BASIC TELEVISION MULTIPLE CHOICE – GROB TV 1. Which of the following camera tube uses lead oxide (PbO) for the photoconductive target plate? A. Vidicon C. Saticon B. Plumicon D. Image Orthicon 2. Camera signal output without sync is called A. Black burst C. composite video vid eo B. generator lock video D. noncomposite video 3. A low-contrast picture in which white seems flat and lacking in detail suggest A. low beam current C. excessive gamma B. high gain in the amplifier D. insufficient scanning width 4. Which of the following camera tubes has minimum lag? A. Vidicon C. Saticon B. Plumbicon D. Iconoscope 5. The part of the visible spectrum where camera pickup tubes have the greatest output is A. red C. yellow-green B. blue D. infrared 6. Precise scanning size and linearity are most important in A. a black-and-white camera B. a plumbicon C. a single-tube color pickup D. a saticon
BASIC TELEVISION MULTIPLE CHOICE – GROB TV 13. Which of the following is FALSE? A. The lens inverts the optical image o n the faceplate of the camera tube. B. The composite video signal includes the camera signal and sync not blanking C. The standard composite video from a camera is 1V p-p with negative sync. D. The plumbicon uses a lead oxide layer for the target plate. 14. Which of the following is TRUE? A. The diameter of the vidicon image p late is about 5 in. (127 mm) B. The plumbicon camera tube uses a silicon target plate. C. The composite video signal includes the camera signal and sync but not blanking. D. the lens inverts the optical image on the faceplates of the camera tube. 15. Which of the following produces the signal variations for G4 of the vidicon. A. Target plate C. Muzzle B. Wire mesh D. Beam control 16. What is the gamma required for the camera tube? A. 2.2 B. 0.4545 C. 4.545 D. 0.22 17. __________ is more important for the gamma correction for the camera tube. A. Color C. Both Color and monochrome B. Monochrome D. Either of Color or monochrome
7. Beam alignment magnets for the camera tube are adjusted while rocking which control? A. C. Beam current Optical focus B. Electrical focus D. Shading
18. In gamma correction the ___________ is stretched by the picture tube. A. Black B. gray C. white D. red
8. Special effects and production switching are done by the A. CCU C. SEG B. ENG camera D. Sync generator
19. To how many lax units is the illuminator of 3 fs, approximately equal? A. 650 B. 225 C. 65 D. 30
9. The gamma of the picture tube is A. 0.4545 C. 1.4 B. 1.0 D. 2.2
20. What is the diagonal screen size for the 19CP4 picture tube? A. 12 in. B. 16 in. C. 19 in. D. 24 in.
10. If the camera cannot be placed far away enough to include everything in the scene, change the lens to one with a A. lower f rating C. longer focal l ength B. higher f rating D. shorter focal length 11. A typically value of vidicon dark current is A. 0.2 uA C. 8 mA B. about 200 uA D. 800 mA 12. A lens has an 8-cm focal and 4-cm diameter. Its f rating is A. 2 C. 8 B. 4 D. 32
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21. To what deflection angle does a maximum deflection angle of 45 deg either side center correspond? A. 30º B. 90º C. 120º D. 360º 22. What is the usual heater voltage for picture tube? A. 1.6 v B. 5.6 V C. 6.3 V D. 9.3V 23. What is the typical anode voltage for a 25-in color picture tube? A. 10 kV B. 30 kV C. 1 kV D. 30 V 24. Typically, the anode capacitance for a 25-in tube is _____ pT. A. 2 B. 20 C. 205 D. 2000 25. When the TV set was turned on, full power was applied to the heater and the picture appeared within a fraction of a second. A. Ultor C. Instant-on operation B. Implosion D. Screen persistence 2 of 12
