Interference

Introduction::

Thunderstorms, HAM radio operations, AM radio stations, electric motors, electric relays, microwave ovens, electric ranges, light dimmers, night lights, noisy car ignition systems, bad grounding, crosstalk, power factor correction devices to name a few are good sources of RF interference. Add low signal to noise ratios in the 6 db ~ 10 db range to the above factors and you have problems.

Why the twist in twisted pair lines? When a current flows trough a conductor an electromagnetic field is generated around it any other parallel conductor in close proximity will generate an induced current from the expanding and collapsing of the electromagnetic field. This interference is called crosstalk and to reduce it both conductors must be separated further away. Another method of reducing crosstalk is to add a twist to the two wires and the induced electromagnetic field will be cancelled by the other therefore reducing crosstalk. One additional benefit of of adding a twist to wire pairs is that it tends to reduce external interference. Never the less twisted pair phone lines are highly vulnerable to electromagnetic noise.

Thunderstorms:

By far the most important source of EMI (electro magnetic interference) are thunderstorms. The effects of nearby thunder activity in ADSL modems is high CRC/FEC error rates and loss of ADSL synchronization. There is no cure for this type of EMI no matter the S/N. Thunderstorm activity can interfere on ADSL modems as far a 40 miles.

CRC accumulated errors

CRC error rate

Picture 1 of table 1 shows the accumulated CRC errors during a thunderstorm notice the sharp increase in CRC errors as the thunderstorm pass, the ADSL modem was forced to retrain due to the high CRC error rate as shown in picture 2. Some ADSL modems are programmed to re-train when a preset errored seconds threshold is exceeded. Picture 1 shows the accumulated CRC errors rose from 9,000 to 17,000 in a one and a half hour.

Electrical Noise:

Electrical noise categorization is complex but can be traced to two major types, radiated and conducted. Radiated noise is propagated trough the air while conducted noise is present in power and transmission lines.

ADSL modems can be subject of interference by the two major types. Hunting for interference sources can be time consuming especially with intermittent sources. When looking for noise sources it is best to know where to start.

Professionals use expensive equipment like oscilloscopes or spectrum analyzers to identify and detect electrical noise sources but by using a electromagnetic radiation detector like the Winradio ERD-1500 or MFJ Enterprises MFJ-852 or a simple and inexpensive AM or SW radio the average person can locate some of the noise sources.

EMI Filters and ferrite chokes:

We have successfully used EMI filters in several locations to mitigate high CRC errors. One simple way is to install a RF choke to your and to your neighbors' telephone cable drop coming in from the street or telephone pole. PLEASE note that this action requires authorization and supervision from your telephone company and neighbors. IN ADDITION the choke is usually installed outside your demark line (outside your jurisdiction).

A more drastic measure is to install high performance EMI filters in your electrical system. There are many EMI filter vendors like Radius Power. The RP120 is an example of one of their high performance EMI filters. Installing this type of filters require the services of licensed and qualified technicians, this is a time consuming, expensive and dangerous task.

Grounding and shielding:

Proper grounding is probably one of the most over looked aspect in solving electrical noise. The FCC has established the ground conductivity for the US and territories. Conductivity is the inverse of resistivity G = 1/R where G = conductivity and R=resistivity. The unit of conductivity is the mho.

Proper grounding is essential for effective operation in a noise free environment. Improper grounding can lead to potentially dangerous ground loops and susceptibility to interference. To understand the principles involved in shielding and grounding, some terms must first be understood. A ground is a conducting flow path for current between an electric circuit and the earth. Ground wires are typically made with materials that have very low resistance. Because current takes the path of least resistance, the ground wires connected from the system provide a suitable path for electrical noise to ground. Ground wires also safeguard against unwanted common-mode signals and prevent accidental contact with dangerous voltages. Return lines carry power or signal currents. A ground loop is a potentially dangerous loop formed when two or more points in an electrical system are grounded to different potentials.

Multiple grounding rods from old installations.

Single grounding rod Proper grounding.

Grounding at the NID Proper grounding.

13 volts open ground. Can be hundred of volts.

Shot noise:

This type of noise occurs when the finite number of particles that carry energy is small enough to give rise to detectable statistical fluctuations in a measurement. These particles can be electrons in electronic circuits or photons in optical systems. The strength of this noise increases with the average magnitude of the current. As the signal increases more rapidly as the average signal becomes stronger, shot noise often is only a problem with small currents.

The strength of the current fluctuations can be expressed by giving the variance of the current I, <I> is the average (macroscopic) current.

ΔI² ≡〈 (I - 〈 I 〉 )² 〉 and the Fourier transform:
S (t) = (〈 I(t) 〉 I (0) 〉 - 〈 I (0) 〉² )

Thermal, Johnson-Nyquist Noise:

This type of electrical noise is generated when resistors or conductors heat up. Electic ranges, heaters, dryiers, water heaters, etc have resistors (heating element) that produce high levels of electrical noise. High adsl CRC error counts can be generated during the use of these devices. Johnson or thermal noise can be distinguished from shot noise which consists of additional current fluctuations that occur when a voltage is applied and a macroscopic current starts to flow. For the general case, the above definition applies to charge carriers in any type of conductor medium. Thermal noise is generated in resistors.

It can be modeled by voltage source in series with the noise generating resistor. The root mean square (rms) of the voltage, vn, can be calculated from:

v_n = √ 4k_B TR Δf

where k_B is the Boltzman's constant in joules per kelvin, T is the temperature of the resistor in kelvins, R is the resistor value in Ω ohms and Δf is the bandwidth in herz.

RF Noise:

Any unwanted reception of other forms of broadcasts is considered noise. Fadio frequency interference short for RFI or RF noise is a very common source of noise in bad shielded and grounded systems. Shielded and unshielded twisted pair cabling is considered to be very vulnerable to this type of noise.

This type of noise finds its way into the system in many different ways:

• As a result of a strong radio station nearby.

• Due to the lack of propper grounding in the system.

• As a result of unproper shielding in the system.

• Bad maintenance of the receiving system (bridge taps, loading coils, etc)

• Weak signal strength of the receiving system compared to the source of interference.

Impulse Noise:

Impulse noise is also called 'spikes' is the primary source of error in data communications. This type of noise can be identified in radios by the clicking or cracking noise it produces and can last as long as 2/100 of a second. This type of noise is very dirsruptive in digital data transmissions causing errors. This is a very common source of CRC/HEC errors in xDSL broadband systems.

Some sources of impulse noise are:

• Lighting discharges during thunderstoms.

• Abrupt voltage changes in surronding circuits and cables.

• Flourecent lights.

• Bad grounding and shiending.

• Maintenance equipment in the line.

• Other spheric anomalies/phenomena.

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