Guide to Electronics/Electrical Servicing--How to test and measure electronics--part 2

Function generators

Basically, these are signal generators designed to produce sine, square or triangular waves as outputs. Each may be varied typically over a frequency range covering from less than 1 Hz to about 20 MHz. The basic frequency may be generated either by a highly stable oscillator circuit, or by using a frequency synthesizer. The output levels are typically variable between about 5 mV and 20 V peak of either polarity, plus a transistor-transistor logic (TTL)-compatible signal at 5 V. Some of the basic waveforms. The output impedance is nominally 600R and provision is made for driving signals via balanced or unbalanced lines.

++++Function generator waveforms: (a) sinusoidal, (b) positive and negative pulses, (c) positive and negative sawtooths (serrasoids), and (d) compound wave

Each of the basic output signals is capable of being modified. The sine wave may often be phase-shifted, and the square wave mark-to-space ratio is variable so that a pulse stream of variable-duty cycles can be provided.

The triangular wave can be varied to provide a sawtooth of varying rise and fall periods. In addition, it’s also possible to add a d.c. value to each output as an offset. This is valuable for testing circuits that are d.c. coupled and with a frequency response that extends down to zero. The square wave is also differentiated and integrated to generate exponential envelopes (see the block diagram).

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++++ Block diagram of function generator: Buffer (exponential); Buffer (square); Adder Diff/int stage; Square wave generator; Gate controller; Buffer (sine); Phase shifter; Sine wave; oscillator

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An additional feature often allows a second signal component to be gated into the primary waveform, as indicated. A further variation of this is the tone burst that consists of a sine wave modulated with a parabolic envelope, but generated in bursts. This waveform is particularly suited to the testing of full-bandwidth audio systems or those circuits where strange resonances may occur. A variation of this feature uses a form of frequency modulation (FM). Here, a basic sinusoid is frequency-swept between predetermined limits, at either a linear or a logarithmic rate.

The more sophisticated instruments in this range may be equipped for microprocessor control, and have a digital readout of frequency and amplitude, plus a built-in standard frequency source.

It’s most important that the instrument calibration is carefully checked following any repairs. Service work is therefore a very specialized operation.

Integrated circuits are extensively used, together with stabilized switched mode power supplies. The highly critical oscillator circuit is invariably temperature controlled.

Decibel measurements

Measurements of gain or loss scaled in decibels can be made using an intelligent DMM. An intelligent DMM allows for automatic selection of ranges and for true root mean square (r.m.s.) measurements, so that amplitude measurements can be made. Measurements of power loss and gain can be carried out with such an instrument, and some can also measure signal-to noise ratios.

PCs, iPads and SmartPhones as test instruments

With the introduction of the personal computer (PC) to the workshop environment, initially for information only, a wide range of hardware devices that are software controlled became available. These range from fairly simple and low-cost active probes capable of providing the actions of storage oscilloscope, spectrum analyzer, DVM and even transient recorder functions, through DMM, storage scope with spectrum analyzer, transient recorder, plus function generator, to high-cost extensive networked systems that pro vide all of the above, plus remote data logging from test benches, etc.

In general, these all have one point in common: they are attached to the PC via the parallel printer port, a specially provided plug in parallel port card or, more recently, the universal serial bus (USB) connector. For oscilloscope simulation the sampling rate of analogue-to-digital conversion (in MS/s) must be at least twice the bandwidth for sine waves and preferably higher (up to eight times the bandwidth) for a complex signal.

Although there are other suppliers of virtual instrument (VI) equipment, the following devices and systems that run in the familiar Windows environment, with typical task bars and pull-down menus, have been extensively evaluated for the servicing environment. These are manufactured by:

• National Instruments (www.ni.com)

• Fluke

Of the three manufacturers, National Instruments (NI) probably provides the most extensive range of measurement, data acquisition (DAQ), data logging including image acquisition, and signal conditioning accessories for a system that is more likely to be used in a product life testing and monitoring environment. Since the system is designed to operate with real-time applications, in a measurement, control and statistical analysis mode, it’s capable of being coupled into a wide range of communications networks that include back-plane bus extensions, Internet, general-purpose interface bus (GPIB), Ethernet, fire-wire (IEEE1394) and USB.

Standard function generator waveforms including composite video are provided, plus many more that are user programmable. These offer:

• sine and square waveforms at frequencies up to 105 MHz

• linear and logarithmic frequency sweeps and bursts

• multi-device synchronization for channel expansion

• frequency resolution up to 1.07 µHz using direct digital synthesis.

