Beetle Phi (run 5308) 4
3
chip 0 hits
chip 1 hits
1
3
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Figure 4: Tracks reconstructed in the VELO telescope are extrapolated to the BeetlePhi-detector. Corresponding hits in this Phi-detector readout with the Beetle chip 0 and chip 1 are marked red and blue, respectively. The contour of the trigger scintillator (Sc3) is indicated with the dashed line. routines. For a valid Beetle cluster, one channel has to contain an ADC value above threshold. If a channel satisfies this condition, a cluster is made out of this channel and its two direct neighbours, if they exceed a second threshold (hence maximally three channels in one cluster). The first threshold is an absolute one, typically, we choose 40 adc channels, which is about 4 to 5 in signal-to-noise. The second threshold is a relative one, a value of 1 sigma 1 was chosen. Furthermore, we analysed data in combination with the VELO telescope. For events in which the track of the VELO telescope traverses the active area of the BeetlePhi detector we analyse clusters found in the BeetlePhi detector near this VELO track. Another approach is to sum the AdcCor values of the channel that is closest to the VELO track plus its two neighbours, see section 4.4.4. Alignment The clustering of the hits in the Beetle is done using the same clustering routines as the VELO telescope [7],[8]. The clusters of the Beetle are fed into an alignment routine together with the cluster sets of the telescope, described earlier. The alignment routine minimises the residuals of local cluster positions with respect to the tracks found in the telescope by changing the BeetlePhi-detector coordinates, see section 4.4.1.
3 Data The obtained data rate is about 37 events per spill in coincidence mode with the VELO telescope, while in standalone mode the data rate is about 92 events per spill. During earlier lab-tests [3], bias setting scans of the Beetle frontend were made to find some optimal settings with respect to rise time, undershoot and pulse amplitude. After the testbeam period it turned out that the calibration of the DAC’s in the Beetle1.1 is different from that of the Beetle frontend test chip. Hence the applied bias currents are lower than the anticipated settings. The 36/320 bias setting was chosen to demonstrate the capability of producing a fast pulse at the price of some undershoot in the pulseshape. The 63/346 bias setting features a longer tail but without undershoot. Table 2 shows the data taken in coincidence mode with the VELO telescope. 1 In
Veloroot terms this means that NEIGHBOURLEVEL=1
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Figure 5: Pedestals per channel for clock sample 3 of chip 0 (dashed line) and chip 1. The spectra look similar for all clock samples. Beetle bias settings: I pre = 346 µA and Isha = 63 µA. During the testbeam period the VELO group required less stringent trigger conditions for some runs which led to slower data taking. The Beetle DAQ was then switched to a stand-alone mode for an efficient usage of the testbeam period. Table 3 gives an overview of the data taken in stand-alone mode.
4 Analysis The analysis is split into two parts: 1) A stand-alone analysis in which only data measured with the BeetlePhidetector are used. 2) Data acquired in coincidence mode with the VELO telescope is analysed such that the telescope tracks are used to resolve the Beetle signal. These two methods implicitly form a consistency check.
