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High-Speed Cable
High-Speed Cable Assembly System
Electrical Definitions and Options
Performance Issues
Selection of the right cable assembly for the right application can be tricky. Many factors can determine the cable assembly performance.
Often times, the enhancement of one parameter will degrade another.
This section will help to define many of these pertinent issues to effectively choose the right high speed cable assembly for your application.
I. Performance Parameters
A. Definition of applicable cable assembly performance parameters.
B. Issues that affect these parameters.
C. Tactics used to maintain parameters within acceptable limits.
1. Attenuation:
A. Attenuation is the decrease in magnitude of a signal as it travels through any transmitting medium. It is a ratio measured in dB of signal in to signal out. It accounts for absorptive, not reflective losses through the medium and always gets worse with increasing frequency.
B. Length, conductor size, shield, stranded vs solid, dielectric material loss.
C. Keep cable length to a minimum. Use the largest conductor you can afford, and shield if in close proximity to EMI interference. Choose cables with lower dielectric constants.
2. Impedance:
A. Impedance is defined two ways; single ended and differentially. For single ended, this is defined as the impedance between two conductors when they are driven such that one conductor is ground and the other is a signal. Balanced or differential impedance is between two conductors when driven differentially, that is one conductor at +V and the other at -V.
B. Z = square root of L/C
L = Inductance C = Capacitance
C. Use methods to assure minimum impedance mismatch between the connector system and cable.
3. Propagation Delay:
A. Propagation delay is the time it takes for a signal to propagate from one specified point to a second specified point in a system. In other words, it is how fast the system will allow a signal to travel.
B. PropDelay = square root of LC
C. Faster prop delays can be obtained by using cables with lower dielectric constants.
4. Skew:
A. Skew is the difference in propagation delay between any two pairs within the same cable sheath. This is important because errors may result from the fact that signals transmitted in sequence over different wire pairs would not necessarily arrive at the receiver in the same sequence.
B. Within pair - controlled by wire manufacturer.
Pair to Pair - largely controlled by termination method.
C. Wire manufacturers may list skew within pair. Pair to Pair skew can be controlled by assuring minimal length differences between paired wires.
5. Shielding Effectiveness:
A. Shielding effectiveness is a numerical rating in dB of a shield's ability to isolate a region in space from electromagnetic energy. SE is measured as a difference between a system with a shield versus the same system without a shield.
B. It is affected by materials used in the connector/shell system, number of grounding fingers, how well a connector and cable are sealed in the shield and outside EMI interference.
C. If application allows, choose better shields although be aware of their decreased flexibility. The sealing method of the shield and connector system are important. Equally important is how the cable shield integrates with the connector's shielding can.
6. Capacitance:
A. Capacitance is the ability of a dielectric material between conductors to store energy when a difference of potential exists between conductors. In general, the larger the distance between conductors, the lower the capacitance.
B. The larger the area of dielectric material between conductors, the lower the capacitance.
C. If capacitance is a factor that can not be compromised, choose cables with distances between conductors as large as possible for your design.
Quick Note about Rise Time (Tr):
Time required for a digital pulse to rise from 10% to 90% of its amplitude. It's important because the faster the rise time is, the more pulses will be able to fit in a given time frame. These pulses present themselves as large numbers of different frequencies. More high frequency component pulses relate to a faster rise time with a flatter peak. Rise time (Tr) can be related to attenuation by .35/Tr, where a bandwidth of 1 GHz allows a Tr of 350ps.
Electrical Definitions and Options
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• Assembly Overview • Product Performance and Capabilities • Electrical Definitions and Options • Cable Definitions and Options • Mechanical Specifications |
Performance Issues
Selection of the right cable assembly for the right application can be tricky. Many factors can determine the cable assembly performance.
Often times, the enhancement of one parameter will degrade another.
This section will help to define many of these pertinent issues to effectively choose the right high speed cable assembly for your application.
I. Performance Parameters
A. Definition of applicable cable assembly performance parameters.
B. Issues that affect these parameters.
C. Tactics used to maintain parameters within acceptable limits.
1. Attenuation:
A. Attenuation is the decrease in magnitude of a signal as it travels through any transmitting medium. It is a ratio measured in dB of signal in to signal out. It accounts for absorptive, not reflective losses through the medium and always gets worse with increasing frequency.
B. Length, conductor size, shield, stranded vs solid, dielectric material loss.
C. Keep cable length to a minimum. Use the largest conductor you can afford, and shield if in close proximity to EMI interference. Choose cables with lower dielectric constants.
2. Impedance:
A. Impedance is defined two ways; single ended and differentially. For single ended, this is defined as the impedance between two conductors when they are driven such that one conductor is ground and the other is a signal. Balanced or differential impedance is between two conductors when driven differentially, that is one conductor at +V and the other at -V.
B. Z = square root of L/C
L = Inductance C = Capacitance
C. Use methods to assure minimum impedance mismatch between the connector system and cable.
3. Propagation Delay:
A. Propagation delay is the time it takes for a signal to propagate from one specified point to a second specified point in a system. In other words, it is how fast the system will allow a signal to travel.
B. PropDelay = square root of LC
C. Faster prop delays can be obtained by using cables with lower dielectric constants.
4. Skew:
A. Skew is the difference in propagation delay between any two pairs within the same cable sheath. This is important because errors may result from the fact that signals transmitted in sequence over different wire pairs would not necessarily arrive at the receiver in the same sequence.
B. Within pair - controlled by wire manufacturer.
Pair to Pair - largely controlled by termination method.
C. Wire manufacturers may list skew within pair. Pair to Pair skew can be controlled by assuring minimal length differences between paired wires.
5. Shielding Effectiveness:
A. Shielding effectiveness is a numerical rating in dB of a shield's ability to isolate a region in space from electromagnetic energy. SE is measured as a difference between a system with a shield versus the same system without a shield.
B. It is affected by materials used in the connector/shell system, number of grounding fingers, how well a connector and cable are sealed in the shield and outside EMI interference.
C. If application allows, choose better shields although be aware of their decreased flexibility. The sealing method of the shield and connector system are important. Equally important is how the cable shield integrates with the connector's shielding can.
6. Capacitance:
A. Capacitance is the ability of a dielectric material between conductors to store energy when a difference of potential exists between conductors. In general, the larger the distance between conductors, the lower the capacitance.
B. The larger the area of dielectric material between conductors, the lower the capacitance.
C. If capacitance is a factor that can not be compromised, choose cables with distances between conductors as large as possible for your design.
Quick Note about Rise Time (Tr):
Time required for a digital pulse to rise from 10% to 90% of its amplitude. It's important because the faster the rise time is, the more pulses will be able to fit in a given time frame. These pulses present themselves as large numbers of different frequencies. More high frequency component pulses relate to a faster rise time with a flatter peak. Rise time (Tr) can be related to attenuation by .35/Tr, where a bandwidth of 1 GHz allows a Tr of 350ps.