The DMMs have 5.5 digit accuracy for a.c. voltage and current (true r.m.s.), d.c. voltage and current, and resistance. These are used to measure the outputs from many different types of transducer on several channels virtually simultaneously.

The data-capture analogue-to-digital converters (ADCs) or digitizers can be driven from a wide range of transducers via appropriate signal conditioners, ranging from resistance temperature detectors (RTDs) to thermocouples, thermistors, chromatography sensors, strain gauges, force, load and pressure sensors, and linear displacement devices such as the linear variable differential transformer (LVDT). Bandwidths vary from 100 MHz down to 4 MHz, while the corresponding resolution ranges from 8 bits up to 21 bits.

Because of the complex nature of the NI system, the manufacturer pro vides extensive technical backup comprising system development support, technical training and future development. The website also offers training videos online, which are very useful in deciding how to make use of the products. For lower level servicing applications, however, the NI equipment may be seen as over the top for a small workshop.

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The waveform shown can be used to derive the relationship between the peak and r.m.s. value for a symmetrical square wave as follows. Squaring the voltage waveform that swings between _0.4 and _0.4 V produces a new wave for V2 as follows: (_0.4)2 _ _0.16 and (_0.4)2 _ _0.16

Practical:

The average value for V2 is thus 0.16 and continuously positive. Taking the square root of V2 produces the values of 0.4 V, again continuously positive. Thus, for the symmetrical square wave the peak and r.m.s. values are equal. This is only true for square waves; all other shapes have a variation in this relationship. Now suppose that the addition of a d.c. offset produces an asymmetrical square waveform that swings between _0.2 and _0.6 V (amplitude is unchanged). Repeat this exercise and compare the results.

++++CRO display of square wave signal in time domain.

++++

++++ CRO display of square wave in frequency domain.

++++Fluke handheld oscilloscope

The Fluke system provides similar types of function, but with lower accuracy and less sophistication and at lower cost than the NI system. This is an economical alternative to standard test equipment and data acquisition tools.

The system software and documentation are supplied in the most European and Asian languages (or use Google translate), with a backup service via the Internet and website. The system is fully developed to CE under ISO9001 standards. It functions on most of the early PC developments from DOS software and 80386 or higher processors supported by downloadable updates from the website. Some adapters plug into the PC parallel, others into the USB connector, and the power supply of an adapter is taken from the port; no separate supply is needed.

In the time domain, the main screen of the oscilloscope provides for both normal timebase and X-Y operations. The latter is particularly useful when comparing two time-related waveforms such as found with advanced modulation schemes. The spectrum analyzer provides an alternative view of a signal in the frequency domain. Switchable probes of 110 or 10/100 are available, in either 60 MHz or 250 MHz form.

In the digital storage CRO mode, the sampling speeds depend on the model that has been selected. Fluke offers oscilloscope adapters in ranges described, as simple, general-purpose and high-speed, with sampling rates ranging from 20 to 200 MS/s, with comparable analogue bandwidths. Both waveforms and instrument settings can be stored on disk for future study and analysis.

When equipped with suitable sensors/transducers, the data acquisition and logging software can measure temperature, humidity, sound pressure, light, current, resistance, power, speed and vibration levels. This data can be transferred to other Windows applications over local area networks or the Internet using the copy and paste facility. The data can also be displayed in a spreadsheet format, such as Excel, with analysis of data trends.

The Fluke handheld oscilloscope is particularly suitable for field service (used along with a laptop computer). The large probe is plugged into a USB port, needing no other source of power, and requires no additional connections or installation procedures. The design allows for single-handed operation, with control accomplished by a button which is pressed to start the oscilloscope action and which flashes green to indicate that the oscilloscope is active. The tip of the probe is illuminated to make it easier to locate the point at which the signal is to be examined. When the signal has been captured on the computer screen the button can be pressed again to stop oscilloscope action: the button then glows red. If the button is held down it will activate the auto-setup action to con figure timebase and triggering.

The supplied software will provide oscilloscope, spectrum analyzer and metering functions, and additional software can be bought to add data acquisition.

+++ With a PC virtual instrument, connect the system to a working three stage low-frequency amplifier to measure the frequency response, bandwidth and gain of the individual stages. Using the spectrum analyzer and an input from the function generator set to 100 Hz sine and square waves, note the change in the harmonic content at the amplifier output.