4.1 Selection criteria In a first coarse analysis of the testbeam data a few selection criteria have been determined. They provide a method to mask features of the testbeam setup found in the data that are not correlated to the Beetle. An RF generating setup was used to investigate the response of the individual Beetle1.1 channels separately. A network analyser generated an RF signal into an open ended coaxial-line positioned a few mm from the BeetlePhidetector. The RMS of the pick-up signal per channel is shown in Fig. 6. As expected, the intentional 12 floating channels didn’t pick up the RF signal and show a lower RMS value. However, 33 other channels show similar RMS values. Analysis of the testbeam data showed lower noise values and an absence of signal for the same channels. Hence these “low-RMS” channels were disregarded in the analysis. We found a correlation between the state of the last header bit and the ADC value of channels 0, 1 and 2 in both chips. Depending on the status of this header bit, the RMS value of channel 0 shifts about 40 ADC counts. This cross-talk effect is partly due to the cable, see for instance [9]. Therefore channels 0, 1 and 2 of both chips are neglected in the analysis. A known problem of the Beetle1.1 is a shift of the baseline at low trigger rates 2 . The longer the time between two events, the further the baseline drifts away and the more triggers are needed to bring the baseline back to its default value. To avoid the effects of this low trigger rate problem, every first event of a new spill is skipped. In practice this means that we neglect every event with a preceding trigger longer than 100 ms ago. Additionally, every first sample out of the 8 consecutive clock samples is not used because the effects of the baseline drift is already visible 2 This
problem is solved in the Beetle1.2
7
Table 2: Data taken in the October 2001 testbeam. Trigger mode B∩C implies data taking in coincidence with the VELO telescope. Trigger mode B is also a coincidence mode but with a less confined trigger efficiency (the VELO group needs this to readout all their detector channels). Vbias is the Phi-detector bias voltage, V f s and the two currents are Beetle1.1 frontend bias settings. file nr
trigger condition
Vbias [V]
V f s [V]
Isha [µA ]
I pre [µA ]
nr of events
5223-5227
B∩C
58
0
59
346
34k
5228-5235
B∩C
65
0
59
346
30k
5239-5240
B∩C
67
0
59
346
20k
5236-5238, 5241-5248
B∩C
72
0
59
346
48k
5249-5253
B
72
0
59
346
43k
5261-5269
B∩C
72
0
63
346
69k
5273-5279
B∩C
72
0
36
320
68k
5289-5293
B∩C
72
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63
346
31k
5294-5298
B∩C
72
0
36
320
41k
5300-5304
B
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0
36
320
25k
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Figure 6: A sine modulated RF signal picked up by the BeetlePhi-detector. The signal is fitted with a gauss, the sigma (ADC counts) is plotted versus channel-number for chip 0 (left) and chip 1 (right). The 12 floating channels (nrs. 10-15) have a low sigma as expected. However, in total 33 other channels show a similar behaviour. at a trigger rate of 19 Hz which was the maximum trigger rate allowed by our data acquisition. LHCb will trigger with 1 MHz and therefore no problem is expected. After inspection of the noise distribution for the different pipeline-columns we found a few pipeline-columns with a clear offset with respect to the average noise values, see Fig. 7. Two groups of deviating pipeline-columns can be distinguished; pipeline-column numbers 1,2,3 and 39,40,41. These pipeline-column numbers are correlated to the chosen readout latency (∼ 1 µs). It turned out that the monitor signal of both the read and write pointer induce by accident a signal in a PCB-trace close to the silicon detector. Removing the superfluous bond, between the monitor output on the chip and the PCB-trace, after the testbeam experiment solved this inconvenience. These pipeline-column numbers are disregarded in the analysis.
4.2 Noise The typical noise values for chip 0 and chip 1 are 10.4 and 9.2 ADC counts respectively, see Fig. 8. LHCb will use repeater cards with a rad-tolerant amplifier to drive the signals of the frontend chip via 60 m twisted pair cables to the digitiser boards. During the testbeam run of autumn 2001 we used separate amplifiers to drive 8
Table 3: Data taken in October 2001 testbeam in stand-alone mode. Trigger mode A is the stand-alone mode where only the scintillator SC3 and the BeetlePhi-detector are used to take data. Vbias is the Phi-detector bias voltage, V f s and the two currents are Beetle1.1 frontend bias settings. file nr
trigger condition
Vbias [V]
V f s [V]
Isha [µA ]
I pre [µA ]
nr of events
SA 1/4/5
A
72
0
63
320
32k
SA 14/15
A
72
0
85
346
20k
SA 13/16
A
72
0
51
346
20k
SA 11/12
A
72
0
63
241
19k
SA 7/8
A
72
0
36
346
23k
SA 2/3/6/17/18/21/22
A
72
0
36
320
111k
SA 9/10/23-29
A
72
0
63
346
120k
Total:
345k
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Figure 7: The pedestal subtracted ADC distribution (for different clock samples and chip channels) fitted with a Gaussian; their mean values are plotted versus the pipeline column numbers for chip 0. the lines between the Beetle1.1 chip output and the Joerger ADC (see section 2.3) because of the low gain of the Beetle1.1 output drivers. The gain of the Beetle1.2 output drivers has been increased with a factor 4; hence in LHCb the amplifier noise of the repeater card will not contribute significantly to the noise. This temporary solution with the separate amplifiers introduced additional noise which we refer to as system noise henceforth. The system noise was determined by analysing data with the Beetle chips switched off. The usual common-mode correction was applied. Still, the width (1 sigma) of the common-mode corrected pedestals was about 6.0 and 4.2 adc channels for chip 0 and 1, respectively. The system noise is obviously not related to the performance of the Beetle chip. In order to present intrinsic chip results, that do not depend on the experimental conditions under which they have been measured, we also have chosen to subtract this system noise when quoting S/N values. The data shown in the figures in this report are not corrected for system noise. Table 4 shows the average strip noise per chip without and with system noise correction.