BASIC TELEVISION MULTIPLE CHOICE – GROB TV 26. What are the phosphor numbers, respectively, for monochrome and color picture tubes? A. P1 and P4 C. P4 and P22 B. P1 and P22 D. P4 and P1 27. What is the color of P1 screen phosphor? A. Red B. Blue C. White D. Green 28. Which of the following is the most negative (or least positive) electrode is the electron gun? A. Cathode C. Screen grid B. Control grid G1 D. Focus grid 29. Which is the most position element in the electron gun? A. Ultor C. Cathode B. Anode D. Ultor or anode 30. Most of the electrons in the beam flow out of which terminal? A. Ultor C. Cathode B. Anode D. Ultor or anode 31. The crossover point for focusing is formed by the _____ electron lens. A. first C. Cathode B. second D. fourth 32. In which method in a G3 voltage of 200 V. used for? A. low-voltage focus C. either low or high voltage focus B. high-voltage focus D. none of these 33. Small magnets embedded in the yoke housing is correct for __________. A. yoke position C. pinoushion distortion B. magnetic deflection D. centering adjustments 34. The coils above and below the electron beam of the picture tube are for ________. A. V scanning C. Either V or H scanning B. H scanning D. None of these
35. Neck shadow result when the deflection yoke is ________. A. too near back C. too far back B. too near forward C. too too far forward 36. In color picture tube, degaussing should be done ______ the color purity adjustments. A. after C. either way B. before D. none at all 37. In color picture tube, degaussing is done with _________. A. direct current C. 60 Hz alternating current B. alternating current D. 90 Hz alternating current 38. When the receiver is first turned on, what current does the ADG circuit have? A. High C. Both high and low B. Low D. Neither high nor Low
BASIC TELEVISION MULTIPLE CHOICE – GROB TV 39. In color picture tube, what does a solid red raster checked for? A. good color purity C. Degaussing B. Convergence D. Resetting the yoke 40. Where do the small, white dots in the picture used for? A. good color purity C. Degaussing B. convergence D. Resetting the yoke 41. What does the color fringing on the edge of the picture shows? A. Misconvergence C. Turned off guns B. Insufficient signal drive D. Red cloud 42. Static convergence is done for the _________ of the screen. A. center B. edges C. left D. right 43. Where the permanent magnets used? A. static convergence C. automatic degaussing B. dynamic convergence D. resetting the yoke 44. In picture, the basic waveform for dynamic convergence is the ___________. A. parabola C. full sine wave B. half a sine wave D. parabola or half of sine wave 45. Pincushion magnets are used for ______ picture tubes. A. monochrome C. both color and monochrome B. color D. either color or monochrome 46. The abbreviation TW is for _____ pincushion correction. A. C. horizontal diagonal B. vertical D. either way 47. The typical dc grid bias for a 19-in, picture tube. A. -4 V B. -40V C. -6 V D. -60 V 48. Typical ac signal drive for a 19-in, picture tube. A. 8 Vp-p C. 120 Vp-p B. 40 Vp-p D. 140 Vp-p 49. For the picture tube, the brightness control varies the ___ bias signal drive. A. dc C. either ac or dc B. ac D. both ac and dc 50. The R, G, and B screen grid adjustments are set for ___ in the picture. A. visual cutoff C. brightness control B. white highlights D. gray-scale tracking 51. The R, G, and B video drive controls are set for ___ in the picture. A. gray C. black dark B. white D. green 52. Background controls of many picture tubes are for the A. ac bias C. ac video signal B. dc bias D. dc video signal 4 of 12
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BASIC TELEVISION MULTIPLE CHOICE – GROB TV 53. The G2 master screen control of picture tubes vary the _________. A. ac voltage C. ac video signal B. dc voltage D. dc video signal 54. In the sawtooth waveform for linear scanning A. the linear rise if for flyback B. the complete cycle includes trace and retrace C. the sharp reversal in amplitude produces trace D. the beam moves faster during trace than retrace. 55. Given a 635 us vertical retrace time, the number of complete horizontal lines scanned during vertical flyback is A. 10 B. 20 C. 30 D. 63 56. One-half line spacing between the start position for scanning even and odd fields produces A. linear scanning C. fishtailing B. line pairing D. exact interlacing 57. The number of lines scanned per frame in the raster on the picture tube screen is A. 525 B. 262 C. 20 D. 10 58. In the frame for which interlaced scanning is used, alternate lines are skipped during vertical scanning because A. the trace is slower than the retrace B. the vertical scanning frequency is doubled from 30 to 60 Hz C. The horizontal scanning is slower than vertical scanning D. the frame has a 4 : 3 aspect ratio 59. If the horizontal flyback is 10 percent, this time equals A. 10 us B. 56 us C. 6.4 us D. 83 us
