Electronic Test Equipment Primer

Common Misconceptions About Test Equipment

Modern electronic test equipment affords the technician and engineer accuracy and convenience of use far beyond what was ever possible with volt-ohm meters and other analog devices of not-so-long ago. Though VOM's still have their place on the test or repair bench, the introduction of digital technology into these devices has improved reliability and dependability while maintaining a price easily affordable to the average consumer. There are a couple misconceptions concerning test equipment shared by some of those starting out in electronics, either as a student or as a hobbyist.

The most common misconception is the need or requirement for accuracy and precision (which translates directly into $$). These are two very different qualities! As an example, if you fired five shots at a target, and the bullets all passed through the same hole made by the first bullet, you would have great precision. But if the hole was not in the bullseye, you would lack accuracy. If you had five holes in the bullseye, you would have accuracy, but with much less precision. Were the one hole from five shots in the bullseye, you would have both accuracy and precision. So how much accuracy and precision do you need in a piece of test equipment? This, of course, depends entirely upon what your practical needs are.

Most consumer electronics are built mainly from "off the shelf" components having 5% to 10% tolerance ratings. Though this may sound pretty "loose", it is every bit practical. Part of the criteria for a good circuit design is being insensitive to the "tolerance stack" possible when using such parts. This is called a "reproducible" or repeatable design, and is a highly desirable quality.

A 100O resistor with a 5% tolerance could measure anywhere from 95O to 105O and be within its rating. It should therefore stand to reason that two different, though identical electronic devices might show a range of different voltage or other measured value at a given place in its circuitry while constructed of such parts. There goes your need for high accuracy and precision! If a point in the circuitry of 1000 identical amplifiers always measures nominally from 12.5 to 14 volts while functioning as it should, it makes no difference if it is displayed on the meter accurate and precise to three decimal places! There's no real need for a highly precise measurement in this instance. But if you need to tune the output of an oscillator to precisely 72.0125 MHz, you are going to need accuracy and precision in your measuring equipment to get reliable adjustments.

Another common misconception is that the right test instrument will tell you what is wrong with a defective piece of equipment. It seldom, if ever, happens that way! Test instruments allow you to identify symptoms you must recognize and translate into a possible cause of a problem. This process is called "Troubleshooting", and is a cultivated skill based upon knowledge of theory and experience. In a nutshell, it is rarely possible to repair a device having no or insufficient knowledge of how it works.

Examples of Common Misconceptions

Volt-Ohm Meters

Volt-Ohm-Meters (VOM) meters can often measure current in small amounts, usually below 200 mA, but otherwise are for voltage and resistance measurements only. VOMs can offer excellent accuracy, but are somewhat inconvenient to use as compared to a DMM. VOMs use an analog needle movement which must be "zeroed" each time the resistance range setting is changed. Another inconvenience is the scale graduation. A VOM's accuracy is measured at "full range deflection" (FSD) or when it is deflected all the way to the right of the scale. It is best to think ahead! If a voltage measurement is likely to be in the 100V range and you have the meter set to the 30V range, the needle movement may be slammed against the stop when the probes are applied to the circuit. Not good, obviously. Also, the fine graduations on the scale are worth different values depending upon which voltage range the meter is set to.

So why would anyone want a VOM today? Two main reasons: 1) A VOM will "load" a circuit or device under test a bit differently than a DMM. This can sometimes help identify a marginal component or circuit, and provide a good "second opinion" on the device or circuit. 2) A DMM updates the LCD display about 12 times a second. This makes it difficult to see a slow oscillation or unstable voltage in a circuit. A meter movement can rise and fall with a slow change, but the DMM display may just appear unstable and jumping around randomly. So a simple rise/fall trend may not be at all obvious on the DMM.

