Electromagnetic Interference Testing | Troubleshooting Techniques

Troubleshooting Techniques

Electromagnetic : Some of the main duties of a communication expert are to troubleshoot, service, and maintain communication equipment. Most communication equipment is relatively reliable and requires little maintenance. However, equipment does fail. Most electronic communication equipment fails because of on-the-job wear and tear. Of course, it is still possible for equipment to fail as the result of component defects. The equipment might fail eventually because of poor design, exceeding of product capabilities, or misapplication. In any case, you must locate such failures and repair them. This is where troubleshooting techniques are valuable. The goal is to find the trouble quickly, solve the problem, and put the equipment back into use as economically as possible.

General Servicing Advice

One of the main decisions you must make in dealing with any kind of electronic equipment is to repair or not to repair. Because of the nature of electronic equipment today, repairing it may not be the fastest and most economical approach.

Assume that you have a defective radio transceiver. One of your options is to send the unit out for repair. Repair rates run anywhere from $50 per hour to over $100 per hour depending upon the equipment, the manufacturer, and other factors. If the problem is a difficult one, it may take several hours to locate and repair it. Many communication transceivers are inexpensive units that may cost less to buy new than to repair.

There are two types of repair approaches: (1) replace modules or (2) troubleshoot to the component level and replace individual components. Some electronic equipment is built in sections or modules. The module is, in most cases, a separate PCB containing a portion of the circuitry inside the unit. The typical arrangement might be for the receiver to be on one PCB, the transmitter on another, and the power supply on still another, with another unit such as a tuner or frequency synthesizer also separate. A fast and easy way to troubleshoot and repair a unit is to replace the entire defective module.

If you are a manufacturer repairing your own units in volume for customers, or if your organization does many repairs of a similar nature on a particular brand or model of equipment, repair at the component level is the best approach.

Common Problems

Many repairs can be made quickly and easily because they result from problems that occur on a regular basis. Some of the most common problems in communication equipment are power supply failures, cable and connector failures, and antenna troubles.

Power Supplies

All equipment is powered by some type of dc power supply. If the power supply doesn’t work, the equipment is completely inoperable. Therefore, one of the first things you should do is to check that the power supply is working.

If the unit is used in a fixed location and operates from standard ac power lines, the first test should be to check for ac power and the availability of the correct dc power supply voltages. Is the unit plugged in, and if so, does ac power actually get to the outlet? If ac power is indeed available, check the power supply inside the unit next. These power supplies convert ac power to one or more dc voltages to operate the equipment. Open the equipment and, using the manufacturer’s service information, determine the power supply voltages. Then use a multimeter to verify that they are at the correct levels.

Most power supplies these days are regulated, and therefore the voltages should be very close to those specified, at least within 65 percent. Anything outside that range should be suspect. Any voltages that are obviously quite different from the specified value indicate a power supply problem.

Another common power supply problem is bad batteries. With continuous usage, batteries quickly run down. If primary batteries are used, the batteries must be replaced with new ones. If secondary or rechargeable batteries continue to fail even after short periods of use after charging, it means that they, too, should be replaced. Most rechargeable batteries can be charged and discharged only so many times before they are no longer effective.

Cables and Connectors

. Perhaps the most common failure points in any electronic system or equipment are the mechanical components. Connectors and cables are mechanical and can be a weak link in electronic equipment. Once it has been confirmed that the power supplies are operating correctly in the equipment, the next step is to check the cables and connectors. Start by verifying that the connectors are correctly attached.

Another common problem is for the cable attached to the connector to break internally. Most of the time the cable does not break completely, but one or more wires in the cable may be broken while others remain attached.

Occasionally connectors get dirty. Removing the connector and cleaning the connections often solve the problem. It may be necessary to replace the connectors to ensure a reliable physical connection, however.