+++

Servicing information:

With the introduction of computing systems, the lifetime of the hard-copy service manual is nearly over. Much of today's service information is now provided either on CD-ROM or via the Internet. In earlier times, the service technician's manual carried not only the original service diagram and data, but also a lot of personal information that he or she had gleaned over time about that particular system. Trying to read today's manual from a monitor screen is certainly a different experience. Following the diagram of such as a television receiver, with interactions across many screens, appears at first sight virtually an impossibility. Even printing out sections of the diagram and gluing them together with sticky tape is not conducive to best use of the servicing bench. Fortunately, most electronic-based service information provides space in memory for the technician to add further comments as aids for future use. However the service information is provided, via the web, CD, YouTube, DVD or even satellite links, new skills will have to be developed to make the most of what could be information overload.

Technicians servicing domestic electronic equipment are generally well provided with circuit diagrams and data sheets, but to produce a thoroughly reliable and economical repair that will generate customer loyalty, they need additional skills and support. Not only is it necessary to understand how the system works, there is also a need to have a knowledge of the system's historical reliability and its particular points of failure. In the past, when systems were constructed largely from discrete components, many of the failures occurred in a regular manner, giving rise to the stock faults.

Indeed, many service departments found these to be a source of good business that created a sound reputation for doing a good job.

With today's extensive use of dedicated ICs the system reliability has improved considerably. However, stock faults still occur, but repeat very much less frequently, making a good memory an additional requirement for service personnel. One of the best service aids is a subscription to the journal Television (see below). This acts as a clearing house for the hints, tips and solutions to problems encountered by many practicing service technicians. Fortunately, there are now a number of computer system databases available that have been designed to aid the servicing of such equipment.

While these can provide almost instant access to many of the stock faults, it must be emphasized that these should be used in conjunction with the manufacturer's circuit diagrams and data sheets.

The website (www.servicemanuals.com) extremely useful for servicing information that is hard to get or out of print, and for anyone in a hurry, a manual can be downloaded for almost immediate access.

Of these databases, two have been tested and found to be invaluable to the busy workshop.

The first one tested was formerly provided on CD-ROM and is now available from the magazine publisher. THIS publishes, among others, the magazines Electronics World and Television and Electronics Online. The fault-finding and repair hints represent the knowledge gleaned from manufacturers, dealers and repair centers, and cover about 1 million entries from more than 400 manufacturers. Unusually, the database information is covered in most European languages. Originally, information was sent out on CD-ROM, but this has now been superseded by a completely online ser vice, for which you need to register to obtain servicing tips (although lists of stock faults can be seen without registration). The requirements for your PC are:

• Windows PC, Linux and Apple

• 32-bit operating system (Microsoft Windows XP or higher, Apple OS)

• graphics card high color or higher

• resolution 800 x 600 or higher

• browser: Microsoft or Netscape version 4.01 or higher

• Java Script active

• cookies allowed

• Java Engine installed and active.

Some other useful servicing-related websites for technical data, service sheets, spares and data are listed at the end of this section.

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Compare information from several different sources in relation to a fault condition.

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Web links:

• Engineering information, BBC:

- bbc.co.uk, www.bbc.co.uk/enginfo

• Transmitter information:

(transmitter alignment program)

• Service support database:

• Service information forum:

- www.repairworld.com

• Specialist repair services:

• Test equipment:

• Manufacturers' services:

- philips.com

- ti.com (Texas Instruments)

QUIZ:

1 The safe operating area (SOA) for a transistor is located:

(a) at the lowest possible value of bias

(b) when the transistor is nearly saturated

(c) between the cut-off and saturation points

(d) when the current is a maximum.

2 All types of passive components can be tested using:

(a) a multimeter

(b) a bridge type of circuit

Multiple-choice revision questions:

(c) an oscilloscope

(d) an ohmmeter.

3 You would use a counter/timer typically for:

(a) measuring the time a task took

(b) measuring the frequency of an oscillator

(c) measuring the number of times a fault occurred

(d) measuring the mean time before failure.

4 An LF generator will probably use:

(a) an LC circuit

(b) a shift register

(c) a Schmitt trigger

(d) a Wien bridge circuit.

5 An advantage of using virtual instruments is:

(a) they can make use of a spare computer

(b) they are more precise

(c) they can record readings as well as measuring them

(d) they make use of software.

6 The main advantage of getting servicing information from the Internet is that:

(a) it’s more accurate

(b) it’s more up to date

(c) it’s faster and requires no storage

(d) it’s better illustrated.

Next: coming soon

Prev: part 1

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