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Figure 8: Measured noise distribution and the Gaussian fit results; noise for chip 0 (1) is N = 10.4 (9.2) ADC counts for bias settings Isha = 63 µA and I pre = 346 µA. Table 4: Strip noise per chip without and after system noise correction. Setting Isha /I pre [µA] Chip Strip noise Strip noise (corrected) 63/346 0 10.0 8.0 63/346 1 8.7 7.4 63/346 0+1 9.3 7.7 36/320 0 10.2 8.2 36/320 1 8.8 7.8 36/320 0+1 9.5 8.0 Combined 0+1 9.4 7.8
4.3 Stand-alone 4.3.1 Signal; 1 and 3 strips In the stand-alone analysis we deal with signals of a single strip and cluster signals formed by 3 adjacent strips. Hits in the BeetlePhi-detector are selected by defining a threshold (typically 40 ADC counts). A signal is considered a hit if it exceeds this threshold in clock sample 4. The contents of all 7 other corresponding clock samples are plotted also, resulting in a pulseshape as shown in Fig. 9. The threshold applied is visible in clock sample 4. Events in which more than 4 channels exceed the threshold are discarded and labelled noisy (about 0.4% of the events). Figure 10 shows the summed signal charge of the strip that fired and its two neighbours. 4.3.2 Landau Fit The energy loss distribution of a MIP passing through a thin material obeys the Landau probability density function. The Moyal function represents a simple analytic approximation of the detector response to a MIP passage [10]: f (x) v(x) v(x)
−v(x)
)/2 , where ∝ e−(v(x)+e = (x − x0 )/w, forx0 ≥ 0 = −(x − x0)/w, forx0 < 0
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Figure 9: Signal of single strip for bias setting Isha = Figure 10: Signal of 3 adjacent strips for bias setting 63 µA and I pre = 346 µA. Isha = 63 µA and I pre = 346 µA.
Counts
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where x is the charge in ADC counts, x0 the most-probable-ADC value and w the width of the Landau distribution. The fit is executed in two steps; in a first iteration the Moyal fit results in a most probable (MP) ADC value. This value sets the fit interval in the second iteration (Lower bound ADCsecond f it = MPADC f irst f it − ADCo f f set ). Figures 11 and 12 show the fitted Landaus for two selected Beetle1.1 bias settings (see section 4.3.3). The MP ADC values fitted with this Landau are used to calculate the S/N. The χ2 distributions in Fig. 13 indicate that reliable fit results have been obtained in the signal region. 120
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Chi2 / ndf = 145.5 / 105 const = 148.4 ± 3.935 m.p.ADC = -106.4 ± 1.141 width = 15.28 ± 0.4044
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Chi2 / ndf = 115.5 / 111 const = 102.8 ± 3.572 m.p.ADC = -122.2 ± 2.133 width = 16.58 ± 0.6741
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Figure 11: Landau fit for single strip signal (left) and for 3 strips signal (right) with bias setting Isha = 63 µA and I pre = 346 µA.