Documentation

Before you begin any serious detailed troubleshooting and repair of communication equipment, be sure that you have all the necessary documentation. This includes the manufacturer’s user operation manual and any technical service manuals that you can acquire. Nothing speeds up troubleshooting and repair faster than having all the technical information before you begin. Manufacturers often regularly identify common problems and suggest troubleshooting approaches. But perhaps most important, manufacturers provide specifi cations as well as measurement data and procedures that are critical to the operation of the equipment. By having this information, you will be able to make the necessary tests, measurements, and adjustments to ensure that the equipment complies
with the specifi cations.

Troubleshooting Methods

There are two basic approaches to troubleshooting transmitters, receivers, and other equipment: signal tracing and signal injection. Both methods work equally well and may often be used together to isolate a difficult problem.

Signal Tracing

A commonly used technique in troubleshooting communications equipment is called signal tracing. The idea is to use an oscilloscope or other signal detection device to follow a signal through the various stages of the equipment. As long as the signal is present and of the correct amplitude, the circuits are good. The point at which you lose the signal in the equipment or at which the signal no longer conforms to specifications is the location of the problem.

To perform signal tracing in a transmitter, you need some type of monitoring or measuring instrument: an RF voltmeter or an oscilloscope, an RF detector probe on an oscilloscope, a spectrum analyzer, and power meters and frequency counters. Fig. 22-28 is a block diagram of a dual-conversion communications receiver. To troubleshoot this receiver, you need an RF signal generator to supply a low-level input to the antenna terminals. Modulate the input with an audio signal. Then using an oscilloscope, spectrum analyzer, or another instrument, follow the signal through the various stages of the receiver. This procedure assumes that the power supplies are working and all other circuits get the correct DC voltages.

As a first step, check the output of the local oscillator or frequency synthesizer to verify that a signal is reaching the first mixer. If the input signal passes through the input RF LNA, check the output of the mixer to be sure the correct IF is obtained. Then trace the signal through any IF filters or IF amplifier stages to the second mixer. Be sure the local oscillator is providing a signal to the second mixer. Then test to see if the correct second IF appears at the mixer output.

The demodulator may be a DSP in a modern receiver. Here all you can do is a test for digital output and output from any DAC. If audio stages are involved, test them next. If you modulate the RF input signal with an audio signal, you may hear the output on the speaker. Otherwise, trace the audio signal through any audio and power amplifi er stages.

If you encounter a problem, the trouble is probably a bad transistor or IC. Active components such as transistors and ICs fail more often than passive components such as resistors, capacitors, and inductors or transformers. You should verify bias voltages to determine that they are correct; but if they are and the circuit still does not operate, usually there is an open or short-circuited transistor.

Electromagnetic Interference Testing | Troubleshooting Techniques

Other components may also fail. Capacitors fail more often than any other type of component except semiconductor devices. Resistors are less likely to fail but can open or change in value. Inductors rarely fail. Delicate, sensitive components such as crystal, ceramic, and SAW filters can also break. Once you isolate the problem to a particular component, turn off the power, replace the suspected part, and repeat the tests. Continue testing until the receiver works.

Signal Injection

The signal injection is somewhat similar to signal tracing. It is normally used with receivers. The process is to use signal generators of the correct output frequency to inject a signal into the various stages of the receiver and to check for the appropriate output response, usually a correct signal in the speaker.

The signal injection is the opposite of signal tracing, for it starts at the speaker or other output and works backward through the receiver from output to antenna. If the output is audio, signal injection begins by testing the audio power output amplifi er. You inject a 1-kHz sine wave from an audio oscillator or function generator into the input of the amplifier. Follow the receiver documentation’s information with regard to how much signal should be present at the outputs of each stage. If an audio signal is heard, the speaker and power amplifi er are OK. Then inject the audio signal into any other audio amplifier stages back to the demodulator circuit output.

