When Bad Harmonics Happen to Good People
The complexity of qualifying and maintaining the copper network for more demanding broadband services such as VoD and IPTV comes at a time when many telco field technicians are inexperienced and minimally trained, and competitive pressure is high. Thus, a clear understanding of pair qualification and maintenance should be a priority for all technicians.
Interference on Copper Pairs
Alternating current (AC) interference on telephone pairs is the chief threat to the quality of both voice and data services. This interference, commonly measured using a spectrum analyzer, can occur at various frequencies called harmonics, which are multiples of 60 Hz or 60-cycle power. AC harmonics are the individual components of what is generally referred to as power influence (PI), also known as noise-to-ground or common mode noise. This unwanted AC energy can be induced on telco cables by adjacent and parallel power lines, and occurs at greatly increased levels when cable shield bonds or multi-grounded neutral (MGN) connections are faulty or open.
Power utilities distribute 60-cycle energy to residential and business customers beginning with three-phase, two-phase, and finally single-phase lines generally with a neutral (ground) return wire. Single- and two-phase power lines are by design unbalanced; only three-phase power can be balanced.
While high-voltage, three-phase power transmission lines are impressive and perhaps intimidating, they are nearly always balanced, and thus, are seldom a source of line noise on copper pairs. A balanced (three-phase) line has equal current (measured in amperes) on each of the three distribution wires, with (theoretically) zero amps or no ground return current.
In the plain old telephone service (POTS) setting, electrical interference can be manifested as high PI and circuit noise when a cable pair is not perfectly balanced. But in the high-speed, high-frequency realm of data communications, electrical interference may be detected only as bi-polar violations, low noise margin, a bad signal-to-noise ratio, or impulse noise. Correcting poor cable bonds or shield grounds may mitigate both voice and data transmission problems. The latter are often intermittent in nature.
Voice band communication on a copper telephone cable pair without load coils occurs at frequencies between 300 Hz to 4,000 Hz, and this same bandwidth is used for caller identification (CID) and 56 kb dial-up modems. Harmonic interference of sufficient magnitude at any frequency within this range may cause interference to data signals or generate audible noise affecting telephone customers.
Both AC volts and PI must be measured on a pair in order to establish a complete picture of line condition. While a 60 Hz, a 180 Hz, or even a 300 Hz signal on a cable pair may not be audible, such low harmonics can exist at a high enough magnitude (voltage) to cause electronic equipment to malfunction or fail, and may create a safety hazard for customers and field craftspeople.
Figure 3. The Dynatel 965AMS DSL Spectrum Analyzer from 3M is a diagnostic tool for voice and data services on copper pairs that measures and identities harmonic interface, bonding and grounding problem, and interfering signals.
A Tutorial on Harmonics
A spectrum analyzer is used to measure harmonics or power influence (PI) on telco pairs. (See Figure 3 image above and The Five-Step Plan below.) This device is a highly sophisticated and frequency-specific volt/ohm meter (VOM). A traditional VOM measures voltage on a cable pair primarily at 60 Hz, and while it determines the voltage on a conductor, it cannot distinguish the harmonic makeup (and thus the likely source) of that voltage. In contrast, a spectrum analyzer can measure and display voltages at all harmonic frequencies, individually and selectively, helping determine the source of the interference.
For the purposes of copper pair trouble diagnosis, the harmonic content of what we call PI is measured in AC volts and converted to decibels (dB). More specifically, for telco purposes the signal strength of a harmonic is converted to dBrnC. That is, decibels (dB), in reference (r) to noise (n), with message weighting (C). This weighting is accomplished by a filter incorporated in the telephone test set that restricts dB measurement at harmonic levels that are generally either too high or too low in magnitude to be audible over a twisted copper telephone cable pair.
For practical reasons, harmonic or frequency interference that occurs at a level below that which can be detected by the human ear should not be classified as noise. Thus, circuit (metallic) noise, PI, and cable pair balance are expressed in terms of dBrnC in order to evaluate the practical performance of a voice-grade telephone line. However, xDSL circuits are not as forgiving as the human ear is in rejecting unwanted harmonic frequencies that may be carried on cable pairs. Thus, a new performance standard is required for pairs carrying both voice and DSL signals.