4.3.3 Pulseshape A bias setting scan with the Beetle1.1, described in [3], revealed a large variety of pulseshapes. Two favourite bias settings were chosen according to a pulseshape without and with undershoot, as shown in Figs. 14 and 15, respectively. The points in these figures represent the fitted MP ADC values of the charge distributions per 2 ns bins. We can conclude from these plots that more than 90% of the charge is collected in the single central strip. 11
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Chi2 / ndf = 145.1 / 113 const = 141.2 ± 3.81 m.p.ADC = -109.2 ± 0.9304 width = 15.03 ± 0.3819
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Chi2 / ndf = 122.1 / 117 const = 104.5 ± 3.718 m.p.ADC = -119.4 ± 2.454 width = 17.06 ± 0.6702
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Figure 12: Landau fit for single strip signal (left) and for 3 strips signal (right) with bias setting Isha = 36 µA and I pre = 320 µA. 14 12 10
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Figure 13: χ2 for bias setting Isha = 63 µA and I pre = 346 µA; single (left) and 3 strip cluster signal (right). The spillover and rise-time of the pulseshape are used to compare the performance of the Beetle1.1 chip for different frontend bias settings. The spillover is defined as the fraction of the peak amplitude 25 ns after the peaking time, the rise time is the time it takes the pulse to rise from 10 % to 90 % of the peak amplitude. To obtain these characteristics from the pulse shape, first the peaking time and peak amplitude are determined by fitting a parabola to the peak of the pulseshape. This parabola also gives the time coordinate of the 90 % point. The 10 % point in time is obtained with an arctan fit to the rising edge of the pulseshape. The time difference between to the 10 % and 90 % points determines the rise time. An arctan fit to the falling edge of the pulseshape gives the spillover fraction. Table 5 shows the characteristics of the pulseshapes produced by the Beetle1.1 for our two favoured and some extra Beetle1.1 bias settings. The measured rise time and spillover fulfil the LHCb requirements. The rise time and spillover correspond to former lab measurements done with an identical frontend on the BeetleCO1.0 chip [3].
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Figure 14: Pulseshape for a single strip (left) and 3 adjacent (right) strips signal with bias setting Isha = 63 µA and I pre = 346 µA.
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Figure 15: Pulseshape (with undershoot) for a single strip (left) and 3 adjacent strips (right) signal with bias setting Isha = 36 µA and I pre = 320 µA. 4.3.4 Sticky Charge In the analysis of the data we observed a correlation between the analog values of the current and previous clock sample, even if the previous clock sample belonged to the former event. It was found that for each of the 128 analog channels of the Beetle the current clock sample is changed by addition of -30% of the charge of the previous clock sample. This effect is independent of the time between the triggers. Because it seems that after resetting the readout amplifier there remains a residue from a previous readout (RC time too short), the problem is baptised sticky charge. Since the trigger in our case is 8 clock cycles long, as described in section 2.3, the pulse shape as measured is influenced by this effect. For pulseshapes with a rise time faster than 25 ns, only the tail of the pulseshape is changed: the observed fall time is too low. This results in an underestimation of the spillover ratio up to 15-20 % in absolute value. For the non-consecutive readout this problem is solved in the Beetle1.2 and for the consecutive readout (if you send during the readout of a previous trigger a new trigger) this problem will be solved in the Beetle1.3.
13
Table 5: Stand alone analysis; single strip characteristics for different bias settings. The rise time is defined as the time interval in which the signal rises from 10 to 90% of the signal peak. The spillover is the remainder of the signal 25 ns after the signal peak. Isha [µA] I pre [µA] Rise time [ns] Spillover % after 25 ns 63 242 19 24 36 320 21 21 63 320 18 21 36 346 20 19 51 346 18 25 63 346 19 24 85 346 22 26 4.3.5 S/N The S/N numbers measured with the stand-alone BeetlePhi detector are sumarized in Table 6 for different FE bias settings. Our 2 favourite bias settings (36/320 and 63/346) result in a S cluster /Nstrip of 14.8 and 15.9 respectively. Table 6: Stand alone S/N without and with system noise correction. Isha µA
Ipre µA
Sstrip /Nstrip
Scluster /Nstrip
Sstrip /Nstrip
Scluster /Nstrip
(corrected)
(corrected)
63
242
101.0/9.3=10.9
111.4/9.3=12.0
101.0/7.7=13.1
111.4/7.7=14.5
36
320
109.6/9.5=11.5
118.4/9.5=12.5
109.6/8.0=13.7
118.4/8.0=14.8
63
320
103.5/9.4=11.0
116.7/9.4=12.4
103.5/7.8=13.3
116.7/7.8=15.0
36
346