The next injection takes place at the input to the IF amplifier or the second mixer if the receiver has one. Set the RF signal generator to the IF with appropriate audio modulation. You should hear the audio tone in the speaker. If you do not, check the local-oscillator and IF stages. Do not overlook the demodulator. Keep in mind that you may not actually be able to get to the inputs and outputs of some circuits because they may be contained within an IC. If this is the case, you can restrict the injection to the IC input while monitoring the output. If you get no output, replace the IC.

Next, inject a signal of any higher IF with modulation at the input to the first mixer. You should hear the tone; but if you do not, inspect and test the first IF stages and local oscillator. Finally, test the RF amplifier with a signal at the receive frequency.

Modern Troubleshooting

Although signal tracing and signal injection are valid troubleshooting approaches, they are more difficult than ever when applied to a modern communications device. Examples are cell phones and wireless LAN transceivers or line cards in a router. These devices use only one or more ICs to implement the entire signal chain. It is not possible to access the internal circuits of the chips, making the procedures described previously impossible, if not more difficult. These techniques have limited use in such highly integrated devices, although the principles are still valid.

Boundary Scan and the JTAG Standard

Modern networking and communications equipment are made up primarily of digital ICs. These are large-scale devices made for surface mounting on a printed circuit board (PCB). It is diffi cult and sometimes not possible to access the pins on an IC, making signal tracing or injection worthless in
troubleshooting. Realizing this, IC designers eventually figured out a way not only to test the chip after it is made and packaged but also to test the equipment built with these chips. This method is called boundary-scan.

The purpose of boundary-scan is to provide a way to observe test points inside the chip that are not normally accessible and to observe signals at IC pins inaccessible because of their surface mounting on a PCB. And it provides a way to apply test signals to selected points in the circuit and then to monitor the results. Fig. 22-29 shows the boundary scan circuitry that is built into many complex ICs. The logic circuitry to be tested is in the center and referred to as core logic. Surrounding the core is a series of boundary-scan cells (BSCs). Each BSC is made up of a pair of flip-flops and some multiplexers that can receive inputs and store them or output their contents when interrogated.

The BSC flip-flops are also connected to form a large shift register, the boundary scan register. This register provides the basic storage unit for inputting and outputting data. The input and output are serial. Serial data can be input into the register via the TDI line.

Electromagnetic Interference Testing | Troubleshooting Techniques

Data stored in the register can be read out serially by monitoring the TDO line. An external clock signal TCK and a test mode select (TMS) signal control the serial data speed through the test access port (TAP) controller. The function of the boundary scan circuitry is determined by a binary-coded instruction that is entered serially.

To read the data at the internally monitored points, the boundary scan circuits are fed a serial instruction called INTEST. This is stored in the instruction register and decoded. The internal circuits are then configured by the logic to read the data from the core logic and store it in the BSCs. The data in the boundary scan register is then shifted out and read serially. This serial data is generally sent to a computer where a program uses it to determine if the results are correct.

Other instructions may also be entered to control the various functions. A SAMPLE/ PRELOAD instruction is used to set the core logic to some desired state before a test is run. The CLAMP instruction can also be used to set the core logic levels to some predetermined pattern. The RUNBIST instruction causes a predetermined, built-in self-test (BIST) to be executed.

The boundary scan circuitry and its operation have been standardized by the Institute of Electrical and Electronics Engineers (IEEE) and is designated as the Joint Test Action Group (JTAG) 1149.1 standard. Most large complex ICs now contain a JTAG interface and internal circuitry. It is used to test the chip. Then later it may be used by the equipment manufacturer as a way to implement a larger test program for the equipment. Because a computer is programmed to use the JTAG interface, most testing and troubleshooting can be automated. An engineer or technician can actually sit in front of a computer, running the test software and exercise and test the equipment and monitor the results.

