Автор: san
Дата: 13.01.12 02:01
Цитирую Генри Отта, вырезал по минимуму, но все равно много получилось
Раздел 2.15.2
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2.15.2.1 Low-Frequency Cable Shield Grounding.
The main reason to shield cables at low frequency is to protect them against electric field coupling primarily from 50/60-Hz power conductors.
As was discussed in Section 2.5.2, a shield provides no magnetic field protection at low frequency.
This points out the advantage of using shielded twisted pair cables at low frequency: The shield protects against the electric field coupling and the twisted pair protects against the magnetic field coupling.
Many low-frequency circuits
contain high-impedance devices that are susceptible to electric field coupling,
hence, the importance of low-frequency cable shielding.
At low frequency, shields on multiconductor cables where the shield is not the signal return conductor are often grounded at only one end.
If the shield is grounded at more than one end, then noise current may flow in the shield because of a difference in ground potential at the two ends of the cable.
This potential difference, and therefore the shield current, is usually the result of 50/60-Hz currents in the ground.
In the case of a coaxial cable, the shield current will produce a noise voltage whose magnitude is equal to the shield current times the shield resistance, as was shown in Eq. 2-33.
In the case of a
shielded twisted pair, the shield current may inductively couple unequal
voltages into the twisted pair signal conductors and be a source of noise (see
Section 4.1 on balancing).
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2.15.2.2 High-Frequency Cable Shield Grounding.
At frequencies above about 100 kHz, or where cable length exceeds one twentieth of a wavelength, it becomes necessary to ground the shield at both ends.
This is true for either multiconductor or coaxial cables.
Another problem develops at high frequency; stray capacitance tends to complete the ground loop, as shown in Fig. 2-47, which makes it difficult or impossible to maintain ground isolation at the unterminated end of the shield.
It is therefore common practice at high frequency, and with digital circuits, to ground the cable shield at both ends.
Any small noise voltage caused by a difference in ground potential that may couple into the circuit (primarily at power line frequencies and its harmonics) will not affect digital circuits and can usually be filtered out of rf circuits, because of the large frequency difference between the noise and the signal.
At frequencies above 1 MHz, the skin effect reduces the common impedance coupling from the signal and noise currents that flow in the shield.
The skin effect causes the noise current to flow on the outside surface of the shield and the signal current to flow on the inside surface of the shield.
The multiple grounds also provide magnetic field shielding at frequencies above the shield cutoff frequency.
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2.15.2.3 Hybrid Cable Shield Grounding.
Although single-point grounds are effective at audio frequencies and below, and multipoint grounds are effective at high frequency, what does one do when the signal contains both high- and low-frequency components, for example, a video signal?
Most audio equipment today also contains digital circuitry for signal processing, so even in audio equipment, high-frequency signals are often present.
Although, in this later case, the high-frequency signals may not be intentionally in the cable, they can unintentionally be coupled as a common-mode signal to the cable, and the cable shield may then be needed to prevent the radiation of these highfrequency signals.
In these situations, the circuit shown in Fig. 2-47 can be taken advantage of by replacing the stray capacitance with an actual capacitor (i.e., 47 nF), which forms a combination or hybrid ground. At low frequency, a single-point
ground exists because the impedance of the capacitor is large. However, at high frequency, the capacitor becomes a low impedance, which converts the circuit to one that is grounded at both ends.
Александр Пупена.
Кафедра АКИТ,Национальный университет пищевых технологий. Киев.
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