Most audible interference issues in the voice band are on a cable for a sufficient time that circuit noise, power influence and balance can be detected and measured using a spectrum analyzer. However, the same poor bond or ground connections that create audible noise will also allow AC power spikes or transients to disrupt or corrupt data transmission at the much higher frequencies used for high- speed data or video deployment.
Spectrum analysis equipment designed for telcos use can measure and identify harmonic interference in the range of 20 kHz to 2.2 MHz, and identify unwanted interfering signals that originate from adjacent cable pairs including HDSL and T1 lines. Such equipment can also detect spikes or transient voltages from power lines if the test set is connected when such an event occurs. Used in the low-frequency (60 Hz to 3,000 Hz) mode, a spectrum analyzer detects electrical interference caused by parallel power lines, bad bonds, or poor grounding to the MGN.
Unlike internal interference, most electrical noise problems stem from power line transients, lightning that damages equipment, or corrupt data transmissions occur very rapidly, coming and going in milliseconds. Electrical interference may occur only at odd times, once or twice or perhaps dozens of times a day. For this reason, it is rare that power line troubles that can potentially degrade DSL or video service can be detected using the DSL (high-frequency) spectrum analyzer alone.
Fortunately, or unfortunately, as the case may be, the problem of PI may help technicians isolate and fix such intermittent problems. Unlike transients, PI is on cable pairs for sufficient durations to be reflected in cable sheath bonding and/or improper grounding problems. Low frequency harmonics (from approximately 60 Hz to 3 kHz) can be measured and analyzed with the voice band (VB) spectrum analyzer function.
Using the impulse noise test function, a technician can use a spectrum analyzer to count the number of noise incidents exceeding an adjustable threshold over a selected period of time and, as a result, determine the impact of noise on customer data throughput rates. The ability to select the noise threshold level allows users to test against the requirements of the pertinent service (ADSL, ADSL2, etc.).
Open or loose cable bonds or poor grounds that allow interference in the voice band also result in transient high frequency interference to video and DSL signals. If the cable sheath is not delivering uninterrupted PI reduction, providing electrical protection for electronic circuits and eliminating transient voltages, cables cannot deliver reliable video or DSL services.
It should not be assumed that nearby single-phase or two-phase power distribution lines are necessarily the primary source of electrical interference on cable pairs. Many times, these one- or two-phase power lines create a severe imbalance that reflects back to the three-phase circuit feeding them. In such cases, an open cable bond under a three-phase power line segment, perhaps close to a central office, can be a potential source of electrical interference.
Without spectrum analysis capabilities, the only thing technicians can know for sure is that there is PI on cable pairs. Still, they have no clear means of determining the origin of such unwanted AC energy. The ability to determine which harmonic or set of harmonics is causing a problem is of great value in isolating and fixing electrical interference or PI problems. Specifying harmonics in detail also helps determine if an interference problem is the result of poor bonding or grounding or some malfunction in the power distribution system.
Figure 1. Using a spectrum analyzer for power interference and noise measurements can help technicians gather information that indicates and specifies harmonics.
Figure 1 illustrates two representative spectrum analysis screen shots used in troubleshooting. The image on the left displays a traditional Power Influence, Noise, and Balance reading. Even though the PI is very high, these measurements identify a defective cable pair. This is made evident because the balance is less than 60 dBrnC.
Fixing the cable pair to improve the tip-to-ring electrical similarities lowers only the noise and increases the balance to improve the customer's service. To lower the PI, it is necessary to identify the harmonic to determine if good cable bonding along with MGN grounding mitigates the PI or if this issue may be resolved only by working with the local power company.
The image in Figure 1 is used to indicate and specify harmonics. In this case, the reading on 1017 Hz should lead a technician to know that improved cable shield bonding along with connections to multi-neutral grounds can lower this harmonic by at least 12 dB.
Figure 2. A circuit that tests with little noise and PI show these spectrum analysis values.