112.9/9.4=12.0
119.3/9.4=12.7
112.9/7.8=14.5
119.3/7.8=15.3
51
346
107.5/9.3=11.6
123.3/9.3=13.3
107.5/7.7=14.0
123.3/7.7=16.0
63
346
109.2/9.3=11.7
122.2/9.3=13.1
109.2/7.7=14.2
122.2/7.7=15.9
85
346
100.0/9.5=10.5
113.2/9.5=11.9
100.0/7.9=12.7
113.2/7.9=14.3
4.4 Telescope Analysis In the telescope analysis the BeetlePhi detector is considered as an extra station of the VELO telescope. The selection criteria discussed before are applied in the telescope analysis. Tracks found in the VELO telescope are used to optimise this analysis. The polarity of the pulseshape is inverted in the figures of the telescope analysis; this has no physical meaning. 4.4.1 Alignment A Beetle hybrid class3 is added to the veloroot software to access the tracking methods of this package. This class describes the mapping from the routing lines to the Beetle chip channels. The existing class Dt_PhiDetTyp1.cxx describes the mapping from the local geometry of the detector strips to the routing lines. The BeetlePhi detector can now be aligned with respect to the telescope by using the telescope tracks. The alignment method in the BeetleTree class uses these tracks to determine the residuals with respect to the hits in the BeetlePhi detector. 3 Dt_PhiDetTyp1HybBeetle.cxx
14
The pedestals and common mode noise (see section 2.4) are subtracted from the raw Beetle data. Clusters at the BeetlePhi detector are produced with a signal to noise cut of 2 (see section 2.4). The Beetle alignment routine uses telescope tracks fitted [8] through clusters on 8 telescope detectors with χ 2 ≤ 50. These telescope tracks are extrapolated to the BeetlePhi detector and give a track intercept at the BeetlePhi detector. The residual is given by the closest distance (in the detector z-plane) between the track intercept and a strip on which a Beetle cluster is detected [8]. The calculated residuals amount at maximum to 100 µm. For this set of selected tracks a χ 2 is formed by summing the residuals. Of the 6 degrees of freedom describing the position of the BeetlePhi detector only 3 (x, y, β [5]) are used as free parameters in the minimisation of the χ2 . The results of the alignment of the BeetlePhi detector are shown in Fig. 16. The quality of the alignment is sufficient for the cluster analysis. The sigma of the distribution of the residuals is 35.1 µm for chip 0 and chip 1.
hnew Nent = 1046 Mean = -5.108 RMS = 35.95 Chi2 / ndf = 151.3 / 133 Prob = 0.1324 Constant = 13.77 Mean = -4.875 Sigma = 35.09
25
20
hnew Nent = 1299 Mean = 0.2575 RMS = 36.28 Chi2 / ndf = 145.6 / 135 Prob = 0.2539 Constant = 17.67 Mean = -0.4532 Sigma = 35.1
25
20
15 15 10 10 5
0 -100
5
-50
0
50
0 -100
100 Residual [um]
-50
0
50
100 Residual [um]
Figure 16: Distribution of the residuals (in µm) after alignment of the BeetlePhi detector for Beetle chip 0 (left) and chip 1 (right), σ = 35.1 µm for both chips.
4.4.2 Cluster signals In the MakeBeetleClusters routine the parameter sigtonoise is set to 2 and the threshold for the neighbouring channels is set to 1 sigma to generate Beetle clusters. The typical number of tracks found per event in the VELO telescope and number of clusters per event in the BeetlePhi detector are shown in Fig. 17. A datafile containing about 10000 events is reduced to about 6000 events after merging the Beetle and VELO telescope data. From these events about 54 % contain at least one telescope track and one Beetle cluster. The cluster signals found in the BeetlePhi detector are plotted in Fig. 18. The Landau fit (see section 4.3.2) for one of the favourite bias settings is shown in Figs. 19 and 21 for single strip and 3 strip clusters respectively. Part of the corresponding pulseshapes are plotted in Figs. 20 and 22 where the most probable (MP) ADC value of the pulse height found for the Landau fit is used. The Landau fits are acceptable for a bin size of 4 ns and the MP-value is a good indication for the Beetle signal. 4.4.3 S/N with the telescope analysis The S/N values found in the telescope analysis without and with system noise correction are listed in Table 7. A cluster signal incorporates the charge collected with at most 3 strips, the strip signal only reckons the centre strip corresponding to the largest signal of the cluster. The cluster centre is within 100 µm from the telescope track. The bin size of the fitted signal slices is 4 ns. After system noise correction it turns out that the S/N performance
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TrackMulti
ClusterMulti 0.45
TrackMulti 0.5
Entries
ClusterMulti
0.4
5958
Entries
5958
0.35 0.4 0.3 0.25
0.3
0.2 0.2
0.15 0.1
0.1 0.05 0
0
1
2
3
4
5
0
6 7 8 9 number of tracks per event
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 number of clusters per event
ADC counts
Figure 17: Distribution of telescope tracks vs normalised number of entries (left). Beetle clusters vs normalised number of entries (right).
