HIGHSPEED DIGITAL DESIGN CHAPTER 10 Ribbon Cables

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HIGH-SPEEDDIGITAL DESIGNCHAPTER 10Ribbon
Cables
2002. 11. 4. Control Systems Laboratory Kim Han
Me E-mail hanme_at_hyowon.pusan.ac.kr Homepage
http//www.shinbiro.com/ctchen
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Contents

• Introduction
• 10.1 RIBBON CABLE SIGNAL PROPERGATION
• 10.1.1 Ribbon Cable Frequency Response
• 10.1.2 Ribbon Cable Rise Time
• 10.1.3 Measuring Rise Time
• 10.2 RIBBON CABLE CROSSTALK
• 10.2.1 Basic Calculation of Crosstalk
• 10.2.2 Effect of Multiple Grounds
• 10.2.3 Effect of Twists
• 10.2.4 Measuring Crosstalk
• 10.2.5 Stacking Ribbon Cables

10.3 RIBBON CABLE CONNECTORS 10.3.1 Connector
Inductance 10.3.2 Connector Capacitance
10.3.3 Staggering Connections to
Reduce Parasitic Effects 10.4 RIBBON CABLES EMI
10.4.1 Flat Foil Wrap 10.4.2 Flat Shield on
One Side 10.4.3 Folded(Round) Shielded Cables
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Introduction
Advantages of Ribbon Cable
(1) Simultaneous crimping action (2) Very
Cheap (3) Uniform separation
Ribbon cables make excellent transmission lines.
Why ???
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10.1 RIBBON CABLE SIGNAL PROPAGATION
The rise time of a ribbon cable
To be able to apply to all cable.
Basic frequency response shape
Q) What follows from a shape invariance?
If we change K but then compensate by changing
the L so that the overall ratio rise time
remains fixed, the frequency response stays the
same.
The key to understanding cable attenuation.
We get the same frequency response using a long
coax or a short ribbon cable.
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10.1.1 RIBBON CABLE FREQUECY RESPONSE
Disadvantage for resistive termination
- To occur bump - To introduce attenuation at DC
Using a line receivers
Impact of Cables dielectric configuration
- To control the signal propagation velocity.
If there has Cables supporting wires on a thin,
flat plastic sheet
Low permittivity high speed.
- To control the attenuation.
Skin effect cause to go up the attenuation.
To be influenced attenuation by changing the
cables characteristic impedance.
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10.1.2 RIBBON CABLE RISE TIME
These long tails introduce significant
interference in long-distance transmission
systems.
About Conductive cable
For normal digital applications
You must keep the system clock much slower than
the 10-90 R.T. of cable.
Why???
To avoid overlap between pulses.
As the cable length grows, the rise time
stretches even further.
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10.1.2 RIBBON CABLE RISE TIME
How do you predict cables rise time?
Step 1.
- Find the value of K
In order to make a complete freq. response curve.
Step 2.
- Find the rising time.
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10.1.3 MEASURING RISE TIME
Step by Step
Terminate the cable at its far end with a
resistor.
1.

• Why???
• The source impedance must be low compared to
• the characteristic impedance of the cable.

2.
Reflection occurrence
3.
The input must be a step function with a r.t.
much shorter than the cable r.t..
Use oscilloscope and pulse generator to be faster
than cable.
4.
Use the best scope probes.
Special low-capacitance, active high-frequency
probe.
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10.2 RIBBON CABLE CROSSTALK
- Crosstalk in ribbon cables varies with the
placement of grounds among the signal
conductors.
Forward coupling
Reverse coupling
Step for the calculation of inductive reverse
coupling

• Any level of crosstalk attenuation is achievable
  
• given enough grounds.

