Noise and interference from coaxial cable (2023)

Compared to UTP, coaxial noise and interference environment is a little easier. For example, near crosstalk and alien crosstalk problems do not exist in coaxial systems. The crosstalk between adjacent cables is so low that it can be completely ignored in typical digital LAN applications. Furthermore, the near echo problem, which is a major concern in bidirectional UTP systems, rarely occurs in coaxial applications. This is because most applications use coaxial cable unidirectionally.

The main remaining sources of noise and interference in high-speed coaxial systems are

1. Reflected noise at the other end and
2. R.F.I.

10.2.1 Coaxial: noise reflected at the other end

Far-end reflected noise works mathematically the same for coaxial cable as it does for UTP cable (see Section 8.3.1, “UTP: Far-End Reflections”). However, in practical coax systems, this is less of an issue, as coax cable is generally manufactured to much stricter impedance standards than UTP cable.

For example, the popular RG-58 cables (RG-58 U, RG-58 A/U, and RG-58 C/U) meet a strict impedance requirement of 50 ± 2 ohms. When two such cables are coupled together, the worst-case reflection factor is 4%. Double bounces that bounce off two of these transition points appear at the other end of the wire with a relative magnitude of no more than 0.0016 (ie, 4% squared), a value small enough to ignore in most applications.

POINT TO REMEMBER

• Coaxial cable is generally manufactured to much stricter impedance standards than UTP cable.

10.2.2 Coaxial: high frequency interference

Coaxial cable has reasonably good natural immunity to external noise due to the physical symmetry of the signal current conductor and the return current conductor (the concentric shield). This symmetry cancels out all first-order effects of external electromagnetic fields. Any residual susceptibility in a coaxial cable results from deficiencies in its shielding.

At frequencies up to a few megahertz, coaxial susceptibility is proportional to the cable shield resistance. The end-to-end resistance of the shield creates a small end-to-end residual voltage in the cable shield when it is excited by the large common-mode currents that can be induced by RFI. This residual voltage appears to the receiver as a source of noise. Immunity issues due to shield resistance are most common in the sub-30MHz band. To overcome low-frequency sensitivity issues, use a thicker, lower-resistance outer braid or switch to a larger cable (which has a larger, lower-resistance braid).

Higher frequency electromagnetic fields can escape directly through screen holes. To overcome RF sensitivity issues, specify a cable with thick braid and solid shielding. Solid foil shielding is usually wrapped around the dielectric just below the heavy braid. The combination of an aluminum screen and heavy low-impedance braiding is particularly good at combating external noise, although a thin aluminum foil slightly increases the resistance of the screen's skin effect and slightly degrades high-frequency attenuation.

In all cases, when working with fast digital systems, specify a good connection. Do not use connectors with braids, pins, or small tabs connecting the coaxial shield to the enclosure. Get a connector that makes 360-degree body contact around the plug body.

10.2.3 Coaxial: Radiation

The key to achieving good radiated performance is specifying adequate coaxial shielding. This problem is the same as the problem of protecting your system from RFI and the same solutions apply.

You need a cable with low transfer impedance. The transmission impedance of a coaxial cable is the ratio of the voltage developed longitudinally along the shield divided by the current signal flowing in the cable. This parameter is usually specified on a frequency basis. Quote from ISO/IEEE 8802.3 (1996): "The EMC performance of a [coaxial] cable is largely determined by the value of the transmission impedance of the cable."

Larger, heavier braids, or multiple braids, or a combination of foil and braid windings, are commonly used approaches to reducing transmission impedance. Data encryption is generally implemented above 100 MHz to ensure that normal cables do not exceed FCC or EN limits. [86]

[86] Unencrypted transmission systems emit horrible radiation because the simple repetition structures within the data stream, such as the idle pattern, tend to concentrate all the radiated power on the repetition rate harmonics of the fundamental pattern. These bundled harmonics come out of the coaxial cable, where they can be easily detected by FCC or EN test antennas. In contrast, encrypted transmission systems distribute their radiated power over a wide range of frequencies and limit the maximum radiation in any given radio frequency band.

POINT TO REMEMBER

• The RF sensitivity and radiation in the coaxial cable is a result of imperfections in the shielding.

10.2.4 Coaxial cable: security issues

Wherever a coaxial connection is terminated in your equipment, you have two options for dealing with the coaxial ground connection: connect it or not connect it to your equipment chassis.

Coaxial ground treatment is generally the same as signal conductor treatment. Figure 10.7 illustrates the direct connection method. [87] If you connect the signal directly, you must also provide a low impedance direct path for the signal current return.

[87] The connection is not shown for schematic clarity, but I think you get the point.

Figure 10.7. A direct attach coaxial cable requires a low impedance connection between the cable shield and the product chassis. If the impedance of the shield connection is too high, the signal return current will be encouraged to flow along alternate return paths. Alternate loop current paths often act as highly efficient radiating antennas.

If you block the direct signal current path with an isolation device such as a transformer, opto-isolator, or differential receiver, you can isolate the coaxial ground from the equipment ground for signal integrity (Figure 2). 10.8), Creating an insulated cable. Traditionally, in a unidirectional connection, you connect the sending end directly and isolate the receiving end. This is a good arrangement because, as discussed in Chapter 6, Section 6.12.2, “Large Ground Displacement Immunity”, it is never a good idea to make direct ground connections between systems with separate AC inputs.

Figure 10.8. An insulated coaxial cable is not connected to the product chassis. It does not allow signal current to flow into the system. All signal flows are returned to the source on the input cable.

A common-mode inductor blocks the flow of ground currents between enclosures in a different way. The common-mode coil is similar to a transformer, but it is wired differently (see Figure 10.9). Normal signal current flows forward through one winding and then backwards through the other. The magnetic fields of the current following this path are exactly opposite and cancel perfectly. Therefore, the coil has no net effect on normal signal current flow.

Figure 10.9. A common-mode inductor dampens the ground currents between the cabinets.

The coil affects any current that enters the system through one winding and then attempts to exit via any path other than the return winding. These currents are contained and prevented by the total inductance of the coil. With enough inductance in the coil (several Henries), you can dampen the flow of ground currents between the cabinets while still providing a good high frequency path for digital signals. For this approach to work, the inductor must have a primary winding impedance of several thousand ohms at 60 Hz. It must also have a small enough leakage inductance to pass your high-speed digital signals. Designing such a choke is a challenging project.

A balanced DC signal (see table) offers more flexibility in cable shielding treatment. Balanced DC signals carry very little signal power at frequencies below a certain predefined f DC cutoff frequency. Therefore, a balanced DC coaxial transmission system does not need grounding at frequencies below fDC because there is no reverse current at these low frequencies. For example, a 10 MHz Manchester encoded signal has a lower frequency limit on the order of 1 MHz. If the transmission system breaks below this frequency, the received signal changes little. With this system, you should consider making a connection between the coaxial cable ground and the system chassis that is low impedance at high frequencies, but high impedance at 60 Hz. This connection can be made with a capacitor, provided the capacitor has a low enough series inductance for your application (see Section 9.4, “150 W STP-A: Radiation and Safety”).

Balanced DC signals are perfect for connecting through transformers.

POINTS TO REMEMBER

• If you block the direct current path of the signal with an isolation device such as a transformer, opto-isolator, or differential receiver, you can isolate the coaxial ground from the equipment ground for signal integrity reasons.
• A common-mode inductor can also block the flow of ground current between enclosures.
• Balanced DC signals are perfect for connecting through transformers.