Module 16—Introduction to Test Equipment
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3-1 to 3-10
, 3-11 to 3-20
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,3-31 to 3-34
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Upon completing this chapter, you should be able to:
1. Define and explain the use of the terms "dB" and "dBm" as they apply to power measurements.
2. Describe the use of resistive loads, bolometers, and thermocouples in power measurements.
3. Explain the measurement of mechanical rotation using the tachometer, stroboscope, and the strobotac.
4. Explain the measurement of frequency in various ranges using vibrating reeds, tuned circuits, heterodyne frequency meters, absorption wavemeters, cavity wavemeters, and frequency counters.
5. Describe the use of frequency-measurement devices, oscilloscopes, and spectrum analyzers in waveform analysis and maintenance.
6. Describe semiconductor testing and applicable terms in maintenance.
In chapter 1, you studied test equipment administration and the basic measurements that all technicians are responsible for performing. Chapter 2 presents miscellaneous measurements that are fairly common; keep in mind, however, that you may not routinely perform these measurements in your particular job. This chapter introduces you to several test instruments and components found in those test instruments. It will also serve as a review of some of the basics of electronic theory related to test equipment.
You may be required to check the power consumption and the input-signal power levels of electronic equipment. The determination of dc power is fairly simple; recall that the unit of power, the watt, is the product of the potential in volts and the current in amperes (P = E x I).
As discussed in NEETS, Module 2, Introduction to Alternating Current and Transformers, the phase angle of the voltage and current must be considered for accurate ac power measurements. The measurement of ac power is further complicated by the frequency limitations of various power meters. If there is no phase angle difference, you can compute ac power in the same manner as dc power; that is, by determining the effective value of the product of the voltage and current.
For equipments that operate in the audio-frequency (AF) range, power levels have to be determined in the performance of routine checks and during corrective maintenance procedures.
Power measurements for AF circuits are usually indicated in terms of decibels (dB) or decibels referenced to 1 milliwatt (dBm). Because the actual calculation of decibel measurements is seldom required, the following explanation is somewhat simplified. Most test equipment is designed to measure and indicate decibels directly. This eliminates the need for you to perform complicated calculations. Nevertheless, a basic explanation of the decibel measurement system is necessary for you to understand the significance of dB readings and amplifier-gain ratings that are expressed in decibels.
THE DECIBEL SYSTEM
The basic unit of measurement in the system is not the decibel; it is the bel. The bel is a unit that expresses the logarithmic ratio between the input and the output of any given component, circuit, or system. It may be expressed in terms of voltage, current, or power. Most often, it is used to show the ratio between input and output power to figure gain. You can express the power gain of the amplifier (N) in bels by dividing the output (P1) by the input (P2) and taking the base 10 logarithm of the resulting quotient. The formula for determining this gain is:
If an amplifier doubles the input power, the quotient of P1 to P2 will be 2. If you consult a logarithm table, you will find that the base 10 logarithm of 2 is 0.3, making the power gain of the amplifier 0.3 bel.
Q-1. What is the logarithmic ratio between the input and output of a given circuit called?
Experience has shown that because the bel is a rather large unit, it is difficult to apply. A more practical unit, and one that can be used more easily, is the decibel (1/10 bel). You can convert any figure expressed in bels to decibels by multiplying that figure by 10 or simply by moving the decimal point one place to the right. Applying this rule, we find that the above ratio of 0.3 bel is equal to 3 decibels.
The decibel (dB) cannot be used to represent actual power; only the ratio of one power compared to another. To say that an amplifier has a 3 dB gain means that the output power is twice the input power. This gives no indication of the actual power represented. You must be able to state the input power for it to be meaningful. In many applications, a mathematical expression represents the actual power, not a power ratio. One standard reference is the dBm.
The dBm is an abbreviation used to represent power levels above or below 1 milliwatt. Negative dBm (-dBm) represents power levels below 1 milliwatt, and positive dBm (+dBm) represents power levels above 1 milliwatt. In other words, a dBm value is a specific amount of power; 0 dBm is equal to 1 milliwatt. Briefly stated, the amount of power in a given value of dBm is the power which results if 1 milliwatt is amplified or attenuated by that dB value. For example, 40 dBm represents an actual power level (watts or milliwatts) that is 40 dB above 1 milliwatt, whereas -10 dBm represents a power level that is 10 dB below 1 milliwatt. The formula for finding dBm is a variation of the dB power formula:
Q-2. What term is used to represent power levels above or below a 1-milliwatt reference?