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10.2 RIBBON CABLE CROSSTALK
The calculation of inductive reverse coupling
Step 1.
Model the magnetic field patterns emanating from
the signal wires.
Step 2.
Integrate to find the total flux captured between
the receiving wires.
Step 3.
Convert the change in flux/unit time to a voltage.
The simplest geometry example for computing
crosstalk having the four-wire
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10.2.1 BASIC CALCULATIONof CROSSTALK
Magnetic field intensity, varies as 1/X. as a
function of distance from each wire.
Magnetic field from the signal wire and its
return-current path partially cancel. The
integrated flux over any small area remote
from A B will vary as 1/X2.
The partial cancellation of fields from signal
and return wires is proportional to the
separation between them, called ?1.
The total flux captured between C, D is
proportional to the separation between them,
called ?2.
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10.2.1 BASIC CALCULATIONof CROSSTALK
The overall reverse coupling coefficient between
two wire pairs as
In a cable having many ground wires, the
coupling ratio above Equation is diminished at
least by one-half and perhaps to one-fourth of
its size.
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10.2.2 EFFECT of MULTIPLE GROUND
Effect 1. - Returning signal current always
splits among all the ground paths.
Effect 2. The amount of ground current on
nearby wires dominate Crosstalk
on any signal wire.
Ex) In a sparsely grounded cable, doubling the
number of grounds, which halves both ?1 and ?2,
cuts the crosstalk between remote wires by ¼.
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10.2.3 EFFECT of TWISTS
A unique advantage of Twisted cable
In order to cancel crosstalk, Twisted cables
must be maximumly reduce the separation ? between
them. And every signal wire in one should
have its own individually twisted ground return
wire. Communication twist cable is reduced
electromagnetic emissions. When used in
conjunction with differential transmission,
twisted cabling really shines. When it used to
differentially type, crosstalk is very low.
Remember!!!
The magnetic fields from the two wires have
opposite polarity and nearly cancel.
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10.2.4 MEASURING CROSSTALK
Initial Condition!!!
A G-S-G configuration. The driving signal
traverses wire 6, with crosstalk displayed
from wire 8. Both ends of wire 8, as well as
the far end of wire 6, terminate in the
characteristic impedance of the cable,
100(Ohm).
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10.2.4 MEASURING CROSSTALK
The near-end(reverse) crosstalk on wire 8.
The ratio of crosstalk to signal is about
2.5. This pulse length equals 22ns, twice the
one-way delay time of the cable. The one-way
delay time must therefore be 11ns.
The effective relative permittivity of the cable.
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10.2.4 MEASURING CROSSTALK
Far-end crosstalk
Reaching a maximum amplitude of 1.6, the
far-end crosstalk for this configuration
causes less trouble that near-end crosstalk. We
may measure these data lines safely after only
a short delay, because the far-end crosstalk
decays quickly. Far-end crosstalk follows the
derivative of the driving signal and so grows
without bound when we shorten the driving rise
time.
Far-end crosstalk accumulates as it propagates
down the cable. Far-end crosstalk on a shorter
cable would be much less, and on a longer
cable much more.
Near-end crosstalk stays the same amplitude
regardless of cable length but elongates in
time as the cable is stretched.
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10.2.5STACKING RIBBON CABLES
If you stack Ribbon cables
Crosstalk increases markedly when wires come
close together. Consequently,, When folding a
cable, you will observe a similar increase in
crosstalk. Therefore,, Use cable spacers.
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10.3 RIBBON CABLE CONNECTORS
Mass termination connector
Insulation displacement connectors
It form a permanent gas-tight seal.
Ribbon cable connectors always have parasitic
inductance and capacitance.
As with any other connector, the performance of
your digital signaling loop degrades because
of these parasitic effect.
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10.3.1 CONNECTOR INDUCTANCE
Estimation of the self-inductance
Where??? - a loop formed by a single signal pin
and one ground pin.
G-S-G from Figure 10.2
If multiple nearby grounds, halve this
approximation result.
Ex) R0.0125, x0.4, H0.1 L8nH
Estimation of rising time
Our 8(nH) single-pin inductance in series with a
100(Ohm) line generates a rise time of
100(ps). In a G-S-G configuration, the
degradation would be less.
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10.3.2 CONNECTOR CAPACITANCE
Equation of parameter estimation of the
parasitic capacitance
Where??? - between a single signal pin and one
ground pin.
G-S-G from Figure 10.2
Ex) r0.0125, x0.4, H0.1 C0.136nH
If multiple nearby grounds, more than double.
Estimation of rising time
When??? A capacitance of C(F), shunted across a
transmission line of Z0(Ohm)
Our 0.136(pF) single-pin capacitance, when
shunting a 100(Ohm) line, generates a rise time
of 15(ps). In a G-S-G configuration, the
degradation would be greater.
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10.3.3 STAGGERING CONNECTIONS to REDUCE
PARASITIC EFFECTS
When working at subnanosecond speeds, Parasitic
connector effects play a big role.
To reduce overall parasitic effects.
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10.4 RIBBON CABLE EMI
Ribbon cables suffer from severe EMI problems
when routed between cabinets.
Prevention method
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10.4.1 FLAT FOIL WRAP
In order to eliminate EMI
With a spiral wrap, be sure that each joint
between overlapping spiral layers makes firm
electric contact with the previous layer.
Returning current has to spiral around the signal
wires to get back to its source.
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10.4.2 FLAT SHIELD on ONE SIDE
A flat copper braid offers several advantage.
What are those???
1. The close proximity of the copper braid acts
like a ground plane, reducing crosstalk
between individual wires in the cable.
Better than foil wrap. 2. The copper braid
provides a low-inductance path for returning
signal currents.
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10.4.3 FOLDED(ROUND) SHIELDED CABLES
These cables have both the advantages of mass
termination connectors and shielding.