Digital Multimeters (DMM)

The digital multimeter is the most widely used test instrument today. They can range in price from under $20.00 to hundreds of dollars. The DMM is referred to as a "multimeter" because along with voltage, resistance, and current, they often can measure transistor beta, inductance, capacitance, frequency, continuity up to a specific resistance, and have a diode checker. The accuracy and precision of most DMMs above the $20.00 price tag are more than satisfactory for the great majority of repair and troubleshooting needs. There are also "auto-ranging" meters that automatically select the proper voltage or resistance range when the leads are first connected to the device. True RMS meters will measure AC voltage and current at much higher frequencies than a standard DMM, which are mostly intended to measure AC below 1000 Hz.

Signal/Function/Audio Generator

Available in audio and radio frequencies, these devices produce repeating waveforms, often more than one type. They are most commonly used in conjunction with an oscilloscope. Common waveforms include sinusoidal, triangle, sawtooth, and square wave. The different waveforms are provided as some problems may be more readily observed in one type waveform than in another. Generators are available with analog and LCD displays, and generally have fine adjustments for output level and frequency.

Sweep Function Generator

A sweep function generator starts at a preset frequency, and sweeps up through a second preset frequency, at a given sweep rate. This can be used to show a circuit's frequency response in real time, and response anomalies.

Oscilloscope

The oscilloscope is used to view an AC waveform in "real time" on a screen, as it actually happens in the circuit. This can be extremely useful (or absolutely required) when making some types of repairs or designs. For example, the audio output of an amplifier can be viewed on an oscilloscope to see how or if it is distorting the input signal. This is done by putting a known audio frequency waveform into the amp, usually a sinewave. If the amplifier does not create an output, the oscilloscope can be used to identify the last stage where the sinewave was present. This can quickly lead the repair technician to the part of the circuitry that is malfunctioning. A viewed waveform that does not match the shape of the input waveform is often an indication of a problem as well. The voltage and frequency of an unknown waveform can easily be measured with an oscilloscope. Oscilloscopes can have "dual traces" to allow you to view two waveforms for comparison, have high frequency capability (100 MHz or more), sample/download features, etc.

AC/DC Clamp Meters

This device clamps around a single conductor and is able to measure the voltage and current flowing through it. Measuring high current can be dangerous with a regular DMM as all of the current needs to flow through the meter to be able to measure it. This requires breaking into the circuit and placing the meter in series, which is often very inconvenient as well! The clamp meter calculates the voltage and current levels by measuring the electric field it produces around the conductor.

Fox and Hound

The Fox and Hound Wire Tracing System is a unique device allowing you to trace the path of a wire. The "Fox" injects a three-tone signal into the wire; the "Hound" will pick up the signal inductively from up to 12" away through wood, drywall, or any non-metallic material. The Hound detects the tone, which gets audibly louder the closer the pickup is to the conductor carrying the signal. This also allows you to trace and find the specific conductor when many are present or bundled together such as in telephone or computer networking hubs.

Frequency Counters

A frequency counter simply measures the frequency of a repeating waveform. These are used to measure IF oscillators in radio and TV equipment, or any application where is a frequency measurement is required.

Computer Monitor Tester

The computer monitor tester connects directly to the input of a standard computer monitor, relieving the inconvenience of having to connect it to a computer. It produces several test patterns and will work with virtually any monitor from MDA to SVGA. These testers can reveal distortion in the picture by displaying various lines and shapes, and problems in color reproduction in standard color bars.

Circuit Testers

These are simple devices which usually test for the presence of voltage or continuity, indicating such with an audible beep or glowing lamp. They are simpler to use than VOMs or DMMs, especially when only the presence of voltage or continuity is being tested for.

LCR Meter

LCR meters measure L (a coil's inductance) C (a capacitor's capacitance), and R (resistance) across a broader range of values (and generally with greater accuracy) than a standard DMM. A DMM may have a low-range resistance of 200 ohm, which would measure any resistance up to 200. Such a meter would not measure a .22-ohm resistor with much accuracy. An LCR meter may have a 20-ohm range, which could provide much better accuracy at such low R values.