Basic test instrumentation and its role in measurements

DAVID HERRES | CONTRIBUTING EDITOR

It can be helpful to understand the differences between common test gear used for bench-top development tasks.

Here's a quick review of some basic instrumentation common to most engineering work benches.

AMMETER

Typical clamp-on ammeters.

The ammeter is the basis for many other electrical measuring instruments. Whether you are measuring volts or ohms, essentially inside the instrument you are measuring current. Measurement of current in a circuit is somewhat problematic because all the electrical energy to be measured must pass through the meter, so there is the inconvenience of cutting open and later re-terminating the circuit. Another problem is that conventional ammeters, as incorporated in the ubiquitous multimeter, cannot dissipate heat that is greater than just a few amps.

The clamp-on ammeter is a work-around. It solves both problems by measuring the magnetic field that surrounds any currentcarrying conductor. The instrument is calibrated to read amps. The user closes the jaws around an insulated currentcarrying conductor. It doesn’t matter whether the conductor is centered within the jaws, and it may pass through at an angle. For low-amp measurements, the conductor may be coiled, multiple turns passing through the jaws in the same direction, and then the total reading divided by the number of turns. A hand-held clamp-on ammeter (trade name Amprobe) can be rated as high as 600 A, making it useful for large three-phase motor work. Specialized Hall-effect instruments can read dc amps.

VOLTMETER

A solenoid voltmeter (tradename Wiggy) useful in checking for presence or absence of voltage and the approximate level (120 or 240 V) in residential, commercial and industrial branch circuits and load centers.

Unlike the ammeter, which is a series instrument, the voltmeter is placed in parallel across a component, conductor, circuit or power source. The full current does not pass through the instrument, only a small fraction of it. The exact amount depends upon the voltage being measured and the impedance of the voltmeter. The input impedance rating of the instrument is all-important and determines how accurately a given circuit can be measured. A low-impedance meter places a heavy load on the circuit under investigation. Used beyond its rating, or with a high-impedance circuit, the large voltage drop can damage the circuit.

A high-impedance voltmeter is (relatively) invisible to the circuit under investigation. Nevertheless, it should not be used at voltages exceeding its rating. CAT ratings, which vary with precisely-defined electrical environments, must be observed. These ratings are generally printed adjacent to the inputs.

A low-impedance instrument, such as the solenoid voltmeter (tradename Wiggy) is useful in checking for presence or absence of voltage and the approximate level (120 or 240 V) in residential, commercial and industrial branch circuits and load centers. The loud buzz for ac and single click for dc means you needn’t keep an eye on the readout, and the distinct vibration is useful in noisy locations. This lowimpedance meter is useful for checking GFCI (ground fault circuit interruption) protection downstream from the device. Placing one probe on the neutral wire (white) and the other on the equipment ground (green or bare) or on the equipment chassis will cause the device to trip out if it is receiving power and working. The instrument is not to be left connected to a power source for long, or it will overheat.

OHMMETER

The most common type of ohmmeter for general use is incorporated in the digital multimeter. Analog meters, with moving needles rather than digital readouts, are also available, and they are preferred by some old timers. They have the advantage of being more accurate outdoors in cold weather. A reflective surface behind the needle aids in eliminating error by facilitating straight-on alignment. Digital multimeters are far more widely used.

Bench-type multimeters incorporate the four-wire (Kelvin) option, which is essential for precise low-resistance measurements. Four separate probes, with alligator clip attachments, plug into four dedicated ports, and they are connected to the resistance under investigation. The four-wire setup substantially reduces the effect of cumulative resistance due to measuring leads, contact resistances and electrical paths within the meter. One pair of leads carries the test current from the meter and the other pair measures the voltage drop across the resistance under investigation. This arrangement excludes the unwanted cumulative resistance.

OSCILLOSCOPE

A Tektronix MDO3104 oscilloscope, frequently used for general-purpose work.

The oscilloscope is by far the most versatile and frequently used (with the possible exception of the multimeter) of our many electrical instruments. It is essentially a voltmeter, although equipped with a current probe it can read amps, and in conjunction with another probe reading volts, it can be configured to graph power.

In its most widely-used mode, the time domain, the oscilloscope displays a graph of amplitude in volts along its vertical Y-axis, plotted against time in seconds along its horizontal X-axis. Fractional units such as milliand micro-volts and seconds automatically appear when appropriate.

Through the miracle of triggered sweep, a rapidly oscillating periodic signal can be displayed as a single stable waveform. Two external or internally-generated signals can be displayed in separate channels and in the Math mode they can be added, subtracted, multiplied and divided. Other functions, applicable to single waveforms, include square root, integration, differentiation and logarithmic displays.

Besides seeing displays in the time domain, the user by pressing a button can instantly see the Fast Fourier Transform of the same signal, displayed in the Frequency Domain, where amplitude as power is plotted on the Y-axis (linear or logarithmic scale) and frequency on the X-axis. This is used for viewing harmonics and calculating total harmonic distortion. Additionally, in the X-Y mode, Lissajous figures are displayed for one signal triggered by a second signal applied to a second channel. These figures change depending upon the amplitude and frequency relations and phase angles.

Early analog oscilloscopes applied the external signal more or less directly to the vertical deflection plates, and an adjustable time base to the horizontal deflection plates. In response, the electron beam wrote the uniform waveform trace on the phosphor coating on the inside of the glass screen, through which it could be viewed as visible light.

Today’s digital instruments achieve the same effect with many more functions and analytic capabilities. The signal from each analog input, after preconditioning including amplification or attenuation as needed, goes to a separate analog to digital converter (ADC) in which sampling occurs. The digital output goes to processor, memory and display.

