1055497-1 Datasheet, PDF (274/320 Pages) Tyco Electronics – RF Coax Products

RF Coax Connectors
Appendix A - Theory and Application (Continued)
Fast rise times and short pulse widths require high fre-
quency components.
Two frequent causes of digital signal degradation can be
(1) high capacitance of the transmission line and (2) impe-
dence mismatches of connector transmission line or I/0
devices. Selection of an impedance-matched connector
on a digital line, especially if short cable assemblies are
used, can be as important as connector selection for an
RF modulated line. Reflected pulses out of phase with
the original pulse can cause false signals or high error
rates in a digital system.
Since pulses with fast rise times are necessary in high-
speed computers, any circuit element which could reduce
or attenuate high frequency response is undesirable.
Signal Integrity and Propagation
To explain how to maintain signal integrity, it is necessary
to review how the signal is configured in a cable and how
it propagates. Ignoring digital signals for this discussion
we will identify the issues that deal with the integrity of a
sine wave. Consider a coaxial cable consisting of an inner
conductor surrounded by a dielectric material and then an
outer conductor (See Figure 13). The outer conductor may
be a braid, a foil, or a solid metal.
Outer
Conductor
A
S3=1+3
1
3
S3
B
S5=1+3+5
d
D
Dielectric
Material
Center
Conductor
Figure 13
Diagram of a Cable
An electromagnetic wave traveling in a coaxial cable pro-
duces an electric and a magnetic field between the inner
conductor and the outer conductor (Figure 14). The electric
(E field) is radial and varies in time. An alternating current
flows along the inner conductor and the outer conductor. An
oscillating magnetic field (H field) circles the inner conductor.
E
H
5
V
S5
C
S7=1+3+5+7
7
Figure 12
Development of a Square Wave
Figure 14
Electric field (E) and magnetic field (H)
belonging to the principal mode in a coaxial line.
The alternating current on a conductor is not spread
throughout the conductor but is strongest at the surface
and decays exponentially at points further into the conduc-
tor. This is called the skin effect. At a frequency of 1MHz,
three skin depths is 0.0078” (95% of the current is within
three skin depths of the surface) and at 10GHz three skin
depths is 0.00078”. As a result, the current is on the outer
surface of the inner conductor and the inner surface of the
outer conductor over the entire range of interest for most
RF systems. The dimensions and material beyond several
skin depths have no effect on the wave; gold plated plastic
will propagate as well as gold plated copper at sufficiently
high frequencies.
274
Catalog 1307191
Revised 3-07
www.tycoelectronics.com
Dimensions are in millimeters
and inches unless otherwise
specified. Values in brackets
are standard equivalents.
Dimensions are shown for
reference purposes only.
Specifications subject
to change.
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