A Ham S Guide To Rfi Ferrites Baluns And Audio-Books Pdf

A Ham s Guide to RFI Ferrites Baluns and Audio
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Understanding and Solving RF Interference Problems Page 2. close to resonance will work better than one that is not Long antennas tend to pick up more RF. than short ones Think about these fundamental principles when trying to diagnose which cables. are bringing your RF into a given system or radiating their trash into your receiving antenna. A path to ground or the power system is not always needed to produce antenna action The whip. antenna on our VHF and UHF handheld radios uses the radio capacity coupled to our hand that. holds it as a counterpoise that is to provide the other half of the antenna All that is required. for this to work is that the size of the counterpoise must be a significant fraction of a quarter wave. or larger so that it can sink the antenna current, Common Mode and Differential Mode Signals A differential mode signal is one that exists be. tween the conductors of a cable At any given point along the cable current flowing on one con. ductor is precisely balanced by current flowing in the other direction on the other conductor The. intentional signals carried by cables are differential mode signals the audio or video signal in a. home audio system Ethernet signals on CAT5 6 cable and the RF signal carried by the feedline. connecting our antennas to our transceivers, A common mode signal is one that places equal voltage on all conductors that is the voltage be. tween the two ends of the cable are different but there is no voltage between the conductors An. tenna action produces a common mode voltage and current along a cable The antenna current. induced on audio and video wiring is a common mode signal That is with ideal cable there is no. differential voltage between the signal conductors as a result of this antenna action If the cable is. shielded nearly all of this current flows on the shield and skin effect causes it to flow on the out. side of the shield If the shield is ideal that is if the current is distributed with perfect uniformity. around it the field inside the shield will be zero and thus none of this antenna current will flow. inside the cable Conversely when a cable shield is carrying differential mode current as in the. case of coax skin effect will cause that differential mode current to flow on the inside of the shield. The real world is not ideal so most interfering signals will simultaneously exist in both common. mode and differential mode but in most real world conditions one or the other mode dominates. Several cable defects essentially manufacturing tolerances certainly can and do convert this. common mode antenna current to a differential signal that is a voltage between the signal con. ductors but that is rarely the most powerful coupling mechanism One common defect that af. fects both balanced and unbalanced cables is imperfect construction of cable shields In even the. best real world balanced twisted pair cables there are imbalances in the capacitance between. red and black conductors to the shield on the order of 5 B Whitlock JAES June 1995 In. balanced paired cables that use foil drain shields there is even more imbalance in the inductive. coupling between each conductor and the shield Noise or RFI coupled by this mechanism is. called shield current induced noise or SCIN N Muncy JAES June 1995 All three of these. mechanisms convert shield current to a differential signal at system input and output terminals. If the cable is an unshielded pair loudspeaker cable for example RF will be induced approxi. mately equally on both conductors but depending what the input circuit of the equipment looks. like at RF current flow into the equipment may not be equal on both conductors This can also. produce a differential voltage at the input or output terminals. Output Wiring is Important Too It is well known for example that RF interference is often cou. pled into the output stage of audio equipment for example the power amplifiers that feed loud. speakers or headphones There is always feedback around that output stage so RF present at the. output will follow the feedback network to the input of a gain stage where it will be detected and. amplified This problem is made much worse when parallel wire cable zip cord is used to feed. the loudspeakers or headphones and can usually be solved simply by replacing the zip cord with a. twisted pair of POC plain ordinary copper Pseudo scientific advertising hype for exotic cables. notwithstanding it was shown nearly 30 years ago that 12 copper twisted pair or 10 for very. long runs is a nearly ideal loudspeaker cable R A Greiner Amplifier Loudspeaker Interfacing. JAES Vol 28 Nr 5 May 1980 As we will discuss later the twisting of a pair greatly reduces the. level of RF that the wiring couples to circuitry, Power Supply and Control Wiring can also act as antennas When I bought the house I owned in. Understanding and Solving RF Interference Problems Page 3. Chicago I upgraded the electrical wiring and put all of it in steel conduit EMT This shielded the. wiring so that only the short power cords between equipment and the wall outlets could act as. antennas The house I recently bought in California is wired with no conduit using unshielded. parallel conductors Thanks to its length and the fact that it is not shielded this wiring acts as an. effective receiving antenna for the RF I am transmitting and an effective transmitting antenna for. the RF trash generated by computer equipment power supplies for low voltage lighting fixtures. and even battery chargers, Current Returns to its Source Current flows in a complete circuit that includes the source of the. current The circuit will couple noise inductively and also by antenna action The cause of many. RFI and noise problems as well as the solution to them lies in identifying and controlling these. circuits Always ask Where does the noise or RFI current flow. Loop Area One of the most fundamental laws of electrical circuits is that the current that is mag. netically induced between two circuits is proportional to the loop area of each circuit Making the. loop area small also minimizes the extent to which the wiring can act as an antenna When we use. a closely coupled pair of conductors to form a transmission line we are reducing the loop area. which reduces the total current induced in the loop by an interfering signal and the total magnetic. field produced by current in that loop The transmission line of course has other useful proper. ties More on this later on, The equipment designer can also use multilayer printed circuit techniques to place a ground ref.