BASIC TELEVISION MULTIPLE CHOICE – GROB TV 64. Which of the following is faster in horizontal scanning? A. trace C. flyback B. retrace D. retrace or flyback 65. Which of the following takes more time? A. H retrace C. V retrace B. H trace D. V trace 66. What does an interlaced scanning require for the number of the horizontal lines? A. odd C. Both odd and even simultaneously B. Even D. Either odd or even 67. How many horizontal lines are in the odd or an even field? A. C. 525 lines 10 ½ B. 262 ½ lines D. 600 lines 68. How many H lines are there in a complete frame? A. 19 lines C. 262 ½ lines B. 21 lines D. 600 lines
69. How many H lines are there in each field? A. 8 ½ lines C. 262 ½ lines B. 10 ½ lines D. 325 lines 70. How many H lines are there in each V retrace? A. One C. three B. two D. four 71. What are the frequencies of V scanning, V sync, and V blanking? A. 30 Hz B. 60 Hz C. 90 Hz D. 360 Hz 72. What are the frequencies of H scanning, H sync and H blanking? A. 15,750 Hz C. 31,500 Hz B. 16,750 Hz D. 30,050 Hz
60. Which of the following is NOT true? A. line pairing indic ates poor interlacing. B. People look too tall and too thin on a square raster on the picture tube screen. C. A person can appear to have one shoulder wider than the other because of nonlinear horizontal scanning. D. the keystone effect produces a square raster.
73. In video signal analysis, what are the three parts of the composite video signal, for two horizontal lines in the picture? A. Camera signal C. H blanking B. H sync D. all of the above
61. The width of a vertical sync pulse with its narrations includes the time of A. sux half-lines, or three lines B. five lines C. three half-lines D. five half-lines
74. In the IRE scale for composite video signal, list the number of IRE units used for sync, black setup, and the camera signal? A. 40, 7.5, 92.5 respectively B. 92.5, 40, 7.5 respectively C. 7.5, 40, 92.5 respectively D. 92.5, 7.5,40 respectively
62. Sawtooth generator circuits produce the scanning raster, but the sync pulses are needed for A. ity C. keystoning linear B. timing D. line Pairing
75. What are the approximate time periods for the width of H blanking pulse in microseconds? A. 5 B. 0.93 C. 10.2 D. 53.5
63. Which of the following frequencies is wrong? A. 15,750 Hz for horizontal syn c and scanning B. 60 Hz for vertical sync and scanning C. 31,500 Hz for equalizing pulses and serrations in the vertical sync pulses D. 31,500 Hz for the vertical scanning frequency 5 of 12
76. What are the approximate time periods for the visible H trace? A. 5 B. 0.93 C. 10.2 D. 53.5 77. What are the approximate time periods for the width of H sync pulse? A. 5 B. 0.93 C. 10.2 D. 53.5 6 of 12
BASIC TELEVISION MULTIPLE CHOICE – GROB TV 78. Which pulses in V blacking correspond to the 3H lines wide? A. V sync C. Equalizing B. V blanking D. all of the above 79. Which pulses in V blanking correspond to the 21H lines wide? A. V sync C. Equalizing B. V blanking D. all of the above 80. Which pulses in V blacking correspond to the 31,500 Hz? A. V sync C. Equalizing B. V blanking D. all of these 81. Give the maximum number of picture details for each horizontal line. A. 338 C. 125,000 338 B. 426 D. 144,000 82. Give the maximum number of picture details for total picture area. A. 338 C. 125,000 B. 426 D. 144,000 83. The average dc level is close to the blanking level for a ________ scene? A. light C. either dark or light B. dark D. neither dark nor light 84. In what condition does the picture tube reproduce black? A. maximum beam current C. zero beam current B. minimum beam current D. none of these 85. The gamma effects the _________ of the picture tube. A. contrast C. dullness B. Brightness D. either the brightness or contrast 86. Picture tube has games which is __________. A. greater than 1 C. infinity B. less than 1 D. zero 87. Brightness variations of the picture information are in which signal? A. 1 B. Q C.Y D. R-Y 88. The hue 180º out of phase with red is A. cyan C. green B. yellow D. blue 89. Greater p-p amplitude of the 3.58MHz chrominance signal indicates more A. white C. hue B. yellow D. saturation 90. The interfering beat frequency of 920 kHz is between the 3.58 MHz color subcarrier and the A. 4.5 MHz intercarrier sound C. lower adjacent sound B. picture carrier D. upper adjacent picture 91. The hue of color sync phase is A. red B. cyan C. blue D. yellow-green 7 of 12