The display is a reliable, user-friendly flat screen with no high-voltage deflection needed. The most common liquid crystal displays (LCDs) now found in these instruments typically have LED backlighting.

The mixed-domain oscilloscope (MDO) displays the same signal in split-screen format in time and frequency format. The mixedsignal oscilloscope does the same for two separate signals. This is a great diagnostic tool because it correlates in real time digital glitches with intermittent power-supply or other anomalies.

SPECTRUM ANALYZER

A Tektronix RSA5000 Real-Time Spectrum Analyzer.

The spectrum analyzer resembles its close relative, the oscilloscope, the principal differences being:

Model for model, the spectrum analyzer is significantly more expensive.

The spectrum analyzer generally displays waveforms in the frequency domain only, while the oscilloscope displays waveforms in the time domain and the frequency domain.

The spectrum analyzer has more features, greater analytic capabilities and potentially higher bandwidth and advanced specifications compared to the oscilloscope.

Experienced technicians and engineers often find themselves turning away from the oscilloscope in favor of the spectrum analyzer for the most advanced work.

The spectrum analyzer front panel has numerous controls that are less intuitive and self-evident than those of the oscilloscope, but a lot of initial difficulties are resolved by consulting user manuals, available for free download at the manufacturers’ websites.

As with the oscilloscope, the immediate challenge is obtaining a meaningful display. For the oscilloscope, the answer is to press Default Setup and Autoset. For the spectrum analyzer, to display a non-sinusoidal signal in the frequency domain and see the full range or harmonics, it is necessary to first display the Frequency/Span drop-down menu. Typical menu items are Center Frequency, Span, Start Frequency and Stop Frequency. (R to Center can be temporarily ignored. It has to do with positioning a reference marker at the center of the screen.)

Spectrum analyzers fall into three basic categories, swept-tuned spectrum analyzer, vector signal analyzer and real-time spectrum analyzer.

The swept-tuned spectrum analyzer incorporates a superheterodyne receiver, which makes use of a local oscillator to downconvert progressive portions of the signal under investigation to display its frequency spectrum as a function of time. You can watch this sweeping action as it moves across the screen. The only disadvantage in this otherwise ingenious arrangement is that during the time required for the sweep to be completed, short duration events are sometimes lost.

The vector signal analyzer is a variation of the spectrum analyzer that displays the amplitude and phase of a signal at a single frequency, rather than showing the larger spectral context. The principal application is determining modulation quality in design prototypes, using superheterodyne techniques.

The real-time spectrum analyzer samples the entire received RF spectrum in the time domain and uses Fast Fourier Transform algorithms to create overlapping spectra so there are no gaps and no missed short-term events.

CURVE TRACER

The curve tracer is a member of the oscilloscope family of measuring instruments. In its CRT or flat screen display, the user sees an I-V graph characterizing (typically) a discrete semiconductor. This display differs from a timedomain or frequency-domain display in that current is plotted against voltage. The curve tracer applies this voltage or current to the device under test, which can be an IC, discrete transistor, motor or solar array, among others.

In testing a field-effect transistor, for example, test voltage from the curve tracer is applied to one input terminal and the common terminal. This voltage is swept and the amount of current at the output is shown in the instrument’s screen, plotted against the applied voltage. This is the I-V curve. In testing a bipolar junction transistor, rather than voltage, a stepped current is applied at the input terminals.

Two devices under test can be connected simultaneously to the curve tester, and as the user toggles a switch, their separate I-V curves are displayed and can be compared. The test is valuable in evaluating proposed devices for differential amplifiers, where close matching is required.

The curve tracer is widely used by photovoltaic array designers and technicians to test the performance of a prototype or existing installation where the effect of ambient conditions must be distinguished from design or installation faults. As an example, partial shading of an array reduces maximum power output at various times during the day. The curve tracer facilitates PV array diagnosis. Without re-arranging electrical connections, individual panels can be masked to block light, aiding in efficient troubleshooting.

POWER FACTOR METER

A power factor meter.

The Power factor meter, also known as the Cos Phi Meter, is a type of dynamometer wattmeter. The basic principle is that when the field associated with a moving system comes into juxtaposition with the field associated with a fixed coil, the pointer attached to the moving system, properly calibrated, quantifies the deflecting torque in terms of power factor.

A dynamometer-type power factor meter consists of two series-connected fixed coils carrying a specific fraction of the load current. Two nearby identical moving coils mounted 90° apart, deflect the needle. This indicator reads power factor 1 at the center of the dial, 0.7 lag at the left and 0.7 lead at the right.

When the supplied power is sinusoidal and the load is purely resistive, with neither inductive nor capacitive components, power factor is 1, which is ideal. A load that has inductive or capacitive components is characterized by a lower power factor, falling between -1 (worst) and 1 (best). Most loads having a power factor other than 1 are inductive, and can be corrected by adding power-factor correction capacitors to the mix. They are usually brought on and off line automatically to correct for intermittent inductive loading.

The problem with a power factor less than 1 is that it indicates voltage and current are not in phase, so the average product of the two is less than if they were in phase. A load with a poor (less than 1) power factor generates power that then flows back to the source without performing useful work. To prevent overheating in this case, larger conductors and distribution equipment are required. Moreover, there is excessive drag on the generator.

Using a power factor meter, utility workers measure the power factor at industrial facilities, which receive a bill for a power factor penalty in negative territory. Therefore, it is in the interest of plant owners to install power factor correction capacitors as needed.

FREQUENCY COUNTER

A Tektronix MCA3000 frequency counter.

Reasonably accurate frequency counters are incorporated in highend multimeters such as the Fluke 287 and oscilloscopes such as the Tektronix MDO3000 Series instruments. For laboratory applications, there are the Tektronix MCA3000 Series Frequency Counters, selling for $11,200 to $16,500.