Electromagnetic Interference Testing

A growing problem in an electronic communication is electromagnetic interference (EMI). Also known in earlier years as radio-frequency interference (RFI) and TV interference (TVI), now EMI is defined as any interference to a communication device by any other electronic device. Since all electronic circuits and equipment emit some form of EMI, they are potential sources of interference with sensitive communication devices such as cell phones, cordless phones, radio and TV sets, pagers, and wireless LANs. As the number of computers has increased and as more and more cell phones and other wireless devices have come into use, the EMI problem has become a major one. The problem is so great that the FCC has created interference standards that must be met by all electronic devices. Before any electronic equipment can be sold and used, it must be tested and certified by the FCC to ensure that it does not emit radiation in excess of the allowed amount. Such strict rules and regulations have made design and manufacturing more difficult and have increased the cost of electronic products. But the result has been fewer interference problems between electronic products and a reduction in disruption of their use.

Today, a large part of the work of a communication technician or engineer is EMI testing and EMI minimization in products. This section briefly describes EMI, its sources, common techniques for reducing EMI, and EMI testing procedures.

Sources of Electromagnetic Interference

Any radio transmitter is a source of EMI. Although transmitters are assigned to a specific frequency or band, they can nevertheless cause interference because of the harmonics, intermodulation products, or spurious signals they produce.

Receivers are also a source of EMI. A local oscillator or frequency synthesizer generates low-level signals that, if not minimized, can interfere with nearby equipment. Almost all other electronic devices can also generate EMI. Perhaps the worst offenders are computers. Computers contain millions of logic circuits that switch off and on at rates up to and in excess of 2 GHz. Because of the short pulses with fast rise and fall times that are generated, these circuits naturally generate a massive number of harmonics. The problem is so severe that computer manufacturers must comply with some of the FCC’s toughest EMI reduction rules. Computer manufacturers use every trick in the book to get the radiation level down to the FCC specifications.

Of course, any electronic equipment that uses an embedded microcontroller, which today is almost every electronic device, is a potential source of EMI. Another major source of EMI is switching power supplies. More than 80 percent of all power supplies in use today are of the switching variety, and this percentage is increasing. Power supplies that use switching regulators generate very high levels of pulse energy and radiate many high-amplitude harmonics. Inverters (dc-to-ac converters) in uninterruptible power supplies (UPS) and dc-to-dc converters also use switching methods and therefore produce harmonics and radiation.

The 60-Hz power line is another source of interference. Electrical transient pulses caused by high-power motors and other equipment turning off or on can disrupt computer operation or create a type of noise for communication equipment. Just the large magnetic and electric fields produced by the ubiquitous power lines can cause hum in stereo amplifiers, noise in sensitive medical equipment, and interference in communication receivers.

Another form of EMI is electrostatic discharge (ESD). This is the dissipation of a large static electric field. Lightning is the perfect example. The huge pulse currently produced by lightning generates an enormous number of harmonics that show up as noise in radio receivers. Anyplace where static buildup occurs can produce ESD, which not only can destroy integrated circuits and transistors but also can generate pulses that manifest themselves as noise in a receiver.

EMI is transmitted between electronic devices by several means. Interfering signals may travel by way of electromagnetic radiation (radio) from one unit to another. EMI may also be passed along by inductive or capacitive coupling when two units are close to each other. Cross talk on adjacent cables is an example. And interfering signals may be passed along from one piece of equipment to another by way of the ac power line that both uses as the main power source.

Reduction of Electromagnetic Interference

The three basic techniques for reducing the level of EMI are grounding, shielding, and filtering. All these methods are used in the design of new equipment as well as in reducing EMI in applications in which the equipment is already deployed. Here is a brief summary of the techniques most often used.

Grounding

A poor electrical ground often causes EMI. As you know, the ground is the common reference point for most, if not all, voltages in a circuit. This ground shows up in many different physical forms. It may be a metal chassis or rack, the metal frame of a building, or water pipes. The best ground is an earth ground formed when a long copper rod is driven into the ground. Inside the equipment, the ground is formed on the PCBs on which the components are mounted. The ground is usually a wide copper strip or in some cases, a broad copper ground plane formed on one side of the PCB.

