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Author Message   Tech237 Moderator Username: Tech237 Post Number: 1595 Registered: 4-2004 Posted on Friday, July 20, 2012 - 8:40 am:        RFI/EMI and Cable Systems Contra to popular opinion, TVs attached to Cable can experience interference like any other TV. Cable is a closed system until something goes wrong and leaves an opening for signal leakage. Cable can even be a source of interference. It is likely that you will take a call from a customer suffering from interference. This document is intended to enable you identify the issues as interference. Have a good idea of what sort of interference and direct the customer to the resolution. What is RFI/EMI interference? RFI is an unwanted signal, natural or manmade, that causes issues with receiving the intended signal. Manmade noise includes radio transmitters, electric motors, the TV, and thermostatic switches. Natural sources include lightning strikes, wind generated static, and solar noise. With care, these can be reduced if not completely eliminated. RFI stands for Radio Frequency Interference and EMI for ElectroMagnetic Interference. The two are pretty much interchangeable, and we’ll just refer to them as EMI. How Does EMI Propagate? EMI propagates by three methods, and any instance of EMI may use one or more of these methods. The methods are – direct radiation, conducted and inducted. Direct Radiation: This is where the interfering signal enters the affected device by being transmitted through the air. Although more commonly associated with radio transmitters, it can be caused by other sources. The crashing noise, heard on radios, from a lightning strike is an example of natural radiated interference. TVs are prone to issues caused by direct radiation for two main reasons: 1) TV’s are a broadband device, and they are capable of receiving signals over a wide range of frequencies. In turn, this means they are more susceptible to interference from strong nearby signals (called Front-end or Fundamental Overload). This can be defined as “interference to a receiver caused by a strong signal that overpowers the receiver’s RF amplifier.” (Courtesy ARRL) 2) To keep costs down, manufacturers leave out the shielding that blocks interfering signals. Direct radiation can show several different symptoms ranging from lines across a TV screen to voices out of the speakers. No matter the symptom, if it is Front-end overload the fix has to be at the affected device. The signal that is creating the issue is the signal the generating device is supposed to be creating. This is called its Fundamental frequency. The owner of the receiving device should to contact the manufacturer for assistance. FCC Rules Part 15 (http://www.fcc.gov/encyclopedia/rules-regulations-title-47) has information concerning signals received that are not part of normal operations. AM/FM radios, amplifiers and even phones can also be subject to front-end overload problems. Some examples of radioed Interference: Black or white screen with flashes of color (TV). This is typical of front-end overload. The colour flashes are in synch with the transmitter’s modulation. It is commonly noticed on the lower channels. Sometimes, this is accompanied by garbled sound from the speakers. This can be reduced or stopped by installing a good quality High Pass Filter close to the TV’s antenna/cable connector. Inside the TV right at the tuner is better still, but best left to a professional technician to install. This only works, if the generating signal is lower than the TV’s frequency coverage. There are two ham bands that may cause front-end overload issues (even with a TV connected directly to a Cable System). These are the 2m (144 MHz to 148 MHz) and 6M (50 MHz to 54 MHz) bands. Several channels (cable channel 18 for example) are either in or just on the edges of these bands, and are used by both Cable and Over the Air TV providers. The reduction of front-end overload in these bands may require some special filters that remove a very narrow band of frequencies while leaving the desired ones untouched. These filters (called Stub Trap) may need to be homemade and adjustable. (See: http://www.radagast.org/~dplatt/hamradio/K6OIK-filters.pdf (heavy technical) for more information) Harmonics from HF radios may also cause direct radiated interference. If this is the cause, the first step would be to add a properly installed Low Pass filter as close to the radio as possible. A Herringbone Pattern Over the Picture. This form of interference is generated by an FM transmitter, and maybe be from Broadcast Stations, Amateur Radio Stations and even Commercial Two-Way radios. Again better shielding and RF traps at the TV are the cure. Shimmering Picture (sometimes with a ghost). Now this type of interference is often caused by a moving object reflecting a weaker version of the channel or nearby signal to the TV. By nature it is usually of a relatively short duration (minutes). The ‘shimmering” is due to the reflected signal strength and phase relationship (how much later than the real signal) of it reaching the TV, changing as the reflecting object moves. Airplanes are one cause of this type of interference. The ghost appears when; the reflected signal is the same signal as the one intended to be received, but is received slightly behind the direct signal. This mainly affects TV’s on antennas, but may also affect the signals that we pick up via satellite. In fact once or twice, we may hear of this issue due to aircraft running flight and communication tests in the area. Strange Sounds from an Audio Amplifier: Sound systems can also suffer from Front-end Overload. Mainly due to the systems being built to a price and the protection needed is left out. The usual symptom sounds like distorted sound (SSB signals) or a soft hissing sound that follows a rhythmic pattern (Morse code). Again applying filters to the input of the radio and/or amplifier is the first step. We need to select a filter that will allow a narrow band of frequencies through while blocking those outside of that bandwidth. These are called Bandpass Filters. Direct the customer to a qualified technician to install these. If the problem is poor shielding, refer to the equipment manufacturer or qualified technician. Phones often experience interference from nearby AM Broadcast Stations and sometimes from SSB transmitters in the HF bands (Ham and CB). Sometimes this interference is an issue with the phone(s) installed, sometimes with wiring (any unused pairs in the phone cable should be grounded), and sometimes an issue at the stations transmitter. Technicians from the phone company and the radio station need to work together to resolve this type of