The transition from analog to digital signaling in broadcast that began many years ago introduced an entirely new set of components and challenges. To optimize signal quality from input to output, engineers and users need to familiarize themselves with how reflections and parameters such as group delay ultimately determine the quality of the transmitted image.

As the industry increasingly embraces 4K and UHD formats, the move from a 3Gb/s infrastructure to a 12Gb/s patching infrastructure is motivated by the need to transmit uncompressed UHD and 4K digital TV signals over single-link Serial Digital Interfaces (SDI). By single-link, we mean a single cable through the infrastructure from source to destination. However, 4K SDI patching systems today employ a quad-link architecture, which requires four parallel 3Gb/s coaxial connections to carry a 4K signal.

The 12Gb/s Solution

Whether 3Gb/s, quad-link 4K or single-link 12Gb/s, these signals are digital, which can render them more immune to noise, interference and distortion than their analog counterparts. Even so, broadcasters must understand how parameters such as reflections, variable group delay and loss may affect the performance of jacks and other high-speed components within the patching architecture regardless of the overall system format and design.

Because 4K and other UHD formats are the future, let’s examine quad-link 4K and single-link 12Gb/s transmission technology. A quad-link architecture has 3Gb/s signals moving through four parallel connection paths. These four synchronous signals must be generated by the serializer at the source by decimating the 12Gb/s signal. At the receiver the signal must then be re-clocked, synchronized and de-serialized. Figure 1 illustrates the difference between these two approaches.  This technique enables the transmission of a 12Gb/s SDI signal over a 3Gb/s patching infrastructure. 

There are multiple disadvantages of moving a 12Gb/s signal with the quad-link approach.  Detrimental factors include:

• Increased hardware complexity at the source and destination.
• Increased patching system size, cost and complexity.
• Sensitivity in the synchronization process caused by differences in the properties of the four separate transmission paths.

The objective is to create a robust 12G patching infrastructure that will enable the routing of 4K SDI signals over a single-link. Naturally, proper synchronization is all about timing, and that timing becomes increasingly challenging with increasing clock frequency. At the higher data rate, the signal artifacts introduced by parameters such as reflections, group delay, loss and impedance variations increase and become more pronounced. Hence, it is important to understand the impact of these parameters on the end-to-end performance of the data link, which ultimately determines the quality of the transmitted signal.

Reflections and Loss

Reflections are caused by impedance discontinuities in a transmission system. In the frequency domain, reflections translate into loss. In the time domain, reflections, which, if not properly terminated and absorbed, may cause decoding errors at the receiver.

Signals seldom move across a perfect path. Each segment of a path has cable twists and turns causing the frequencies to bounce around along each cable. They reflect off each other and off the cable shield. As these frequencies bounce, they reflect back into each other creating phasing issues similar to what happens to sound reflections in an enclosed room.

In an enclosed room, a speaker’s voice hits the opposite wall and is reflected back to the original source. In the process the reflected signal becomes out of phase with respect to the original voice signal. A similar outcome takes place for these much higher frequency signals in a coax cable.

With time-domain parameters like group delay and frequency dispersion, it’s helpful to understand more specifically what happens to these signals along the path. In group delay, the electrical signals are moving slightly out of phase. The key time-domain parameter, group delay, is the time it takes a narrow group of frequency components of the input waveform to travel through a patch bay jack. Therefore, group delay is the parameter that characterizes the amount of input-to-output delay in a linear system. This is the time that it takes a pulse comprising a group of frequency components to propagate from input to the output of the transmission line.

Constant group delay ensures that the different frequency components of an input signal reach the output together. If the magnitude of the system input-to-output transfer function is relatively constant over the required range of frequencies, then the signal pulses traverse the transmission line without significant distortion. Conversely, if the group delay is not constant over the proper range of frequencies, the output signal can exhibit significant distortion in the time domain.

An important distinction between the time domain transmitted signal distortions caused by group delay variations and those caused by insertion loss variation that is proportional to the square-root of frequency. While a receiver’s adaptive equalizer can significantly improve the latter type of distortion, it can do nothing about the former. This is because the adaptive equalizer can assume the square-root of frequency dependence of the insertion loss and equalize those types of signal distortions.  But in the case of group delay, the receiver has no information that could be used to adjust the signal.