You do not need to use the formula in most applications. The following shows conversions of dBm to mW:
+20dBm = 100mW
+10dBm = 10mW
+7dBm = 5mW
+6dBm = 4mW
+4dBm = 2.5mW
+3dBm = 2mW
0dBm = 1mW
-3dBm = .5mW
-10dBm = .1mW
For a +10 dBm level, start with the 1 milliwatt reference and move the decimal point one place to the right (+10 dBm = 10 mW). Another 10 dB increment brings the power level to +20 dBm, thereby moving the decimal point another place to the right (+20 dBm = 100 mW). For a -10 dBm level, again start with 1 milliwatt, but this time move the decimal point one place to the left (-10 dBm = .1 mW). An additional 10 dB decrease results in another decimal point shift to the left (-20 dBm = .01 mW).
For a 3 dB increase, you double the power. For a 3 dB decrease, you reduce the power by one-half (+3 dBm = 2 mW and -3 dBm = .5 mW). A +6 dBm level is an additional 3 dB change from +3 dBm. In this case, you just double the power level of the +3 dBm (+6 dBm = 4 mW).
Q-3. What milliwatt value is equal to +6 dBm?
The dB change can be made in either direction. For example, +7 dBm is a decrease from +10 dBm. Reducing the +10 dBm power by one-half, we have +7 dBm, or 5 mW. A +4 dBm power level is a 3 dB decrease from +7 dBm (+4 dBm - 2.5 mW). By using this simple method, you can quickly find any power level that corresponds to a given dBm.
Some test instruments you will be using are calibrated in decibels and have a 1 milliwatt zero reference level. Figure 2-1 illustrates such an instrument. Notice that this is an ac voltmeter in which the upper scale of the meter indicates ac voltage and the lower scale indicates decibels. The zero power-level indicator on the decibel scale is located at, or near, center scale. If the power in the line being measured is more than the reference value, the meter will indicate a value to the right of the zero mark (+dB). If the power is less than the reference value, the meter will indicate a value to the left of the zero mark (-dB). Such meters are useful when recording measurements where a direct indication in decibels is desired. However, you must remember that this meter is still a voltmeter and that power measurements are not meaningful unless the circuit impedance is known. If you feel the need to review how to calculate power in ac circuits, refer to NEETS, Module 2.
Figure 2-1.—AC voltmeter.
At radio frequencies below the UHF range, power is usually determined by voltage, current, and impedance measurements. One common method used to determine the output power of radio-frequency (RF) oscillators and radio transmitters consists of connecting a known resistance to the equipment output terminals. Current flowing through this resistance is then measured and the power is calculated as the product of I2R.
Because power is proportional to the current squared, the meter scale can be calibrated to indicate power units directly. A THERMOCOUPLE AMMETER can be used in this manner for measuring RF power. The resistor used to replace the normal load is specially designed to have low reactance and the ability to dissipate the required amount of power. Such resistors are commonly called DUMMY LOADS or DUMMY ANTENNAS.
Q-4. What name is given to a resistor used to replace the normal load in a circuit?
In the UHF and SHF frequency ranges, accurately measuring the voltage, current, and resistance is difficult. These basic measurements can vary greatly, depending on where in the circuit the measurements are made. They are also affected by small changes in parts placement in the vicinity of tuned circuits.
To measure the output of microwave radio or radar transmitters, you can use test instruments that convert RF power to another form of energy, such as light or heat. These instruments can be used to indirectly measure the power. A method used to measure the effect of a resistor load on a stream of passing air can also be used to indirectly measure power. Accurate measurement of large-magnitude power also can be achieved by measuring the temperature change of a water load. The most common type of power meter for use in this frequency range employs a BOLOMETER.
The bolometer is a loading device that undergoes changes of resistance as changes in dissipated power occur. The two types of bolometers are the BARRETTER and the THERMISTOR. The barretter is characterized by an increase in resistance as the dissipated power rises. The thermistor decreases in resistance as the power increases. In either case, resistance is measured before and after the application of RF power. If the same change in resistance is then produced by a variable dc source of power, then the RF power is equal to the measured dc power. This relationship makes possible the direct calibration of a bridge circuit in units of power. In other words, one condition of balance exists when no RF power is applied; but in the presence of power, a second condition of balance exists because of the resistance changes of the bolometer. It is this change of resistance that is calibrated in power.
