1. A systematic study of cross-talk limitations in RPC timing
D. Gonzalez-Diaz, A. Berezutsky and M. Ciobanu
for the CBM-TOF working group

2. 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.

3. Basics (RPC rise-time)
avalanche growth until onset of Space-Charge
if Space-Charge present
already at the level of
the comparator, S would be
even higher !
D.Gonzalez-Diaz, PhD(2006)
P. Fonte et al., IEEE,
Trans. Nucl. Sci. 49,3(2002)881
also
measured with UV source and
double-threshold technique

4. Basics (RPC rise-time)
E=100kV/cm
FT(V)
time domain
frequency domain

5. Basics (Q distribution and usual threshold)
not including streamers
D.Gonzalez-Diaz, PhD(2006)
measured directly in the scope
HADES 2003 prototype

6. Basics (fluctuations in time response)
independent sources!
We believe we can make
σsing;e event~80 ps for large
systems (CBM-Techincal Status Report 2005) ?

7. Cross-talk influence in timing (simple derivation)
space-charge
log(Vsignal)
Vth
exponential regime t
variations in base-line due to cross-talk
variations in time at threshold due to cross-talk

8. 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.

9. Cross-talk influence in timing (simple derivation)

10. Cross-talk influence in timing (simple derivation)
cross-talk: F=10%
subtle cross-talk
(below threshold)
rough cross-talk
(above threshold)
events above threshold
in neighbour: 20%

11. Measurements of cross-talk with RPC mockup

12. Different ways of shielding
shielding vias
shielding strips

13. Comparison with simulation
signal injected from a fast scintillator trise~1ns

14. Cross-talk from cell without shielding

15. Cross-talk from cell with shielding strips

16. Cross-talk from cell with shielding vias

17. Cross-talk from cell with shielding vias (neighbour not terminated on the other side!)

18. Cross-talk from cell with shielding vias (to 1st and 2nd neighbour!)

19. simulation of the S coefficient
scattering matrix coefficient to neighbouring
anode (equivalently: fraction of signal transmitted)

20. Comparison with data from spectrum analyzer
preliminary!

21. simulation of a realistic structure
RPC structure: signal width = 22 cm, gap to next strip = 0.3 cm
16 gaps,
0.16 mm gap
0.3 mm glass
0.86 mm PCB
propagation of exponential signal with 200 ps rise-time in anode and cathode simultaneously (differential mode)

22. Transmission properties (with vias)

23. Transmission properties (with vias)

24. Transmission properties (with vias)

25. Transmission properties (with vias)

26. Transmission properties (with vias)

27. Transmission properties (with vias)

28. Transmission properties (with vias)

29. Transmission properties (with vias)

30. Transmission properties
vias no
vias

31. Transmission properties
vias no
vias

32. Transmission properties
vias no
vias

33. Transmission properties
vias no
vias

34. Transmission properties
vias no
vias

35. Transmission properties
vias no
vias

36. Transmission properties
vias no
vias

37. Transmission properties
vias no
vias

38. all in a nut-shell

39. Dependence with strip length

40. Dependence with strip length

41. Dependence with strip length

42. Dependence with strip length

43. 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.

44. 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.

45. 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).

46. suggestion: place the FEE inside the detector,
Conclusions (III)
suggestion: place the FEE inside the detector,
reduce its input impedance and eliminate
the problem of signal transmission in cables!