BASIC TELEVISION MULTIPLE CHOICE – GROB TV 92. Which signal has color information for 1.3 MHz bandwidth? A. I B. Y C. R-Y D. B-Y 93. Which of the following is false? A. the I video hues are orange or cyan B. The transmitter matrix output includes Y, I, and Q video C. A three-gun picture tube that can serve as a matrix. D. a fully saturated color is mostly white 94. The color with the most luminance is A. red B. yellow C. green D. blue 95. What is the hue of a color 90º leading sync burst phase? A. yellow B. cyan C. Blue D. Orange 96. The average voltage value of the 3.58 MHz modulated chrominance signal is A. zero for most colors B. close to black for yellow C. the brightness of the color D. the saturation of the color 97. The second IF value for color in receivers, for any station, is A. 0.5 MHz C. 3.58 MHz B. 1.3 MHz D. 4.5 MHz 98. if the 3.58-MHz C amplifier in the receiver does not operate, the result will be A. no color C. too much blue B. no red D. too much yellow 99. How many octaves is the frequency range 1 to 8MHz? A. 1 B. 2 C. 3 D. 8 100. Which system can be used for both recording and playback? A. CEO C. Laser disk B. VHD D. VHS 101. How many TV fields are recorded on one slant track of tape? A. 1 B. 2 C. 4 D. 60 102. The video heads rotate at high velocity to increase the A. tape speed C. reel rotation B. writing speed D. tape tension 103. A typical frequency for the FM luminance signal inVCR recording is A. 0.1 MHz C. 10 MHz B. 1.7 MHz D. 680 kHz 104. Which of the following applies to the color-under technique? A. chroma amplitude are decreased B. chroma frequencies are reduces C. luminance frequencies are decreased D. chroma and luminance frequencies are reduced 105. What oscillator frequency is needed to heterodyne 329 kHz up to 3.58 MHz? A. C. 6.3 MHz 3 MHz B. 4.21 MHz D. 10 MHz 8 of 12
BASIC TELEVISION MULTIPLE CHOICE – GROB TV 106. A comb filter is used to A. cancel chroma crosstalk B. separate white from black C. clip the sync from blanking D. separate alternating from direct current 107. Switching for each field is required for the A. audio head C. video heads B. control-track head D. erase head 108. Servocontrol of speed and phase is used for the A. control head C. audio head B. erase head D. video head scanner 109. The part that rotates to meter out the tape at constant speed is the A. control head C. entrance guide B. erase head D. capatan 110. To make the tape speed the name in playback as in recording, the tape speed is regulated by the A. C. video silent tracks audio tra ck B. control-track pulses D. erase head 111. tilting the viseo head gaps is necessary with the A. color-under C. FM luminance signal B. zero guard bands D. long-play tubes 112. Which system uses a laser light beam for playback? A. CED C. tamax Be B. VHD D.VLF 113. In the CED system, the disk capacitance varies with the A. pit depth C. speed of rotation B. disk size D. wavelength of the scanning light 114. The modulated picture carrier wave includes the composite video signal as the A. average carrier level B. asymmetric envelope of amplitude variations C. lower sideband without the upper sideband D. upper envelope without the lower envelope 115. Which of the following statements is true? A. Negative transmission means t hat the carrier amplitude decreases for black. B. Negative transmission means that the carrier amplitude decreases for white C. Vestigial sideband transmission meand that both upper and lower sidebands are transmitted for all modulating frequencies. D. Vestigial sideband transmission means that the modulated picture carrier signal has only the upper envelope.
