Coaxial cable is the backbone of the video interconnect system. Although it has been in existence for more than 100 years, coax cable for high data-rate video applications is obviously a more recent development within this technology. As video formats have evolved from analog to SD digital and now HD formats, coaxial cable performance for broadcast applications has also advanced.
Cable classifications
Common designations for coaxial cable types are the RG type or wire gauge size. These designations, however, do nothing to determine whether a coax is suitable for digital video transmission. They are only categorizations of the size of the conductor and outer diameter. Any given RG type can come in a variety of constructions, such as closed-circuit TV (CCTV), master antenna TV (MATV), RF, analog and HD-SDI.
The measure of the quality of the cable, and thus its appropriate applications, is determined by the performance and precision of its electrical and mechanical specifications. In fact, the design of the cable is built to the requirements of the video signal. For example, cable designed for baseband analog video and close-circuit security applications typically have a reduced bandwidth, a single shield and less stringent tolerances. Accordingly, uncompressed HD formats that operate at high data rates have their own specific set of requirements. Uncompressed SMPTE 292M HD serial digital video operates at 1.485Gb/s, while the new SMPTE 424M HD standard operates at 3Gb/s.
To ensure minimal pulse rounding, back reflections and jitter, the interconnecting cable must be rated to a bandwidth that is equal to or exceeds three times the clock rate (which is one-half of the data rate). For SMPTE 292M formats, this works out to be 2.23GHz. For 424M formats, it's 4.5GHz.
To achieve the performance required of coaxial cables used in HD broadcast standards, the cable must be made to an exceptionally high degree of precision up to 3GHz or 4.5GHz. Each element that makes up the coaxial cable — conductor, dielectric, shield and jacket — plays an important part in the performance and needs to be carefully controlled in the manufacturing process.
Bandwidth and the measure of precision
Bandwidth is a buzzword that is often used in consumer and professional applications, but what does it really mean? Bandwidth refers to the measure of performance wherein all relevant electrical characteristics are compliant with the manufacturer's specifications or relevant industry standards at all frequencies between the high and low points of the rated bandwidth spectrum. Critical specifications relevant to bandwidth typically include characteristic impedance, attenuation and structural return loss (SRL), all three of which are measured over the defined frequency range of the bandwidth.
The most important measurement of these is SRL. (See Figure 1 on page 38.) SRL is a measure of the back reflection of energy from and through the transmission line (cable). Any periodic variation, structural flaw, major impedance variation or attenuation anomaly can manifest itself as a back reflection at some specific frequency within the return loss measurements. These reflections, often referred to as spikes, will typically correspond to an attenuation, impedance and/or mechanical defect at that frequency. The minimum SMPTE 292M and 424M requirement for return loss is 15dB, although manufacturer tolerances often exceed this.
Consistency, consistency, consistency
We have defined what the bandwidth of the cable needs to be, but how is that accomplished from a mechanical and manufacturing perspective? In one word: consistency.
Any mechanical variation will manifest itself as an electrical deviation. A theoretically perfect coax cable is perfectly round, with a perfectly smooth center conductor, which is perfectly centered in a dielectric. The dielectric itself should also be perfectly formed.
Of course, in reality, perfect geometrical dimensions do not exist. The goal in manufacturing HD coax is to minimize the ovality of the dielectric, create a precision diameter conductor that is ideally centered, and make a dielectric that has a consistent cell structure within the foam dielectric.
The center conductor
The heart of the cable is the center conductor. Copper conductors are drawn down from copper rod material into the long thin strands. Unlike the conductor for power and audio applications, a video cable conductor requires a higher level of precision during production.
For video cable, the conductor is initially drawn down to a diameter that is slightly larger than the desired finished diameter. After the initial drawing process, the copper is reduced to its final diameter by other methods to achieve a diameter that is extremely precise, a tolerance within one-half of one-thousandth of an inch. The copper conductor must not only have a precision diameter, but it must also be devoid of surface irregularities (nicks, roughness, chatter, etc.). The precision diameter conductor is the foundation of the consistency of the finished cable.
The dielectric
Just outside the conductor is the second element of the cable, the dielectric. The dielectric insulates the conductor from the shield and determines the capacitance and impedance of the coaxial cable. The exact specifications are affected by the type and amount of plastic used (wall thickness). For HD video coax, a gas-injected dielectric is typically used because it has a lower dielectric constant than a solid compound, which allows for less compound to be used with better high-frequency performance. A gas-injected dielectric typically consists of a mixture of polyethylene and gas nitrogen.
While a solid dielectric has a high degree of structural consistency, gas-injected dielectrics require a carefully controlled process to achieve the consistency required for HD coax. If the gas bubbles (or cells) in the dielectric are not uniform in size, if its shape is not round, or if the ratio of plastic-to-gas density changes, the dielectric may become structurally variant. This variation can result in poor return loss and impedance specifications.
The higher the bandwidth of the cable, the more sensitive the signal becomes to smaller periodic variations in a cable. Thus, making cables to a higher bandwidth requires more than just testing the cable to a higher stop point. Periodic variations that change at higher frequencies must also be minimized or eliminated.
The shield and jacket
Although the conductor and dielectric are the most critical elements to be controlled in manufacturing coaxial cables for high-bandwidth applications, the precision of the shielding and jacketing process is also crucial. Any periodic change in the physical structure of the braid can also cause return loss and bandwidth issues. It is important that the lay, angle and thickness of the foil and braid shield all remain constant.
Although the function of the outer jacket is to protect the cable and does not determine the electrical properties, it is important that the extrusion process of the jacket does not deform the inner components as to create or induce structural flaws. This can be caused by excessive heat, pressure or over handling.
Conclusion
All elements within HD coaxial cables must be precisely controlled to achieve the performance required for high data-rate standards. Although often unseen or immeasurable by the user, the microscopic consistency achieved by cable manufacturers delivers the measurable performance and transmission reliability that broadcast engineers rely on for dependable interconnect systems in broadcast HD applications.
Scott Fehl is product manager for Gepco International.
