Cables (/connectors)

1
Cables (/connectors)

• Signal inventory
• signal types / characteristics
• extra signals of pile-up
• Types of cables
• twisted pair vs. striplines
• Cable test
• Other (open) issues
• grounding shielding
• buffering on flange

2
air
vacuum
Scta beetle
Optional buffer
Receiver in racks
unshielded
shielded
1 .. 2 meter
10 .. 15 meter
3
Signal inventory

• Low voltage power lines
• Analog outputs
• Digital control (lvds / cmos)
• Digital outputs (lvds, pile-up only)
• Other signals
• detector bias
• temperature sensors

4
Low voltage power

• Simple calculation
• 1/2 wheel 16 frontend chips
• 16 500 mW 8 Watts
• If Vdd 2.5 volts gt I 3.2 Amp
• Copper area needed gt 1 mm2
• cooling?
• 1 meter 170 mW

5
Analog output signals

• Balanced output pair
• few hundred millivolts for 10 Mip signal
• Bandwidth gtgt 40 MHz
• sensitive to pickup
• 4 pairs per frontend chip

6
Digital control lines

• LVDS (low voltage differential signalling)
• clock, trigger, reset
• frequency 40 MHz
• line termination needed
• clock / trigger per 4 / 8 / (16) chips
• CMOS-level
• I2C interface
• low frequency
• line termination optional (series termination)
• one set of signals for 1/2 wheel?

7
LVDS
Driver 250 mV into 100 ohms
Receiver 100 mV differential
Maximum cable attenuation 8 dB (Z0 100 ohms)
8
Digital outputs

• Pile-up comparator outputs
• LVDS _at_ 80 Mbit
• 16 pairs per frontend chip, 256 for 1/2 wheel
• extra connectors on hybrid needed

9
Other signals

• Bias for detector
• high voltage, upto 500 Volts ?
• low current
• Temperature sensors
• specification depends on sensor type
• no-high speed, low current
• 1 sensor for 1/2 wheel

10
Summary
11
Cable requirements

• vacuum compatible
• radiation hard
• moderate signal loss
• flexible
• thin
• characteristic impedance of 100 ohms
• polyimide (kapton) isolation

12
Cable specifications

• Cable impedance
• geometry
• dielectric constant of isolation ( 3)
• Loss
• DC resisitive loss
• losses due to skin effect, proximity effect
• dielectric loss
• Dispersion

13
Cable losses
Wire resistance
1.7 ? per meter for 0.01 mm2
10 ?m _at_ 40 MHz
Skin depth
Dielectric loss
low for good dielectrics
dispersion
14
Example

• Diameter 0.1 mm
• Rdc 1.3 ? / m
• skin loss _at_ 40 Mhz adds a factor 2.5
• 20 signal loss for 2 meter
• same copper area for stripline (35 ?m Cu)
• width 0.225 mm
• skin loss _at_ 40 Mhz adds a factor 1.5
• 13 signal loss for 2 meter

15
Impedance of cable
d
Twisted pair
D
For kapton Z0 100 ohms if D 2d
Microstrip line
w
s
w
See figure
h
16
Grounding /shielding

• Questions
• Central (star) ground, where
• shielded cables upto flange
• vacuum cables run close to machine ground

17
Buffering

• Rad-hard components
• cooling problem