----- Original Message -----
From: "Mike Sloane" <[EMAIL PROTECTED]>
I didn't catch the first message from Mr. Jones, but it is completely
wrong.
Hmm... this is the problem with a forum where we have experts and novices in
various areas examining the same issue. When I try to simplify a response
to "Cable Quality makes no difference", I get assaulted with suedo-technical
details.
The short answer is... cable quality DOES make a differnence. The long
version from Adaptec comes from the design of SCSI U320 cables and can be
seen at and partially reproduced below:
http://www.adaptec.com/en-US/products/cables/_education/cables_u320.htm
Adaptec's primary concern is the reflection of the wave when it reaches the
end of a cable at higher frequencies.
(And yes, I agree, Monster Cables for your home stereo is wasted money!)
I have been unable to find supporting links to show that the higher voltage
digital '1' travels faster than the lower voltage digital '0' through copper
wire.
Characteristic impedance
Characteristic impedance is the combined effect of resistance, conductance,
inductance and capacitance in a transmission line.
The characteristic impedance of cables must match the impedance of the
transmitting and receiving circuits. Otherwise, reflections will occur,
causing signal loss and distortion.
Effects on impedance
Impedance is affected by conductor size, insulation material, insulation
thickness, shield proximity and frequency.
Capacitance
Capacitance is the property of an electric circuit that opposes any change
in voltage. Capacitance distorts the signal as it passes through the
transmission line. The lowest possible capacitance is preferred. Capacitance
is frequency dependent.
Effects on capacitance
Many variables in the cable design affect capacitance including insulation
materials and thickness.
Dielectric
The dielectric is the material between conductors in a cable. A dielectric
material is a substance that is a poor conductor of electricity, but an
efficient supporter of electrostatic fields, property that is useful in
capacitors. An important property of a dielectric is its ability to support
an electrostatic field while dissipating minimal energy in the form of heat.
The lower the dielectric loss (the proportion of energy lost as heat), the
more effective a dielectric material. And changes in the dielectric constant
of the insulating material will affect the cable characteristics.
Inductance
Inductance is the property of an electric circuit that opposes current. The
lowest possible inductance is preferred and any conductor possesses this
property with a current flowing through it. Inductance distorts the signal
as it passes through the transmission line. Inductance is frequency
dependent.
Effects on inductance
Inductance is affected by many variables in cable design, chief among them
conductor size, insulation thickness and shield proximity.
Inductor
An inductor is a passive electronic component that stores energy in the form
of a magnetic field. Cables are examples of straight wire inductors with low
inductive characteristics.
Resistance
Resistance is the opposition to the flow of current in an electric circuit.
All metallic conductors have resistance, and the lowest possible resistance
is desired. Resistance wastes transmission line energy in the form of heat.
Skin effect also affects resistance. At higher frequencies, skin effect
increases, causing the signal to concentrate at the outer edge of the
conductor.
Effects on resistance
Many variables in cable design affect resistance, including conductor size,
conductor material, plating material, temperature, and length impact
resistance.
Attenuation or insertion loss
Attenuation, usually measured in dB/ft, is a natural consequence of signal
transmission over long distances. It causes the signal to shrink or shorten
in amplitude, where the wasted signal is lost in the form of heat or
reflections. Attenuation occurs with any type of signal. The lowest possible
attenuation is preferred.
Effects on attenuation
Attenuation is affected by conductor size, conductor/plating material,
insulation material, impedance, and frequency.
Crosstalk
Crosstalk is the effect of a signal traveling in one cable component on the
signal of another cable component. It is a major contributor to noise for
cables. Noise from the signals will affect other signals, producing skew
the difference between two signals at the same location.
Crosstalk is a disturbance caused by the electric or magnetic fields of one
signal on another. The phenomenon that causes crosstalk is called
electromagnetic interference (EMI).
Effects on crosstalk
Cable characteristics such as conductor type, twisting, shielding and
frequency affect crosstalk.
Attenuation to crosstalk Ratio
Attenuation-to-crosstalk ratio (ACR) is the difference, measured in
decibels, between the signal attenuation produced by a wire or cable
transmission medium and the far end crosstalk.
For a signal to be received with an acceptable bit error rate, the
attenuation and the crosstalk must both be minimized. In practice, the
attenuation depends on the length and gauge of the wire or cable
transmission medium, and is a fixed quantity. However, crosstalk can be
reduced by ensuring that twisted-pair wiring is tightly twisted but not
crushed, and by ensuring that connectors between wire and cable media are
properly rated and installed.
The ACR is a quantitative indicator of how much stronger the attenuated
signal is than the crosstalk at the destination (receiving) end of a
communications circuit. The ACR figure must be at least several decibels for
proper performance. If the ACR is not large enough, errors will be frequent.
In many cases, even a small improvement in ACR can cause a dramatic
reduction in the bit error rate.
Time and propagation delay
Associated with skew, time delay is the interval required for a signal to
pass through a transmission line. Skew is the difference between the
earliest and latest signals arrival time at the target or initiator. A
cables propagation delay measures the travel time of a signal down the
transmission line.
Effects on time and propagation delay
Time delay and skew are affected by the insulation material and twisting of
conductors. Unequal wire length in pairs due to twisting, deformation of
dielectric due to twisting, variance in the dielectric constant, air gaps
between conductor and dielectric, unequal pair lengths due to cabling also
adversely impact skew. The likelihood of data clocking errors rises as time
delay and skew increase.
Propagation delay is affected by inductance and capacitance. The velocity
factor measures signal speed as a percentage of the speed of light. The
higher the velocity factor, the lower the propagation delay.
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