interference. Often an RF trap fitted near or even in the phone (by qualified technician) will solve the issue, and other times more extensive cures are needed. Leads at Resonant Lengths: Long leads can also pick up radiated signals. Filters can be fitted to block this signal. Be careful, as some filters have a capacitor at the input and this can cause transistorized amplifiers to be damaged. To reduce this type of EMI, make all leads as short as possible, and avoid resonant lengths. You can wind the lead around a ferrite core forming a trap. These options may take some trial and error to get the best results. Sometimes shortening the lead may not be possible. Try adding an extra 5 or 6ft of cable to change the cable length away from the resonant length. Issues With Other devices: Many other devices (such as touch lamps, phones) are prone to radiated interference. Touch lamps are also a source of radiated interference due to the oscillator that is used to sense when the lamp is touched. All devices are treated the same – fitting AC Line filters in the power cables, and common –mode chokes on any control and power leads. Fit filters and chokes as close to the device as possible. Other devices that can cause direct radiated interference are anything in the home with a thermostat (room heaters, fish tank heaters, air conditioner, etc) or other form of switch (e.g. door bells). As the switch opens and closes sparks can be created between the contacts, which appear as short duration lines on the TV and pops in audio systems. Usually replacing the offending device, cleaning its contacts and/or fitting a capacitor across the contacts (like across an older car’s points) will resolve this cause. Other Instances of Radiated Interference: Recently, I handled an interference issue where a Cable Channel was interfering with an Amateur Radio Net on the 6m Band (50 to 54MHz). It was determined that a stress fracture in the aluminum shield right at the connector was allowing the leakage. A “quick” repair resolved the issue. A person, who lived near the transmitter for an AM Broadcast Station, was having issues with his fluorescent lights flickering even when turned off. The Antenna for this station was actually on his property, and RF was getting into power circuits. He needed to add chokes in all his wiring and capacitors to ground at each light fixture. Finally, a person was having very poor TV reception, despite having line of sight to the transmitter approximately 1 mile away. A test using a portable TV on Rabbit’s ears” produced great reception. Unplugging the coax to his TV created a nice healthy spark. Measuring his coax showed a high AC voltage at 50 Hz on it. Looking at his antenna it was observed that 33KVA Power lines ran less than 100ft away. Placing a 5uF (what we had available at the time) in series with the center conductor, removed the AC voltage and improved his reception 1000%. I surmise that the AC Voltage was causing the AGC (Automatic Gain Control) to shut the TV’s RF Amplifier down, which reduced the level of the wanted signal. Conducted Interference: This interference is conducted along the cables and leads attached to the device having the interference problems. Alternator whine (that high pitched whine on a weak radio station that varies with engine speed) is conducted interference travelling along the power leads in the car. AC hum from a mains power supply is another example of conducted interference. Sometimes RF from a transmitter can be conducted by its power cable to other devices. For maximum reduction of the EMI, steps need to be taken at both ends of the problem. A capacitor connected from the “live” lead to ground is a good start. Make sure the rated voltage is at least 1.5 times the applied voltage. • Use the shortest leads possible. • Often this capacitor is fitted in series with a large value resistor. A common-mode filter may also be helpful. • This consists of a toroid (donut) core with the power lead wrapped around it. Generally it is best to wind half the toroid clock-wise, and the other hall counter-clockwise. A good quality commercial filter is recommended for AC Mains leads. Make sure all grounds within a system are connected together with the shortest, largest gauge conductors possible. • Ground Loops (two or more grounds that have a voltage difference between them) are a form of conducted interference. In most cases of conducted interference getting assistance from a qualified technician is advisable. Inducted Interference: Inducted Interference occurs when one conductor causes a voltage to be created in a nearby conductor. Any unshielded or poorly shielded cable can be both a source and recipient of Inducted Interference. • A poor quality coax can induce a voltage into another nearby poor quality coax, producing interference to the device attached. • This interference can further be rectified in a radio and sent back out as Radiated Interference. • Often this is caused by RF flowing down the outside of the braided shield. • To reduce inducted interference, several steps can be taken. • Use only well shielded cabling. • In certain cases it works better if the shield is grounded at one end only, especially if it is shielding twisted pair cabling. • Try to keep parallel cable runs to a minimum, and well separated. • If cables need to cross each other make that crossing as near 90° as possible. • If two conductors run to the same device twist the conductors around each other (I try for 4 turns per inch). • This helps cancel unwanted signals and noise. • The number of twists per foot will vary depending on the type and gauge of the conductors – the smaller the conductors the more twists per inch you can do. If the above steps have not resolved the issue we would most likely need to have a qualified person investigate this interference. Notice, in the above we had a situation where one type of interference could create a second type. This may make resolution of the issues a little harder. An occurrence of interference may have more than one cause. To find resolution, each cause will need to be treated. This document is not a complete source when dealing with interference. To make it so, would turn it from a quick reference to a major book. If you want to learn more there are several good books including The ARRL’s Interference Handbook, and Interference Handbook by William Nelson. Tech237 N7AUS God made me an athiest, who are you to question his wisdom?   Tech237 Moderator Username: Tech237 Post Number: 1741 Registered: 4-2004 Posted on Tuesday, February 11, 2014 - 10:59 am:        Thanks Chet for posting this here for me, and trying to fix the AWOL formatting. Tech237 N7AUS God made me an athiest, who are you to question his wisdom?