Q-5. What are the two types of bolometers?
BARRETTER.—The construction of a typical barretter is shown in figure 2-2. The fine wire (usually tungsten) is extremely small in diameter. This thin diameter allows the RF current to penetrate to the center of the wire. The wire is supported in an insulating capsule between two metallic ends, which act as connectors. Because of these physical characteristics, the barretter resembles a cartridge-type fuse. The enclosure is a quartz capsule made in two parts. One part is an insert cemented in place after the tungsten wire has been mounted. In operation, the barretter is matched to the RF line after power is applied.
Figure 2-2.—Typical barretter.
THERMISTOR.—A high degree of precision is made possible by the thermistor; therefore, it is widely used. Figure 2-3 shows the typical construction of a bead-type thermistor. The negative- temperature coefficient comes from the use of a semiconductor as the active material. Notice that the
active material is shaped in the form of a bead. It is supported between two pigtail leads by connecting wires. The pigtail ends are embedded in the ends of the surrounding glass capsule.
Figure 2-3.—Bead-type thermistor.
The negative-resistance temperature coefficient of thermistors is desirable. This is because excessive power has the effect of changing the resistance of the thermistor to an extent that causes a pronounced RF mismatch. The resulting decrease in power transfer reduces the likelihood of burnout.
Figure 2-4, views A and B, is an example of a THERMISTOR BRIDGE used for RF power measurements. A thermistor bridge circuit includes other thermistor elements, referred to as compensating thermistors. These thermistors respond to fluctuations in ambient temperature so that the bridge balances and calibration are maintained over a wide temperature range. Compensating thermistors are usually in disc form so that they can be mounted on a flat metal surface, such as a chassis or a waveguide. The thermistor bridge in view B is located in the terminating section of a waveguide and contains RT-2, a bead thermistor, and two compensating thermistors, RT-1 and RT-3, on the outside of the waveguide (view A). RA-1 in view B, a calibrated attenuator, controls the amount of RF energy applied to RT-2.
Figure 2-4.—Thermistor bridge.
Before power is applied, R1 and R2 in view A of figure 2-4 are used to adjust the current through RT-2. When the resistance of RT-2 reaches the equivalent parallel resistance of R6 and RT1 (122.4 ohms), the bridge is balanced. Meter M-1 reads 0 at this time. The RF signal being measured is connected to the test set and applied via the calibrated attenuator to RT-2. This causes the temperature of RT-2 to increase, thus reducing its resistance. The bridge becomes unbalanced, causing meter M-1 to deflect an amount proportional to the decrease in resistance of RT-2. Meter M-1, because of the operation of RT-2, reads average power.
Q-6. As the dissipated power increases, what effect does this have on the resistance of a thermistor?
If the ambient temperature rises, the resistance of RT-1 decreases. This shunts more current around the bridge network and allows RT-2 to cool. The resistance of RT-3 decreases, maintaining meter sensitivity independent of temperature changes. Cavity Z-1 in view B of figure 2-4 is an ABSORPTION- TYPE FREQUENCY METER. This type of meter will be discussed later.
Frequency measurements are an essential part of preventive and corrective maintenance for electric and electronic equipment. Some examples of the various frequency measurements follow:
· Rotation frequencies of some electro-mechanical devices, such as electric motors, must be determined.
· The output frequency of electric power generators is checked when the engine is started and during preventive maintenance routines.
· Equipment that operates in the AF range must be adjusted to operate at the correct frequencies.
· Radio transmitters must be accurately tuned to the assigned frequencies to provide reliable communications and to avoid interference with radio circuits operating on other frequencies.
· Radar sets must be properly tuned to obtain satisfactory performance.
As you can see from the above examples, frequency measurement does indeed play a valuable role in maintenance. These measurements can be divided into two broad categories: MECHANICAL- ROTATION FREQUENCY measurement and ELECTRICAL-OUTPUT FREQUENCY measurement. Depending upon your job and/or the type of command to which you are assigned, you may be tasked with performing one or both of these types of measurements.
MECHANICAL-ROTATION FREQUENCY MEASUREMENT
The rotating frequency (speed in revolutions per minute) of armatures in electric motors and engine- driven generators, as well as the blade speed in turbines, is measured with devices called TACHOMETERS, STROBOSCOPES, and STROBOTACS.