700 Added_ClADCWithTrChip0_90_600 Nent = 14170
600 500 400 300 200 100 0 100
120
140
160
180
200
220
240
260
280 300 time [ns]
Figure 18: Cluster signal with tracking for an optimal bias setting (63/346); ADC counts vs TDC time. Positive pulseshapes are plotted in the telescope analysis. of chip 0 is better than that of chip 1 (except for the Scluster /Nstrip of the 63/346 bias setting). Comparing the S/N without and with system noise correction shows that the system noise disguises the real performance of the separate chips. The S/N of the two bias settings is compatible, as one expects, because of the minor difference in the corresponding preamp bias currents. 4.4.4 Charge collection. The nearest strip to the intercept of a VELO track with the BeetlePhi detector is here considered as a cluster seed. The signals of the cluster seed and its two neighbours are added to measure the charge collected with the BeetlePhi detector, see the results for both chips in Fig. 23. The cluster signals with a bias setting I pre /Isha = 346/63 µA are 117.4 and 109.7 ADC counts for chip 0 and chip 1 respectively. Compared with the preceding tracking method described in section 4.4.2 of which the results are presented in Table 7, it degrades slightly the analysis of the S/N.
16
MP ADC
30 Added_ClADCWithTr_50_500_landau_17 Nent = 245
25
Mean = 139.4
120
100
RMS = 47.46 Chi2 / ndf = 177.2 / 176
20
= 30.93± 1.316
const
80
m.p.ADC = 120.6± 1.392
15
= 15.67± 0.704
width
60
10
40
5
20
0
50
100
150
200
250
300
350 ADC counts
120
140
160
180
200
220 time [ns]
30
MP ADC
Figure 19: Single strip pulse height for the time slice with Figure 20: MP pulse height values of Landau fit to single maximum signal of both chips (36/320 setting). The MP strip clusters of both chips (36/320 setting) vs TDC times. value is 106.3 ADC counts.
Added_MaxClADCWithTr_50_500_landau_17 Nent = 216 Mean = 120.8
25
RMS = 44.47
100
Chi2 / ndf = 159.4 / 166 const
= 29.72 ± 1.225
80
m.p.ADC = 106.3 ± 1.149
20
width
= 16.13 ± 0.6199
60
15
10
40
5
20
0
50
100
150
200
250
300
350 ADC counts
120
140
160
180
200
220 time [ns]
Figure 21: Cluster pulse height for the time slice with Figure 22: MP pulse height values of Landau fit to clusmaximum signal of both chips (36/320 setting). The MP ters of both chips (36/320 setting) vs TDC time. value is 120.6 ADC counts.
