A systematic study of cross-talk limitations in
RPC timingD. Gonzalez-Diaz, A. Berezutsky and M. Ciobanu
for the CBM-TOF working group
15-10-2008
Index
1. Cross-talk and timing. General remarks.
2. Measurements and comparison with FEM (Finite Element Method) description.
3. Design studies for operation in differential mode.
4. Conclusions.
Basics (RPC rise-time)
D.Gonzalez-Diaz, PhD(2006)
measured with UV source and double-threshold technique
tSoeii avalanche growth until onset of Space-
Charge
P. Fonte et al., IEEE, Trans. Nucl. Sci. 49,3(2002)881
if Space-Charge present already at the level of the comparator, S would beeven higher !
also
Basics (RPC rise-time)
ps2009ln
S(E)
)(trise E=100kV/cm
time domain
GHzt
frise
c 75.135.0
frequency domaincmfc cc 17
FT
(V)
Basics (Q distribution and usual threshold)
HADES 2003 prototype
D.Gonzalez-Diaz, PhD(2006)%10/ signalthreshold VV
2.1/
/
promptq
signalV
qrms
Vrmssignal
not including streamers
measured directly in the scope
Basics (fluctuations in time response)
2talkcross
2noiseexternal
2noiseintrinsic
2propag
2RPC
2T,total
independent sources!
2talkcross
2event single
2T,total
We believe we can make
σsing;e event~80 ps for large
systems (CBM-Techincal Status Report 2005)
?
Cross-talk influence in timing (simple
derivation)
log(Vsignal)
t
Vth
variations in base-line due to cross-talk
variations in time at threshold due to cross-talk2
talkcross
space-charge
exponential regime
Cross-talk influence in timing (simple
derivation)Assumptions: Within the same primary collision cross-talk extends up-to infinite time. It does not depend on position. Fluctuations in time of cross-talk signal are smaller than fluctuations coming from the avalanche charge distribution. Pick-up strips are separated by a typical distance bigger than the area of influence of the avalanche. Charge sharing during induction can be neglected!. Cross-talk influences only the first neighbour, coupling to it with a fraction of its amplitude F. Cross-talk is small.
S
F
V
rms
V
Vrms
signal
V
threshold
signal signal
talkcross
Cross-talk influence in timing (simple
derivation)
eventsingletalkcross rmsrms
SrmsV
rms
V
VF
signal
V
signal
threshold )( eventsingle
1
psrms 80event-single 1.0F
Cross-talk influence in timing (simple
derivation)
psrms
psrms
MCtalkcross
analyticaltalkcross
87
60
events above threshold in neighbour: 20%
cross-talk: F=10%
subtle cross-talk(below threshold)
rough cross-talk(above threshold)
Measurements of cross-talk with RPC mockup
Different ways of shielding
shielding viasshielding strips
Comparison with
simulationsignal injected from a fast scintillator trise~1ns
Cross-talk from cell without shielding
Cross-talk from cell with shielding strips
Cross-talk from cell with shielding vias
Cross-talk from cell with shielding vias(neighbour not terminated on the other side!)
Cross-talk from cell with shielding vias(to 1st and 2nd neighbour!)
simulation of the S coefficient
scattering matrix coefficient to neighbouring anode (equivalently: fraction of signal transmitted)
Comparison with data from spectrum analyzer
preliminary!
simulation of a realistic structure
propagation of exponential signal with 200 ps rise-time in anode and cathode simultaneously (differential mode)
RPC structure: signal width = 22 cm, gap to next strip = 0.3 cm
16 gaps,0.16 mm gap0.3 mm glass0.86 mm PCB
Transmission properties (with vias)
Transmission properties (with vias)
Transmission properties (with vias)
Transmission properties (with vias)
Transmission properties (with vias)
Transmission properties (with vias)
Transmission properties (with vias)
Transmission properties (with vias)
Transmission properties
vias
no vias
Transmission properties
vias
no vias
Transmission properties
vias
no vias
Transmission properties
vias
no vias
Transmission properties
vias
no vias
Transmission properties
vias
no vias
Transmission properties
vias
no vias
Transmission properties
vias
no vias
all in a nut-shell
Dependence with strip length
Dependence with strip length
Dependence with strip length
Dependence with strip length
Conclusions (I)
Cross-talk levels at the level of 10% are capable of destroying timing RPC multi-hit performances.
Cross-talk influence depends critically on the ratio Vth/<Vsignal> and the
signal rise-time. For timing RPCs it corresponds to a 'corner frequency' of 1.75 GHz at least (-3dB drop).
Aplac is a good tool for understanding fast signal propagation. For the measured signals the quality of our groundings is well described
by 'ideal grounding' in APLAC. Detector box affects critically to signal properties. Without a proper
description the simulation fails by factors.
Outlook
Elaborate the results a bit further to establish sound requirements. Use this information to build a prototype. Measure cross-talk in a realistic prototype. Measure experimentally the charge-sharing during induction.
Conclusions (II)
Impedance matching does not help at all from the point of view of cross-talk, however it may allow for desired double-hit capabilities.
Detector matching is extremely complicated and does not fit at all to the required granularities. If impedance matching is requested, the granularity at the outer regions would require strips of 4 mm wide and 6 meters long.
How to get a matched detector?? (first of all, the resulting multi-strip configuration is only truly matched after ALL the corresponding cross-impedances are matched):
Build the detector with the impedance of the FEE. Build the FEE and cables with the impedance of the detector. Build the detector and the FEE and match the impedance with some
driver. Do not care about double-hit capabilities in the same strip (HADES).
Conclusions (III)
suggestion: place the FEE inside the detector,
reduce its input impedance and eliminate
the problem of signal transmission in cables!