There are multiple other variables and parameters that affect patching performance. They include:

• Return loss. This is the amount of output power versus the amount of received power.
• Insertion loss. This causes reflections that adversely affect RF power into the connector.

These are all tried-and-true physics that could be explored much further in depth than the length of this article will allow. However, there are solutions to address these problems and minimize artifacts and intrusions along the path.

Environmental Improvements

Preventing these types of problems must start on the cabling side. In a perfect world we could use semi-rigid coax. However, the fact is that such a coax structure will not fit into today’s facilities. Instead, we see these semi-rigid cables, less than 1/8-inch in circumference, used more often for internal routing inside RF and satellite devices – anywhere it is necessary to transport high frequencies. Semi-rigid coax is also used as a conduit for internal circuit paths in patch bay looping plugs.

One means of improving the adverse effect of time-domain parameters is the application of a silver-coated center conductor for the cable. This is a relatively new method that addresses some of the limitations related to power loss and heat associated with a standard bare copper center conductor.

The electric field of electromagnetic fields that are guided by conductors only penetrates the outer surface of the conductor. Because most of the electric field exists within a specific depth from the surface of the conductor, this is where most of the flowing current is concentrated. The resistance that is encountered by this sheet of surface current in flowing through the conductor is known as surface resistance. That surface resistance is proportional to the square-root of frequency, and inversely proportional to the square-root of the conductivity of the metal. 

To reduce the fraction of power that becomes lost to heat in conductors, we need to reduce the surface resistance. One way to achieve this is through the use of higher conductivity metals; the conductivity of silver is 7 percent higher than copper. This may seem minimal, but by switching the metals of a high-quality 12G coax from copper to silver, the insertion loss of a 100-meter cable is reduced by 1.9 dB. That 1.9 dB can make the difference between reception and no reception.

Because current flows on the surface of the conductor, we only need to coat the outer surface of the copper conductor with silver. Another property of silver is that, when exposed to air, it quickly oxidizes to silver oxide, which reduces its conductivity. Therefore, silver coating is a good solution only for covered surfaces, like the center conductor of a coaxial cable, which is sealed and protected by the dielectric.

The effectiveness of the shield design must also be considered. The typical coax cable employs a braided shield, with foil below the surface. As the signal travels down that silver-coated center conductor, it ricochets up to the braid and back down causing a reflection. This is measured as return loss.

Tightly re-braiding the cable creates better precision, which allows for fewer reflections that correlate with a lower return loss, group delay, frequency dispersion and other time-domain parameters. The fewer imperfections in the braid, the cleaner the travel of that wave moving across the cable.

Other Considerations

As we move from 3G to 12G, dielectric loss starts to become a significant contributor to the overall loss of the patching system, which can reduce the effectiveness of adaptive equalization at the receiver and further attenuate the received signals.

Impedance discontinuities create trapped bouncing signals inside patching systems, which leak out and disturb the linear phase with respect to frequency of the desired output signal. This results in group delay variations. Group delay variations create time domain output signal distortions, which adversely affect the ability of the receiver to perform adaptive equalization.

It is therefore important to design impedance-matched systems with minimal impedance discontinuities. It is essential to maintain a constant ratio between the local distributed inductance and the transmission line capacitance inside the patching system.

Finally, it is also critical to the design to use only components that support single-mode propagation with only the lowest order mode. In coaxial systems, this means that we should always operate well below the system’s cut-off frequency.

In summary, 12G patching has stringent performance requirements that require the use of specialized patching components and techniques. Key among these are:

• Wideband impedance matching exceeding the applicable standards’ specifications.
• Low group delay variations and low frequency dispersion.
• Low dielectric loss.
• Single-mode operation.

These performance criteria have been met by Bittree with the 12G+ patching product line introduced in 2017. Combined, they represent the performance requirements to be successful in creating robust and error-free 12G digital patching solutions.

Dimitrios Antsos, Ph.D. of Antsos Consulting contributed to this article.