In the design of equipment, especially RF circuits, the ground is a key consideration. Much care is taken informing it, routing it, and connecting to it. Short, wide, and very low-resistance connections are the best. In equipment that uses both analog (linear) and digital circuits, separate grounding paths are usually formed for each type of circuit. This helps minimize interference to the sensitive analog circuits by the noisier digital circuits.

The two ground systems are eventually connected, of course, but at only one point in the equipment.

Circuit grounds in equipment are in place and cannot be changed. But it is possible for the grounding connections of different pieces of equipment in a system to cause some form of EMI. Many times, the EMI can be eliminated or greatly reduced by simply experimenting with different ground arrangements. These are some useful guidelines:

  1. If a piece of equipment does not have ground, add one. A connection to a large common ground is preferred, especially one connected to the earth ground.
  2. Ground connections should always be kept as short as possible. Ground wires should be no longer than λ/4 at the highest frequency of operation.
  3. Ground cables should be large and have low resistance. Stranded copper wire of a size greater than AWG 10 is preferred. The size should be greater if the ground carries a large current. Copper braid as wide as practical also makes a good ground connection.
  4. If multiple pieces of equipment are involved and signals are passed from one unit to another, ground loops may exist. A ground loop is formed when multiple circuits or pieces of equipment are connected to common ground but at different points. See Fig. 22-30. Current flow in the ground connection can produce a voltage drop across a part of the ground. That voltage then shows up in series, with the very small signals at the inputs to other circuits or equipment causing interference. Ground loops are eliminated by connecting all circuits or equipment to a single point on the common ground. See Fig. 22-31.
  1. EMI is often caused by the incorrect connection of coaxial cable shields. The shielding braid may be broken or open at one of the connectors. In some applications, grounding only one end of the shield instead of at both ends of the cable reduces EMI. This is also true of any shielded wire. Experimentation with the cable shields and grounds can often reduce EMI.
  2. Remember that all ac power connections have a ground associated with them. In two-wire ac connections, the neutral wire is grounded at the entry point of the alternating current. In three-wire systems, the third wire is also a ground used primarily for safety purposes. However, the lack of a third ground can also cause interference.
    Adding the third ground wire often solves EMI problems.
Electromagnetic Interference Testing | Troubleshooting Techniques

Shielding

Shielding is the process of surrounding EMI-emitting circuits or sensitive receiving circuits with a metal enclosure to prevent the radiation or pickup of signals. Often, just placing a metal plate between circuits or pieces of equipment to block radiation is sufficient to reduce or eliminate EMI. The metal reflects any radiated signals and can actually absorb some of the radiated energy.

Almost all communication equipment is made with extensive use of shielding. Oscillators and frequency synthesizers are almost always packaged in a shielded can or enclosure. Individual transmitter or receiver stages are often shielded from one another with blocking plates or completely surrounding enclosures. Switching power supplies are always shielded in their own enclosure. Just remember that when shielding is used, ventilation holes are usually necessary to release any heat produced by the circuits being shielded. These holes must be as small as possible. If the holes have a diameter that is near λ/2 of the signals being used, the holes can act as slot antennas and radiate the signals more effectively. A good initial design prevents this problem.

In some cases, RF signals leak from shielded enclosures where the various panels of metal come together. A continuous shield is the most effective, but most shielded boxes must have a removable panel or two to permit assembly and repair access. If the panels are not securely mated with a low-resistance contact, RF will leak out of the opening. Securely attaching panels with multiple screws and making sure that the metal is not dirty or oxidized will solve this problem. If not, special fl exible metal seals have been designed to attach to enclosures and panels that must mate with one another. These seals will eliminate the leakage.

Finally, radiation EMI can sometimes be reduced in a larger system by moving the equipment around. By placing the offending units farther apart, the problem can be eliminated or at least minimized. Remember that the strength of a radiated signal varies as the square of the distance between the transmitting and receiving circuits. Even a short-distance move is all that may be needed to reduce the interference to an acceptable level.