A tachometer is an instrument that measures the rate at which a shaft is turning. Although tachometers are installed on machinery, such as generators and engines, you may need to determine the speed of a rotating machine that is not equipped with a tachometer. In these instances, you will be required to use a PORTABLE TACHOMETER. Portable hand-held tachometers measure speed by direct contact with the shaft of the measured unit. Portable tachometers are for use only during testing and should not be used continuously. The common types of portable tachometers are the CENTRIFUGAL and the CHRONOMETRIC.
CENTRIFUGAL TACHOMETER.—A centrifugal-type tachometer is illustrated in figure 2-5, view A. View B shows the internal arrangement of the centrifugal tachometer; refer to view B in this discussion. In the centrifugal tachometer, centrifugal force acts upon fly weights that are connected by links to upper and lower collars. The upper collar is affixed to a drive shaft; the lower collar is free to move up and down the shaft. A spring, which fits over the shaft, connects the upper and lower collars.
Figure 2-5.—Centrifugal tachometer.
Each portable centrifugal tachometer has a small rubber-covered wheel and a number of hard rubber tips. You fit the appropriate tip or wheel on the end of the tachometer drive shaft, and hold it against the shaft to measure speed of rotation. As the drive shaft begins to rotate, the fly weights rotate with it. Centrifugal force tends to pull the fly weights away from the center, causing the lower collar to rise and compress the spring. The lower collar is attached to a pointer, and its upward motion, restricted by the spring tension, causes an increase in the indication on the dial face.
When properly used, a centrifugal tachometer will indicate correct shaft speed as long as it is in contact with the machine shaft under test. A portable centrifugal tachometer has three ranges: low (50 to
500 rpm), medium (500 to 5,000 rpm), and high (5,000 to 50,000 rpm).
CHRONOMETRIC TACHOMETER.—The chronometric tachometer (figure 2-6) is a combination watch and revolution counter. It measures the average number of revolutions of a shaft per minute. The chronometric tachometer also comes with hard rubber tips, which must be inserted over the drive shaft.
Figure 2-6.—Chronometric tachometer.
When applied to a rotating shaft, the outer drive shaft of this tachometer runs free until a starting button is depressed to start the timing element. In figure 2-6, note the starting button beneath the index finger. The chronometric tachometer retains readings on its dial after its drive shaft has been disengaged from a rotating shaft and until the pointers are returned to 0 by the reset button (usually the starting button). The range of a chronometric tachometer is usually from 0 to 10,000 rpm and from 0 to 3,000 feet per minute (fpm).
The rotation frequencies of recording devices and teletypewriter motors can be measured by the use of a STROBOSCOPE. The stroboscope is an instrument that allows you to view rotating or reciprocating objects intermittently and produces the optical effect of a slowing down or stopping motion. For example, electric fan blades revolving at 1,800 rpm will appear stationary if you look at them under a light that flashes uniformly 1,800 times per minute. At 1,799 flashes per minute, the blades will appear to rotate forward at 1 rpm; at 1,801 flashes per minute, they will appear to rotate backward at 1 rpm.
When the flashing rate of the light is adjustable, you can calibrate the control in flashes (or revolutions) per minute. The stationary image you see when the rate of the lamp and the rotational rate of a shaft are equal lets you record a very precise speed measurement.
The STROBOTAC (figure 2-7) is an electronic flash device in which the flash duration is very short (a few millionths of a second). (Table 2-1 contains a description of the controls and indicators shown on the strobotac in figure 2-7.) Because of this short flash duration, the strobotac can measure very rapid motion. The box contains a swivel mount with a STROBOTRON LAMP in a reflector, an electronic pulse generator to control the flashing rate, and a power supply that operates from the ac power line. The flashing rate is controlled by the large knob; the corresponding speed (rpm) is indicated on an illuminated dial that is viewed through windows in the knob.
Introduction to Matter, Energy, and Direct Current,
to Alternating Current and Transformers, Introduction to Circuit Protection,
Control, and Measurement
, Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading
, Introduction to Generators and Motors
Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to Solid-State Devices and Power Supplies
Introduction to Amplifiers, Introduction to
Wave-Generation and Wave-Shaping Circuits
, Introduction to Wave Propagation, Transmission
Lines, and Antennas
, Microwave Principles,
, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros
Introduction to Test Equipment
, Radar Principles,
The Technician's Handbook,
Master Glossary, Test Methods and Practices,
Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics