DOCSIS 3.1 noise mitigation: Check your grounds

Jan. 30, 2018
As technologies advance, grounding and bonding practices must advance with them. This is particularly important for noise mitigation in a DOCSIS 3.1 environment. Read on to learn more.

Since the dawn of time, individuals have known that they need to protect themselves from lightning. In the beginning, humans were only concerned with protecting themselves. As time went on and infrastructures were constructed, it became evident that those things needed to be protected, too. Through trial and error, society figured out how to design and construct lightning rods that could take the energy generated from lightning and harmlessly return it to the earth.

Over time, electrical communications circuits were invented. Initially, these circuits were as robust as humans, so the grounding technology that was developed to protect the human body could also protect them. Eventually, as technology evolved and circuits became even more high-tech, humans were no longer the most delicate part of the communications circuits. This spurred the need to adopt more stringent grounding standards to protect them.

Protection

Protection from electricity that ranged from lightning, with its mega-voltages and current, to the micro-current of a small static charge was now needed. Even though it is widely known that grounding, and its twin concept, bonding, are important, it wasn't until recently that we could get by with grounding and bonding that did not meet the standards that had been set.

Today, there are very high-speed data services that are digital - bits or plus-and-minus voltages that are impacted by noise. These digital services can overcome steady state noise, but the random noise spikes of impulse noise knock out data bits.

Impulse noise always comes from outside the cable, and it is always a sign of bad grounding and bonding. If the cables were bonded and grounded properly, the impulse noise would go to ground via the shield and not interfere with the transmitted signal. Therefore, mitigation of impulse noise means providing good bonding and grounding.

As data rates increase, the same length impulse noise, also known as "spike," now knocks out more data bits. In addition, architectures and compression protocols allow each of these bits to carry more information, more data, so the same length impulse now knocks out more data, creating a double whammy.

With the advent of faster and faster data, pixelation on TV screens had to be addressed, as well as much slower effective data rates and other symptoms of inadequate bonding and grounding that customers of MSOs and other Internet providerswere noticing. It was imperative for communications companies to get their grounds and bonds in shape.

However, as cable TV providerswere getting their grounds to meet specification, it was discovered that there was something new to be concerned about with respect to grounding. The new concern was "balance."

A balancing act

With the advent of DOCSIS 3.1, companies not only have to be concerned that the grounds in hubs and headends are adequate in the sense that they meet the absolute ohm specification of the ground for safety of people and protection of property, but also that the various elements of that ground are balanced. That means that they must make sure that each of the various metallic "runs" that make up the ground have the same resistance.

Elements of the hub and headend ground

There are various elements that make up the hub and headend ground, including shelves that are bonded to racks with screws and wires, racks that are bonded together to make aisles, as well as aisles that are bonded to bus bars.

It is difficult to get these elements balanced, to make sure that each has the same resistance. Each of these runs may have wires, bars, screws, posts and bolts made of different materials that are bonded using various methods and substances. Moreover, if these runs are to be balanced, each one of them has to have not only a low enough resistance to meet the absolute safety specification, but also almost the same resistance as the other runs that make up the ground.

Each place that these elements are bonded is an ingress point for noise. There are thousands of them. If there is a loose fit, a bad connector, scrape on a shield or anything that compromises the connection, the chance of ingress is better.

What balances or unbalances the elements?

Assume that the grounds are connected to one another. Assume again that they are effective and meet all safety specifications. However, sometimes there are aspects that can cause bonds to possess slightly different resistances from like bonds, including paint, torque, washers, length of cable, and the makeup of ground wires and cables.

Why balance is important

Balancing the elements of a ground is always important because of the antennas that imbalances create for RF. But we had enough power difference between the signal and the noise to more-or-less harmlessly "absorb" the noise. What changed?

Because of potential energy coming into the plant, when we go from 64-QAM (quadrature amplitude modulation) to 256-QAM channels, we need to lower our noise floor by 3 dB just to stay even with MER (modulation error rate) and BER (bit error rate). Bonding up to 32 of these channels adds to the potential for interference, for noise.