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BASIC TELEVISION MULTIPLE CHOICE – GROB TV 116. With a 2 MHz video signal modulating the picture carrier signal for channel 4 (66 to 72 MHz) which of the following frequencies are transmitted? A. 66 MHz carrier frequency and 68 MHz upper side fre quency B. 71.5 MHz carrier frequency with 69- and 73 MHz side frequencies C. 67.25 Mhz carrier frequency with 65.25 and 69.25 MHz side frequencies D. 67.25 MHz carrier and 69.25 MHz upper side frequency. frequency . 117. With a 0.5 MHz video signal modulating the picture carrier. A. both upper and lower side frequencies are transmitted B. only the upper side frequency is transmitted C. only the lower side frequency is transmitted D. no side frequencies are transmitted 118. In all standard television broadcast channels, the difference between the picture and sound carrier frequencies is A. 0.25 MHz C. 4.5 MHz B. 1.25 MHz D. 6 MHz 119. The difference between the sound carrier frequencies in two adjacent channels is A. 0.25 MHz C. 4.5 MHz B. 1.25 MHz D. 6 MHz 120. Line-of-sight transmission is a characteristic of propagation for the A. VHF band and higher frequencies B. VHF band but not the UHF band C. radio frequencies below 1 MHz D. AM picture signal but not the FM sound signal 121. In channel 14 (470 to 76 MHz), the 3.58 MHz color signal is transmitted at A. 471.25 MHz C. 474.83 MHz B. 473.25 MHz D. 475.25 MHz 122. The difference between the sound carrier and color subcarrier frequencies is A. 4.5 MHz C. 0.92 MHz B. 1.25 MHz D. 0.25 MHz 123. The maximum deviation of the FM sound signal, in kilohertz, is A. 10 C. 75 B. 25 D. 100 124. Which of the following applies for a monochrome picture? A. chroma amplifier on C. picture tube off B. chroma amplifier off D. color demodulator input 125. Which of the following is NOT tuned to 3.8 MHz? A. C. chroma amplifier Burst amplifier B. video preamplifier D. color demodulator input 126. The contrast control is in the A. chroma amplifier C. T video amplifier B. color killer D. delay line
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BASIC TELEVISION MULTIPLE CHOICE – GROB TV 127. The color level control is in the A. demodulator C. AFPC B. BPA D. G-Y amplifier
BASIC TELEVISION MULTIPLE CHOICE – GROB TV 141. The cable converter output for the TV receiver is usually on channel A. 3 B. 6 C. 7 D. 9
128. The color oscillator does not operate. The trouble in A. incorrect hues C. no color B. excessive confetti D. no picture
142. The VSWR for a line terminated in its Zo is A. 0 B. 1 C. 1.5 D. 2
129. The balance for Y video signals to the three guns in the picture tube is set by the A. screen controls C. contrast control B. tint control D. drive controls 130. Which signal needs a 0.6-us time delay? A. 3.58 C. Y video B. B-Y video D. Color burst 131. The output of the burst separator feeds the A. color demodulator C. AFPC for color oscillator B. G-Y adder D. Y video amplifier 132. The output of the color oscillator feeds the A. chroma BFA C. picture tube B. color demodulator D. burst separator
143. How many dBmV units correspond to a 1-mV signal level? A. 0 B. 1 C. 3 D. 6 144. A tap for the subcarrier drop line has a A. high insertion loss C. low tap loss B. high tap loss D. 300 ohm impedance C. chroma BPA 145. The most popular plug for RG-590 coaxial cable is the A. RCA phonograph plug C. F connector B. 4-pin DIN connector D. banana pin 146. Which of the following is TRUE? A. Excessive signal causes in the picture B. A weak signal causes-modulation distortion C. A weak signal causes snow in the picture D. A scrambled signal has excessive sync amplitude
133. Drifting color bars in the picture indicate trouble in the A. Y video amplifier C. color killer B. chroma BFA D. AFPC for color oscillator
147. The upstream signal in two-way cable systems has the frequency of A. 5 to 30 MHz C. 500 MHz B. 3 to 300 MHz D. 13 GHz
134. The best frequency between the 3.8 MHz color subcarrier and the 4.5 MHz sound signal is A. 0.92 MHz C. 4.8 MHz B. 3.0 MHz D. 4.5 MHz
148. A typical value for the IF signal, in megahertz, for up-down cable converter A. 45.75 C. 500.75 B. 300.75 D. 612.75
