In a recent column, we gave a quick review of some basic instrumentation common to most engineering work benches. Here are a few more instruments found in many engineering labs.
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 time-domain 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
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.
Reasonably accurate frequency counters are incorporated in high-end 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.
This is a true microwave counter, capable of measuring up to 40 GHz. It includes an integrated power meter and two additional timer/counter channels capable of capturing very small frequency and time changes. Analysis modes include measurement statistics, histograms and trend plots, enabling the user to analyze a very wide range of signals. Frequency resolution is 12 digit/s. PC connectivity is enabled by the included copy of NI LabVIEW SignalExpress software.
Frequency counters generally operate by counting the number of events within a specific time interval. When that gate time has elapsed, the number of events per second is calculated and it is displayed in the digital readout. The counter is then reset to zero.
To obtain an accurate reading, the time base must be stable, irrespective of power supply and temperature fluctuations and the effects of aging. Often a quartz crystal oscillator is used. It is located within a sealed temperature-controlled “oven”. For critical applications, an external frequency reference is coupled with a GPS-regulated rubidium oscillator. Resolution can be enhanced by oversampling and averaging, particularly when the signal under investigation is subject to jitter.
A multimeter in ohms mode can perform a partially definitive test on a high- capacitance component. If it reads open or shorted, the device is definitely defective. In a high- resistance range, when first connected, current from the meter’s internal battery charges the capacitor, first rapidly and then more slowly as the component approaches its fully-charged state. Reversing the leads, the capacitor discharges at this same exponential rate. Electricians call this strange phenomenon “clocking” and it provides a rough indication of the component’s condition.
Professional-grade multimeters have a capacitance-measuring mode. The capacitor is charged and discharged with a specific current and the rate is measured, a slower rate indicating greater capacitance. The quantity is shown in the readout.
A fully-functional dedicated capacitance meter places the capacitor into a bridge circuit. By varying the values of bridge components until the bridge is in equilibrium, the value of the capacitor being tested is determined. Additionally, the bridge-type meter can measure series resistance and inductance. LCR meters are capable of simultaneously measuring inductance, resistance and capacitance in a single component or circuit.
In this context, network refers to electronic networks located in electrical equipment, as opposed to LAN, WAN or similar networks used for computer connectivity. They characterize devices with two or more ports such as amplifiers and filters. They are effective at frequencies exceeding 1 THz. Most network analyzers are either scalar network instruments, which measure amplitude only, or vector network instruments, which measure amplitude and phase. Vector network analyzers are the more common of these instruments.
The typical network analyzer is comprised of a signal generator, test set, receiver(s) and an LCD, usually incorporated within a single enclosure. Advanced network analyzers include a second signal generator, which is required for a mixer test or amplifier intermodulation testing with two separate tones. Intermodulation distortion is caused by nonlinearity or time variance within the equipment under investigation. Besides conventional harmonics, intermodulation gives rise to sum and difference frequencies. Where they are to be mitigated, they must first be generated and measured in a test environment. The object in a transmitter is to eliminate these spurious emissions, which increase bandwidth usage and power consumption at the expense of the intended signal.
In a vector network analyzer, the receiver measures phase as well as amplitude of the signal. For this, a reference channel is needed. Generally, reference and test channels are down-converted to facilitate more accurate measurements.
Since the network analyzer with display capability is a member of the oscilloscope family, it requires periodic calibration, usually by the manufacturer or in a specialized laboratory. This procedure involves error correction of the instrument, cables, adapters and test fixtures. It is usually performed by expert engineers, sometimes several times per shift. The process is exacting because it involves both internal and external cables, which may cause time delay and phase shift as a function of their length. Moreover, these effects are temperature sensitive.
Q is a measure of the amount of electrical energy dissipated per cycle in a non-linear reactive circuit. (Q is intended to denote quality.) It is expressed in the equation:
Q = 2π × PENERGY STORED /ED
PENERGY STORED = Peak energy stored
ED = energy dissipated per cycle
Q is used in describing bandpass LC filters and resonators. Invented in 1934, Q meters are currently used by radio amateurs. A Q meter consists of a tunable RF generator with low output impedance and a detector with high input impedance. The generator is in series with the tuned circuit under test. The detector measures the voltage across the capacitive component and the meter is calibrated to display the Q of the component or circuit under test.