17
Table 7: Telescope analysis S/N numbers (without and with correction for system noise) for the Beetle1.1 in the X7 testbeam. bias setting Isha /I pre [µA] chip Sstrip /Nstrip Scluster /Nstrip Sstrip /Nstrip Scluster /Nstrip (corrected) (corrected) 63/346 0 109.9/10.0=11.0 120.0/10.0=12.0 109.9/8.0=13.7 120.0/8.0=15.0 63/346 1 94.5/8.7=10.9 116.6/8.7=13.4 94.5/7.4=12.8 116.6/7.4=15.8 63/346 0+1 101.4/9.3=10.9 118.6/9.3=12.8 101.4/7.7=13.2 118.6/7.7=15.4 0 1 0+1
113.7/10.2=11.2 97.1/8.8=11.0 106.3/9.5=11.2
126.6/10.2=12.4 115.1/8.8=13.1 120.6/9.5=12.7
100
ADC counts
ADC counts
36/320 36/320 36/320
0
-100
126.6/8.2=15.4 115.1/7.8=14.8 120.6/8.0=15.1
100 0
-100
-200
-200
-300
-300
-400
-400
-500
-500
-600
-600
100
113.7/8.2=13.9 97.1/7.8=12.5 106.3/8.0=13.3
120
140
160
180
200
220
240
260
280 300 time [ns]
100
120
140
160
180
200
220
240
260
280 300 time [ns]
Figure 23: Charge collection for chip 0 (left) and chip 1; summing the signal of 3 strips near the intercept point of an extrapolated telescope track (bias settings 63/346 )
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5 Conclusions We tested two Beetle1.1 chips bonded to a 70◦ LHCb VELO prototype φ-detector in a CERN testbeam with 100 GeV pions. The data were taken in October 2001 together with the VELO telescope. Two different analysis methods were used to determine the S/N of the Beetle1.1. Tracks found in the VELO telescope were used to select strips in the Beetle detector where signals should be expected. Also stand-alone data (without VELO telescope information) from the Beetle1.1 chips were analysed. Here events were selected of which the signals were well above the noise in the 25 ns wide interval around the signal peak. The other samples were used to follow the time development of the signal. Both analysis methods show similar results for the performance of the Beetle chip. Rise time values4 of 18-22 ns were found for different settings of the preamplifier stage in the Beetle1.1 chip. A problem was found in the calibration and the range of the DACs. This problem is fixed in the Beetle1.2. The fall time of the pulse is important for signal spillover to the next bunch crossing in the LHC. Spillover ratios of 19-26 % were measured for different bias settings of the Beetle1.1. These values should be corrected for an effect that we discovered afterwards; the so-called ’sticky charge’ effect. A charge residue from signals in the preceding readout can cause a serious underestimation of the spillover ratio up to 15-20 % in absolute value. Most of this problem is solved in the Beetle1.2. The entire signal distribution is used to fit a Landau function. The noise was corrected for the system noise of the DAQ system, introduced by the additional amplification of the Beetle1.1 outputs. This system noise contributed about one quarter to the total noise. After system noise correction a S cluster /Nstrip ratio of 14-16 (depending on the length of the read out silicon strips) was measured for the Beetle1.1 chip bonded to a LHCb Phi detector. The signal shape and signal to noise ratio can be optimised by means of the front-end settings. In August 2002 we tested 16 Beetle1.1 chips bonded to a 180 ◦ LHCb VELO prototype R-detector. This test will provide more details of the Beetle1.1 chips and the effect of the use of a single hybrid with 16 chips. The obtained results are important for the final choice of the appropriate front-end chip for the VELO detector of LHCb. Acknowledgements: We like to thank Daniel Baumeister and Sven Lo¨ chner for Beetle support, Raymond Frei for the pitch adapter and the CERN VELO group for support during the testbeam period.
References [1] N. van Bakel et al., The beetle reference manual, v1.0, LHCb 2001-046 Electronics, 2001. [2] http://www.nikhef.nl/pub/experiments/bfys/lhcb/vertex/index.html [3] N. van Bakel et al., Investigation of characteristics and radiation hardness of the Beetle1.0 Frontend chip, revised LHCb 2001-037 VELO, 2001. [4] http://wwwasic.kip.uni-heidelberg.de/lhcb [5] Chris Parkes, Detector geometry vertex locator test-beam software description, LHCb 2000-096 (Version 2) VELO, 2001 [6] V. Wright et al., Study of resolution of VELO testbeam telescope, LHCb-note 2000-103, VELO, 2001 [7] http://lhcbproject.web.cern.ch/lhcbproject/velo/testbeam/Welcome.html -> online description of Veloroot software [8] Chris Parkes, Track fit - Vertex locator test-beam software description, LHCb 2001-038 VELO, 2001. 4 Defined
as the time it takes the signal to rise from 10% to 90% of the peak amplitude.
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[9] J. Buytaert, 40 Ms/s Analog transmission on Shielded Twisted Pair cable, LHCb 1998-032 TRAC, 1998. [10] M. Charles et al. The performance of the SCT128A ASIC when reading out irradiated and non-irradiated VELO prototype detectors, LHCb 2001-041 VELO, 2001.
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