What can be done to reduce noise?

We want to keep the noise floor as low as possible. However, with advanced digital technologies, we need to have less noise just to stay even. By balancing the ground circuits,noise is reduced. Lab experiments and tests in actual hubs confirmed that if an unbalance of 0.8 ohms in the ground circuits can be reduced to 0.3 ohms, the noise floor in the 5 MHz to 50 MHz spectrum can be reduced by 8 dBmV.

Figure 1: A real-life scenario in a headend environment

Figure 1 represents a real-life scenario in a headend environment. There were noise problems affecting a DOCSIS platform in the headend. It was thought that grounding and bonding problems were causing the noise. The resistance of the building ground was measured and found to be acceptable. The continuity of the grounds was looked into for bad bonds. It was discovered that the bonds were good enough and did not adversely affect the protection ability of the ground.

However, the resistance of these daisy-chained grounds varied by 1.0 ohm from rack No. 1 on the left to rack No. 5 on the right. In other words, those grounds were unbalanced. The ground resistance of a shelf in the bottom of the left-most rack, while meeting the specification for ground resistance, even in a best-case scenario, is higher than the shelf at the top of the right-most rack, resulting in unbalanced grounds.

So, what can be done about the imbalance? How can we see if reducing the imbalance will reduce noise?

Figure 1 illustrates a "daisy-chained" ground. Ground circuits can also be "home runs" as shown in Figure 2. This is where each rack has a ground connected in parallel with the other rack grounds connected to the ground bus.

Figure 2: "Home run" ground circuits

As shown in Figure 1, several grounds that are daisy-chained, because of the different number of bonds and greater difference in length, will have more variation in their resistance (be unbalanced) than would several home run grounds with their similar number of bonds and similar lengths. Changing these daisy-chained grounds to home run grounds helps to reduce noise.

Howto determine if grounds are balanced

We cannot measure the resistance value of a ground at a shelf or similar place in a hub or headend. We can, however, easily measure and compare the continuity and balance of the various ground circuits of a hub or headend.

To measure ground balance in a hub or headend, a clamp-around ground tester that measures to a tenth of an ohm is required. All clamp-around testers use Ohm's Law to calculate resistance. If we open the jaws of such a tester, we will see the ends of two coils (Figure 3). One coil of the set, the "transmit coil," induces current of a known voltage onto the circuit under test. This current seeks the path of least resistance back to the other coil, the "receive coil." If the circuit under test is a complete circuit, the receive coil measures the voltage. The test set now knows voltage and current and can calculate resistance.

Figure 3: The coils in a claw tester that is open

If the path of least resistance is all metal (does not involve any soil), the resistance measurement is not the resistance of a ground, but rather of the continuity of the metal circuit. When looking for unbalanced ground circuits in hubs and headends, that is what is done.

If testing shows that the headend and hub grounds are not balanced, what can be done?

Fixing the balance

Daisy-chained ground circuits can be changed to home runs if the headend and hub grounds are not balanced. Bonds need to be double-checked that they are installed to the manufacturer's specifications, and the resistance of individual bonds may be measured with a digital low resistance ohmmeter. It is also important to review grounding and ground testing practices, as well as to ensure that the ground was reconnected if it was disconnected during building renovations. Grounds also need to be checked for continuity.

Other environmental factors also need to be considered. All kinds of noise generators, like air-conditioners and impulse motors, are being installed. Are they grounded and are the grounds tested? These noise sources make it more urgent that grounds be balanced to reduce potential points of ingress for noise.

As you can gather, it is no longer sufficient to have grounds that only protect products and people. We must make sure that they also protect the performance of equipment and products being used. We also must remember that the balance of the elements of a hub or headend ground is important for noise mitigation in modern digital communications circuits. And although it seems as if visually checking these elements is important, it is also important to test them to determine if they are truly balanced.

Ed Rousselot is national communications sales manager for Megger.

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