135. Which control varies the phase angle of the demodulated color video signal A. color level C. drive B. tint D. picture
149. Frequency synthesis is used for A. VCO in the up converter C. fiber-optic cable B. the the trunk amplifier D. microwave
136. Which of the following stages must be on during horizontal flyback time? A. Y video amplifier C. burst separator B. chroma BPA D. R-Y video amplifier 137. Which of the following stages has bias from the ACC and color killer circuits? A. R-Y demodulator C. chroma BPA B. R-Y video amplifier D. Color oscillator 138. A crystal-ringer circuit is used for the A. Y video amplifier C. color demodulator demodu lator B. AFPC on color oscillator D. oscillator D. chroma BPA 139. Which of the following is a midband cable TV channel? A. 6 B. 7 C. A or 14 D. J or 23 140. Coaxial cable for distribution systems has an impedance of A. 50 ohms B. 75 ohms C. 150 ohms D. 300 ohms
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150. For in-band descramblers, the decoding pulses are sent on the A. color subcarrier C. picture carrier B. sound carrier D. H sync pulses 151. Which of the following is NOT true? A. Microwave links can use FM. B. Fiber-optic cables have very high losses C. Supertrunk lines use large cable for low losses D. The value 13 GHz is in the microwave band. 152. A trunk cable run a loss of -20 dBmV. To make up for this loss, the voltage gain of the next amplifier should be at least A. 10 B. 100 C. 200 D. 300
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Electus Distribution Reference Data Sheet: OHMPOWER.PDF (1) OHMS LAW & POWER MEASUREMENTS The relationship between voltage, current and resistance is called Ohm.s Law, so named after its discovery by German physicist George Simon Ohm in 1827. Ohm discovered that there is a relatively linear relationship between the potential difference applied to the ends of a conductor, and the current that is flowing through it. The parameter that relates voltage and current is defined as the conductor.s resistance. From this comes the following formula: R = (V/I) where R is the resistance, measured in ohms (W) V is the potential difference in volts (V) I is the current in amperes (A) As you can see from the formula, the resistance o f a conductor which passes one ampere with a PD of one volt applied between its ends would therefore be 1 ohm. The simple formula can be transposed thus: V = I x R or I = (V/R) which can be very handy! POWER IN A CIRCUIT (DC) When electrical power is dissipated in a circuit, heat is created. The amount of heat is expressed in watts . whether it.s from your electric kettle, a radiator or even your hifi amplifier. Here.s the basic formula: P=VxI where P = power in watts V = potential difference in volts I = current in amps Just like Ohms law this can be transposed to: V = (P/I) or I = (P/V) Other useful formulas relating power, voltage, current and resistance are:
POWER IN AC CIRCUITS The calculations above only relate directly to DC circuit conditions, although they can be applied to a circuit carrying AC provided that: 1. The RMS values of the voltage and current are used (see below); and 2. There is virtually no capacitance or inductance, to produce significant phase shifts. If there is significant capacitance and/or inductance, power calculations become a bit more complicated as we.ll see shortly. MEASURING AC WAVEFORMS Most circuits encountered in electronics are carrying AC, and we often need to measure AC waveforms with a view to calculating circuit component values, signal levels etc. There are several ways of describing an AC waveform, and each one has a use, depending on what you need to know. AC signals are by definition different from one in stant in time to the next. A pure AC signal swings about a zero voltage axis, going positive one moment and negative the next. This means that its average value over a complete cycle is zero, because the positive and negative sides of the waveform cancel out. However the positive-going part of the wave can still deliver energy, and so can the negative part. There is plenty of energy, for example, in a 240V AC power point! So we have to find an alternative way of describing this energy-delivering aspect of AC, other than the average value.
