Coaxial Cable Noise and Interference

In comparison with UTP, the coax noise and interference environment is somewhat simpler. For example, the problems of near-end crosstalk and alien crosstalk do not arise in coax systems. The crosstalk between adjacent cables is so miniscule that for typical digital LAN applications, it may be completely ignored. Also, the near-end echo problem, which causes so much concern in bidirectional UTP systems, rarely shows up in coaxial applications. That's because most applications use coax in a unidirectional fashion.

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

1. Far-end reflected noise, and
2. RFI.

10.2.1 Coax: Far-End Reflected Noise

Far-end reflected noise on coaxial cables works mathematically the same way as on UTP cables (see Section 8.3.1, "UTP: Far-End Reflections"). It is less of a problem in practical coaxial systems, however, because coaxial cables are generally manufactured to much tighter impedance standards than UTP cables.

For example, the popular RG-58 cables (RG-58 U, RG-58 A/U and RG 58 C/U) meet a stringent 50 ±2 ohm impedance requirement. When two of these cables are coupled together, the worst-case reflection coefficient is 4%. Double reflections that bounce off two such transition points appear at the far end of the cable at a relative magnitude no larger than 0.0016 (that's 4% squared), a value small enough to ignore in most applications.

POINT TO REMEMBER

• Coaxial cables are generally manufactured to much tighter impedance standards than UTP cables.

10.2.2 Coax: Radio Frequency Interference

Coaxial cables have fairly good natural immunity to external noise, due to the physical symmetry of the signal current conductor and the returning current conductor (the concentric shield). This symmetry cancels, to first order, all effects of external electromagnetic fields. Any residual susceptibility in a coaxial cable results from imperfections in its shield.

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

Higher-frequency electromagnetic fields can leak directly through the holes in the braid. To conquer high-frequency susceptibility problems, specify a cable with a heavy braid plus a solid foil shield. The solid foil shield is often wrapped around the dielectric, just underneath the heavy braid. The combination of foil shield and heavy, low-resistance braid works particularly well for combating external noise, although a thin aluminum foil will somewhat increase the skin-effect resistance of the shield, slightly worsening the high-frequency attenuation.

In all cases when working with fast digital systems, specify a good connector. Do not use a connector that has pigtails, pins, or little tabs that connect the coaxial shield to the chassis. Get a connector that makes 360-degree contact, all around the connector shell, with the chassis.

10.2.3 Coax: Radiation

The key to obtaining good radiated performance is to specify an adequate coaxial shield. This problem is equivalent to the problem of hardening your system against RFI, and the same solutions apply.

You will want a cable with a low value of transfer impedance . The transfer impedance for a coaxial cable is the ratio of the voltage generated longitudinally along the shield divided by the signal current flowing within the cable. This parameter is usually specified as a function of frequency. Quoting from ISO/IEEE 8802.3 (1996), "A [coaxial] cable's EMC performance is determined, to a large extent, by the transfer impedance value of the cable."

Bigger, heavier braids, or multiple braids, or a combination of foil wrap and braid, are approaches commonly used to reduce the transfer impedance. Above 100 MHz, data scrambling is often implemented to guarantee that ordinary cables will not radiate in excess of FCC or EN limits. [86]

[86] Unscrambled transmission systems radiate horribly because simple repetitive structures within the data stream, like the idle pattern, tend to concentrate all their radiated power at harmonics of the basic pattern repetition rate. These concentrated harmonics then leak from the coaxial cable, where they may be easily detected by FCC or EN test antennas. In contrast, scrambled transmission systems spread their radiated power across a wide frequency range, limiting the peak radiation in any one radio-frequency band.

POINT TO REMEMBER

• RF susceptibility and radiation in coaxial cables result from imperfections in the shield.

10.2.4 Coaxial Cable: Safety Issues

Wherever a coax link terminates on your equipment, you have two choices for the treatment of the coax ground: You may connect it to your equipment chassis, or not.

The treatment of the coax ground generally matches the treatment of the signal conductor. Figure 10.7 illustrates the direct-connection method. [87] If you direct-connect the signal, then you must also provide a direct, low-impedance path for the flow of returning signal current.

[87] For schematic clarity, the connector isn't shown, but I think you get the idea.

Figure 10.7. A directly-connected 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, returning signal current will be encouraged to flow on alternate return paths. Alternate return-current paths often act as very efficient radiating antennas.

If you block the direct path of signal current with an isolating device, such as a transformer, optical isolator , or differential receiver, then you are free, as far as signal integrity is concerned , to disconnect the coax ground from your equipment ground (Figure 10.8), creating an isolated cable . In a unidirectional link, one traditionally directly-connects the transmitting end and isolates the receiving end. This is a good arrangement because, as explained in Chapter 6, Section 6.12.2, "Immunity to Large Ground Shifts," it is never a good idea to make direct ground connections between systems with separate AC power inputs.

Figure 10.8. An isolated coaxial cable does not connect to the product chassis. It does not permit signal current to flow into the system. All signal currents are returned to the source on the incoming cable.

A common-mode choke blocks the flow of intercabinet ground currents in another way. The common-mode choke is similar to a transformer, but connected differently (see Figure 10.9). The normal flow of signal current is in the forward direction through one winding and then in the reverse direction on the other. The magnetic fields from current that follows this path are exactly opposite and perfectly cancel. The choke therefore exerts no net effect on the normal flow of signal current.

Figure 10.9. A common-mode choke attenuates intercabinet ground currents.

The choke does affect any current that enters the system through one winding and then attempts to leave on any path other than by the return winding. These currents are subject to and impeded by the full inductance of the choke. Given enough inductance in the choke (several Henries), you can attenuate the flow of intercabinet ground currents while still providing a good high-frequency path for digital signals. For this approach to work, the choke must possess a primary winding impedance of several thousand ohms at 60 Hz. It must also possess a leakage inductance small enough to pass your high-speed digital signals. Designing such a choke is a challenging project.

A DC-balanced signal (see box), leaves you more flexibility in your treatment of the cable shield. DC-balanced signals carry very little signal power at frequencies below some predefined cutoff frequency f DC . Therefore, a DC-balanced coaxial transmission system does not require a ground at frequencies below f DC because there isn't any return current at those low frequencies . A 10-MHz Manchester-coded signal, for example, has a lower cutoff frequency on the order of 1 MHz. If the transmission system is interrupted below this frequency, it makes little difference to the received signal. With such a system, you might 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 could conceivably be done with a capacitor, provided that the capacitor has a low enough series inductance for your application (see Section 9.4, "150- W STP-A: Radiation And Safety").

DC-balanced signals are perfectly suited for connection through transformers .

POINTS TO REMEMBER

• If you block the direct path of signal current with an isolating device, such as a transformer, optical isolator, or differential receiver, then you are free, as far as signal integrity is concerned, to disconnect the coax ground from your equipment ground.
• A common-mode choke can also block the flow of intercabinet ground current.
• DC-balanced signals are perfectly suited for connection through transformers.