New approach to CPC design
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New approach to CPC design. It is my 1-st presentation from LNF/INFN (Frascati ). The scientific work is still under development and the new ideas have to be tested…. Triggered by P.Campana Why results on cross-talks obtained with small chamber in May 02 are somehow better

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New approach to CPC design

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New approach to cpc design

New approach to CPC design

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

It is my 1-st presentation

from LNF/INFN (Frascati)

The scientific work is still under development

and the new ideas have to be tested…

Triggered by P.Campana

Why results on cross-talks obtained with

small chamber in May 02 are somehow better

than with the large M0 prototype tested in October 02?

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

I’d like to add:

?

?

  • 2 problems observed in beamtests

  • have to be explained and suppressed:

  • rather high cross-talks from wires to pads;

  • - double and multiple TDC spectra

Problems mentioned here were observed already

in M2R1 and other prototypes built at CERN,

as well as in Ferrara’s prototypes;

50% crosstalks observed in some conditions in M3R3

in October 2002 beam-tests at operational HV

What the reason?

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

Cross-talks

?

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

Cross-talks

along the wires (longitudal)

perpendicular to wires (transverse)

Longitudal crosstalks

are less studied

and much less suppressed

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

According to specification (LHCb 2000-061, W.Riegler)

Cross-capacitance Cwp will be increased with pad size, as shown:

LNF M0

Cwp=0.475hw (pF) where h, w are pad height and width (cm)

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

Let’s measure:

(very good agreement with table shown above)

Cwp measured in large M3R3 is absolutely similar

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

The original idea was following:

Cross-talks from wire strips to pads (longitudal) will be reduced with grounding wire strips through HV-capacitors

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

RC-model (Rwire =90 Ohm/m)

Only capacitive coupling

is taken into account

in this model

Assumed that

width of wire strip

is equal to cathode pad

Wire strip with 4 wires

Signal from particle

Cathode

pad

current

source

HV-capacitor

grounded on one side,

as shown

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

Wire strip as a transmission line, i.e. LC-model

(first proposed by LNF group)

Cathode

pad

current

source

Each wire can be considered as a transmission line

Wires in strip are connected in parallel:

L reduced, C increased (product LC is the same)

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

Recently it has been found:

the wire strip is ringing

(response of wire strip in small LNF prototype made with injector)

HV-capacitor 680pF directly grounded

f=95 MHz

The ringing frequency depends on inductance in series

to HV-capacitor (what the reason?)

f=60 MHz

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

Cross-talk profile from wires to cathode pads in the large M3R3 prototype

Volt. step on strip

(strip is floating)

20ns/div

Pad-1 (15%)

Ringing 18 ns

Central Pad

Ratio 2-nd/1-st peak 20%

Pad+1 (20%)

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

Fine waveform structure at voltage rise time 1.5ns:

One can see 4 ns oscillation

High frequency is due to LC of the transmision line itself

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

Response of wire strip in small LNF prototype

made with Current Injector

Ringing 13ns

Strip is grounded through 680pF,

with adding inductance

period is increased

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

Equivalent circuit

1.Inductance blocks

HV-capacitor effect

Wire strip

Terminated end

Cathode pad

High cross-talk:

Compare to ideal case:

2.Another parasite effect

Assumed

But!!!

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

What LC-model shows?

with HV-capacitors grounded

at inductance 3nH in series (perhaps, can be acheaved)

1-side strip termination with 0 Ohm

2-side (peak less factor 2)

Peak=1uA

Ringing= 8ns

Peak=0.5uA

Ringing= 3ns

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

Stray inductance (printed traces) in series

to HV-capacitors and full capacitance of the wire strip

mainly specify the ringing frequency

Green – 3nH

Red – 300nH (can be if width of traces 0.25mm, see M3R3)

1-side strip termination with 0 Ohm

2-side termination

Green peak=1uA

Red peak=1uA

Ringing=30ns

Green peak=0.5uA

Red peak=1uA

Ringing=20ns

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

Correct strip termination with 377Ohm

can not be used

1- signal at

far end

to capacitor

Ohm

2-side termination

through 680pF

At 1-side termination

amplitude will depend

on signal position along

the strip (see next slides)

2- middle

3- near end

No ringing, waveforms are independed to position of the

signal source, but the highest crosstalks will be in this case

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

Inductance in series to HV-capacitors

(1-side termination with 680pF and 50 Ohm)

Same schematics

No ringing

Peak independ on Lstray due to 50 Ohm

, but not enough cross-talk attenuation

3

1

2

Green=3nH

Red=100nH

Scale +/-2uA

10ns/div

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

Inductance in series to HV-capacitors

(1-side termination with 680pF and 0 Ohm)

Same schematics

Better attenuation, but ringing at R=0

1

2

3

Scale +/-2uA

10ns/div

Green=2nH/Ringing 200MHz

Red=100nH/Ringing 70MHz

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

Inductance in series to HV-capacitors

(2-side termination with 680pF and 20 Ohm)

Same schematics

Good cross-talk attenuation factor at 20 Ohm

High inductance leads to ringing even at R=20 Ohm

and drastically reduces cross-talk attenuation

3

1

2

Red=100nH/Ringing 100MHz

Green=3nH/No ringing

Scale +/-2uA

10ns/div

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

Inductance of printed trace (example):

if

w=0.25mm trace width (in M3R3 prototype)

h=1.5mm pcb thickness

l=3-10cm length of trace (in M3R3 prototype)

then L=100-1000nH

if

w=1.2cm

then L=5nH can be achieved

@ C=100pF

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

Ringing on wire strip can double/multiple signals

and TDC spectra

Threshold defined experimentally:

for wire readout –7fC

for cathode (single) – 5fC

FEE noise 50e/pF is not the first reason for threshold choice,

mainly cross-talks define threshold, at efficiency 95%/gap

Dynamic range of signals in CPC is large (100)

Average signal 50fC

So, high probability for after-pulsing can be found

at bad wire strip termination and imperfect layout in CPC

at any frequency of ringing (it depends on design)

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

If Cstrip=Cwire is high?

+/-2uA

Excellent result

+/-0.2uA

No ringing and high cross-talk attenuation

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

!

Conclusion

CPC design can be improved following the way:

No ringing must be on the wire strips at perfect design

1.No wire segmentation is needed in CPC design -

one of theeffective way

No HV-capacitors and resistorrs (cheaper and much easy design), wires are connected to one HV-resistor.

Minimisation of the trace inductances has to be done in the Combined readout chambers, which dumps effect of low impedance.

2.Double Cathode Readout scheme,perhaps, can be used in some cases below M3R3

also effective way at large Cwires(it allows increase threshold at fixed HV)

already tested in M1R1 with excelent results (because very low Cwp and good attenuation of the cross-talks from wires)

Happy end ?

A.P.Kashchuk (LNF/INFN), Frascati)


New approach to cpc design

Summary

Voltage zero must be on the wire strips at perfect CPC design, i.e.

LNF M0

?

Single CRO. No wire segmentation

Double Cathode readout (CRO) below M3R3. No wire segmentation

A.P.Kashchuk (LNF/INFN), Frascati)


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