erence plane next to all signal wiring turning each circuit trace into an unbalanced transmission. line where the return current is carried on the reference plane under wiring A single reference. plane makes a very large reduction in the ability of that circuit trace to receive interference sand. wiching it between two such planes virtually eliminates it These techniques called microstrip one. plane or stripline the sandwich are widely used by better designers They reject noise coupling. both inside and outside the equipment by drastically reducing the loop area of the current path. and have the additional benefit of making high speed data circuits behave better because they are. transmission lines, Loaded Words That Cause Misunderstandings One of the most overused and misunderstood. words in electronics is ground or earth in British English There are several important and. common uses of the words One meaning is an actual connection to mother earth Some common. earth connections include the steel structure of a building a buried conductive water pipe a con. crete encased grounding electrode called a Ufer after its inventor Herbert Ufer and of course. one or more conductive rods driven into the earth Concrete mixes vary widely in their conductiv. ity most we are likely to encounter are highly conductive but some are effective insulators The. primary function of this earth connection is lightning protection. A second common use of the word ground or earth in British English is a third conductor that. is part of the power system wiring that should never carry current except in the case of a fault but. connects the conductive enclosures of equipment to a common point within the power system. This green wire or third pin in the outlet in North American power systems is called the equip. ment ground or protective earth in British English The green wire is required to be connected. to all exposed conductive parts of electrical equipment that might be energized in the event of. equipment failure The purpose of this connection is to provide a sufficiently robust current path. that a fuse will blow or circuit breaker will trip in the event of equipment or wiring failure that. causes the chassis to be hot thus protecting people from electrical shock and preventing fires. A third common use of the word ground or earth in British English is to describe circuit com. mon or circuit reference within equipment Circuit common should nearly always be connected. to the power supply reference and to the shielding enclosure of the equipment If the source of. noise is within equipment circuit common is reference for the noise voltage and current and it is. the point to which that noise current wants to return. A fourth common use of the word ground is as the return for an unbalanced antenna like a ver. tical or long wire In this application the antenna needs some conductor to be a low impedance. sink for the antenna current The radials for an elevated or ground mounted vertical antenna. Understanding and Solving RF Interference Problems Page 4. serve this function There is an excellent discussion of this in http w2du com Chapter05 pdf. Ground Wiring Some hams like to think of the earth as if it were somehow a sink into which all. noise can be poured never to bother us again Indeed you ll find lots of bad advice to solve RFI. problems with a better ground In fact nothing could be further from the truth An earth connec. tion is rarely part of a solution to RF or noise problems Rather we need a better understanding of. the four common meanings of ground that ground is not a single point and that connections. between them change current paths There are exceptions to every rule and this no exception. See Shunting Common Mode RF to Earth later in this tutorial. Consider a noise filter hanging between some piece of noisy equipment and the power line with. capacitors from the hot and neutral to ground What is that ground It is circuit common and. the shielding enclosure of the equipment the green wire in the power cord which is connected to. the equipment ground in the power system which goes to the breaker panel which is in turn. bonded to neutral and a real earth connection at the service entrance to our building and if we ve. done it right there should be a bond between the power system ground and any grounds we ve. added for our radio equipment In most systems the green wire follows a rather long path typi. cally a quarter wave on 80 meters and perhaps even on 160 meters That current path is an an. tenna and any RF current flowing on that conductor will radiate In fact the connection to earth. may increase current flow Like any other radiated RF signal our receiving antennas will hear it All. of those ground connections must be present to have a safe installation but it is the combination. of the high series impedance of the filter s choke and the connection between the filter s ground. and the shielding enclosure of the equipment and it must be very short that suppresses the noise. The earth connection provides lightning safety, This basic scientific fact has major implications in the design of filters intended to prevent noise. coupling from noisy equipment to our ham stations If we add a filter to wiring that enters or. leaves a piece of noisy equipment it is the shielding enclosure for that equipment to which any. ground of our filter should return and of course circuit common should also be connected to. that shielding enclosure All connections between the filter and the noisy equipment should be as. short as possible what my old EE professors liked to call zero length to emphasize the impor. tance of making them short Why First to minimize the loop area and thus the inductance. Second to minimize antenna action More about this when we discuss specific filter designs. Insufficient Input and Output Filtering As hams we know that equipment needs good input and. output filtering to prevent RF from coming in on input and output wiring Beginning in the 1950 s. hams operating the HF bands were deluged with TVI complaints because television manufacturers. failed to include high pass filters in their sets Likewise audio equipment needs good low pass fil. tering to reject our signals Many myopic designers of high futility audio gear and even some. professional gear don t include low pass filters because they don t want to degrade the phase re. sponse of the audio path While good phase response is certainly important so is RF rejection. Good engineering can satisfy both needs without compromise Ever since those early days hams. have always assumed that a good low pass filter will kill RFI in audio systems and a good high. pass filter will kill interference to FM and TV Unfortunately while good filtering is important other. mechanisms are far more important in most real world situations. Shield Resistance adds hum and buzz to unbalanced wiring audio. Understanding and Solving RF Interference Problems Page 2 close to resonance will work better than one that is not Long antennas tend to pick up more RF

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