Filterin

The third method of EMI reduction is filtering. Filters allow desired signals to pass and undesired signals to be significantly reduced in level. Filters are not much help in curing radiation or signal coupling problems, but they are a very effective way from one circuit or piece of equipment to another by actual physical conduction over a cable or other connection.

Some types of fi lters used in reducing EMI include

  1. Bypass and decoupling circuits or components used on the dc power supply lines inside the equipment. Typical decoupling circuits are shown in Fig. 22-32. Instead of using a physical inductor (usually called a radio-frequency choke, or RFC) in series with the dc line, small cylindrical ferrite beads can be placed over a wire conductor to form a small inductance. These beads are widely used in high-frequency equipment. The bypass capacitors must have a very low impedance, even for RF signals; i.e., ceramic or mica capacitors must be used. Any plastic dielectric or electrolytic capacitors used for decoupling must be accompanied by a parallel
    ceramic or mica to make the filtering effective at the higher frequencies.
Electromagnetic Interference Testing | Troubleshooting Techniques
  1. High- or low-pass filters used at the inputs and outputs of the equipment. A common
    an example is a low-pass filter that is placed at the output on most transmitters to
    reduce harmonics in the output.
  2. ac power line filters. These are low-pass filters placed at the ac input to the
    equipment power supplies to remove any high-frequency components that may pass
    into or out of equipment connected to a common power line. Fig. 22-33 shows a
    common ac power line filter that is now built into almost all types of electronic
    equipment, especially computers and any equipment using sensitive high-gain
    amplifiers (medical, stereo sound, industrial measuring equipment, communications
    receivers, etc.).
  3. Filters on cables. By wrapping several turns of a cable around a toroid core, as
    shown in Fig. 22-34, interfering signals produced by inductive or capacitive coupling can be reduced. Any common-mode signals induce voltages into the core,
    where they are canceled out. This approach works on ac power cords or any
    signal-carrying cable.

Measurement of Electromagnetic Interference

The rules and regulations pertaining to EMI are given in the Code of Federal Regulations, Title 47, Parts 15 and 18. Anyone working in the communication field should have copies of the code, which can be obtained from the Government Printing Office. Essentially these guidelines state the maximum signal strength levels permitted for certain types of equipment. The accepted levels of radiation vary considerably depending upon the type of equipment, the environment in which it is used, and the frequency range. The FCC also distinguishes between intentional radiators such as wireless LANs and other wireless units and unintentional radiators such as computers.

Radiation is measured with a field strength meter. It may be a simple device, as described earlier r, or a sensitive broadband communication receiver with a calibrated antenna. In either case, the measurement unit is microvolts per meter (μV/m). This is the amount of received signal picked up by an antenna 1 m long at some specified distance.

Consider the FCC regulations for a computer. Conducted EMI must not exceed the frequency range of 450 kHz to 30 MHz on the ac power line. Radiated emissions may not exceed a specific field strength level at a given distance for specified frequency ranges. The measurement distance is either 3 or 10 ft. At 10 ft the received signal strength may not exceed

100 μV/m between 30 and 88 MHz
150 μV/m between 88 and 216 MHz
200 μV/m between 216 and 960 MHz
500 μV/m above 960 MHz

Several manufacturers make complete EMI test systems that are sophisticated field strength meters or special receivers with matched antennas that are used to “sniff out’’ EMI. Some units have inductive or capacitive accessory probes that are designed to pick up magnetic or electric fields radiated from equipment. These probes are good for finding radiation leaks in shielded enclosures, connectors, or cables. The antennas are directional so that they can be scanned around an area or a piece of equipment to help pinpoint the radiation source and its frequency. Once the nature of the radiation is determined, grounding, shielding, or filtering steps can be taken to eliminate it.

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