The parameter we use instead of the average value is the RMS or .root mean square. value, which is found by squaring the instantaneous values of the AC voltage or current, then calculating their mean (i.e., their average) and finally taking the square root of this . which gives the effective value of the AC voltage or current. These effective or RMS values don.t average out to zero, and are essentially the AC equivalents of DC voltage and current. RMS POWER MEASUREMENT When we use the effective or RMS values of AC voltages and currents, and we have a circuit which is resistive (i.e., with no phase shifts due to capacitance or inductance), we can simply multiply the voltage and current together to give the power dissipated . just as we can for DC. For example an AC waveform of 100V RMS applied across a resistive load (say 10 ohms) would draw the equivalent current (10A RMS) that we.d get from 100V DC, and the load would dissipate the same amount of heat energy: 1000 watts. Another name for this .RMS power . is continuous effective power . Note that although it.s not too difficult to calculate the RMS value of regular repetitive waveforms like sine, sawtooth, triangular or square waves, it.s much more difficult to do so with non-repetitive waveforms such as a music signal with non-repetitive peaks. This is why, for example, amplifier power ratings are calculated and measured with sinewave signals. Although typical amplifiers normal ly don.t handle sinewaves, these waveforms do provide a standardised way to measure and rate amplifier performance. POWER IN AC CIRCUITS WITH REACTANCE If a circuit carrying AC has capacitance and/or inductance as well as resistance, these don.t dissipate power themselves but the added capacitive or inductive reactance produces a phase shift between the voltage and current. This means that the power dissipated in the circuit.s resistance can no longer be calulated simply by multiplying the RMS voltage and current. Doing this merely gives a quantity known as the circuit.s .volt-amps. (VA). The real power in the circuit.s resistance can only be found by multiplying the RMS voltage across it with the proportion of the RMS current flowing through it which is in exactly the same phase. This works out to be: PREAL = (VRMS x IRMS) x cos(f) where f is the phase angle between the voltage and current. This real power will generally be smaller than the VA figure, because the VA turns out to include the energy that is simply stored in the circuit.s reactances during one half of the AC waveform, and returned during the other. PEAK & PEAK-TO-PEAK VALUES There are other ways of describing an AC waveform besides the RMS value, which are sometimes useful. For example it.s often necessary to know a waveform.s half-wave peak level (to requirements for example) or its peak-to-peak level, which is simply the total swing between the positive and negative-going peaks of the waveform. MEASURING AC VOLTAGE & CURRENT Many digital multimeters do not measure the RMS value of AC voltages directly. Often they simply measure the peak value, and calculate the equivalent RMS value . assuming a sine waveform. This calculated value is the one displayed. Older moving-coil meters tend to measure the half-wave average value, but are made to indicate the equivalent RMS. Of course some DMM.s do in fact measure the RMS value of voltages and currents, and these True RMS reading meters are generally the best type to use if you really need to know
the RMS value . especially for non-sinewave voltages and currents. However you.ll find that such meters tend to be somewhat more expensive than the regular type. Incidentally, it.s worth remembering that a True RMS meter will also include the contribution of any DC voltage and current which may be present along with the AC. Happily you can still get a fairly accurate idea of the RMS value of a sine waveform, knowing one of the other measurements such as the half-wave average, peak or peak-to-peak value. This can be done by calculation, or using the handy table on this page. As you can see it.s also possible to work out the RMS value of a few other symmetrical and regular waveforms, such as square and triangular waves, knowing their peak, average or peak-peak values. The important thing to bear in mind, though, when using this type of table is that you do need to know the exact basis on which your meter.s measurement is made. For example if it really measures the peak value, and then calculates and displays the equivalent .sinewave RMS. figure, this means you.ll need to use the table differently from the situation where it measures the half-wave average and calculates the sinewave RMS figure from that. So take care, especially if you.re not sure exactly how your meter works. (Copyright © Electus Distribution, 2001