Figure 2 shows a circuit that has PI well below the norm, with low CN noise and bad balance. If PI is 60 dBrnC or greater, we can use cable pair balance to evaluate the integrity of a cable pair. In this case, if PI increases from 41 dBrnC to 61 dBrnC at times, the noise may increase from 8 to 28 dBrnC, which is unacceptable. In this case, the Longitudinal Balance test should be used to evaluate the quality of the cable pair for good customer service.
When cable pair balance is above 60 dBrnC, indicating an acceptable VB POTS cable pair, there still may be audible noise if the PI is above 80 dBrnC. This unwanted AC electrical interference (PI) will cause problems with high-speed data circuits. ADSL, T1, or HDSL circuits may have intermittent troubles from electrical spikes or surges if cable bonds or grounds are less than perfect.
The higher the harmonic frequency causing a problem, the more effective good cable bonding with proper grounding will be in fixing an AC interference problem. Not all PI problems can be grounded or bonded away; the results are frequency (harmonic)-dependent. However, cables must still be bonded and grounded for telco and customer safety as well as protection of electronic equipment.
Harmonic Signatures and Enlisting Assistance
It might be expected that a single AC harmonic is the cause of noise on a telephone cable. In truth, it is likely that there will be several harmonics on a cable pair under diagnosis. Multiple harmonics, or combinations of unwanted electrical energy, are the components of a harmonic signature; this signature provides clues as to the source of interference. (The sidebar Harmonic Diagnoses lists common harmonic signature patterns and their likely causes.)
By analyzing the harmonic content of cables, the technician can help determine if this electrical interference can best be resolved by improving poor bonds or grounding of cable shields, or must be resolved with the cooperation of the local power utility.
When utility assistance is necessary, it is helpful to communicate with the power company using electrical power language. For example, if a telco representative describes the problem as a noise issue, the utility thinks in terms of radio frequency interference (RFI), which is the type of noise issue they generally deal with. Electrical grid RFI results in television or AM radio interference, while a telco PI problem is not an RFI issue.
Describing a PI problem in terms of dB measurements is also of little value in working with power company employees. Their concerns and maintenance issues are centered on 60 Hz AC and expressed in volts, amperes, and watts. The best approach to resolving harmonics issues caused by electrical utility deficiencies is to build a good relationship with an electrical engineer at the power company, and strive to establish a clear understanding of technical issues on both sides.
What telcos call Power Influence is much more closely related to what the power company knows as Total Harmonic Distortion (THD), which is the percentage of harmonic content in the power distribution system apart from 60 Hz. If you need power company assistance, first confirm that cable shield bonding and grounding are good, and then relay to them the specific harmonic signature you have detected.
Even though the power company can take steps to reduce THD in their system, if telco bonding and grounding are not up to par, there will still be transient problems that could result in data transmission corruption, electrical protection issues, and possibly safety hazards during power line switching, phase wire faults, or thunderstorms. A spectrum analysis test set is useful for locating and resolving such deficiencies.
The Five-Step Plan
Using the spectrum analyzer, work through these five steps to successfully reduce power influence (PI) on cable pairs:
Step 1. Determine which harmonics are causing noise or AC voltage on the cable pairs.
Step 2. Isolate the power line that is emitting the harmonics that were measured on cable pairs.
Step 3. Ground the telephone cable sheath to that particular power line.
Step 4. Verify that cable sheath bonds are good between the new multi-ground neutral (MGN) connections.
Step 5. Recognize that an effective cable shield will have the same harmonic currents on the shield as those present on cable pairs if the shield is properly bonded and grounded.
The results of PI are frequency (harmonic)-dependent, and good bonding with good grounding will reduce 60 Hz interference only by about 0.6 dB. However, these same steps reduce 540 Hz PI by 6 dB and 1020 Hz PI by about 12 dB. Thus, the higher the harmonic involved, the greater the importance of good bonding and grounding practices.
In addition, if cables are not well bonded and grounded, the shield will not carry the high currents and excessive voltages that are present during lightning strikes or power line surges. Instead, such voltages will travel over cable pairs and into central office line cards, remote switch circuit cards, repeaters or doublers, customer-owned equipment, or, in the worst-case scenario, through a technician's body to ground.