NEETS Module 9  Introduction to Wave Generation and WaveShaping
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Q16. What is the filter called in which the low frequencies do not produce a useful voltage? Q17. What is the filter called that passes low frequencies but rejects or attenuates high frequencies? Q18. How does a capacitor and an inductor react to (a) low frequency and (b) high frequency? Q19. What term is used to describe the frequency at which the filter circuit changes from the point of rejecting the unwanted frequencies to the point of passing the desired frequencies? Q20. What type filter is used to allow a narrow band of frequencies to pass through a circuit and attenuate all other frequencies above or below the desired band? Q21. What type filter is used to block the passage of current for a narrow band of frequencies, while allowing current to flow at all frequencies above or below this band? MULTISECTION FILTERS All of the various types of filters we have discussed so far have had only one section. In many cases, the use of such simple filter circuits does not provide sufficiently sharp cutoff points. But by adding a capacitor, an inductor, or a resonant circuit in series or in parallel (depending upon the type of filter action required), the ideal effect is more nearly approached. When such additional units are added to a filter circuit, the form of the resulting circuit will resemble the letter T, or the Greek letter p (pi). They are, therefore, called T or ptype filters, depending upon which symbol they resemble. Two or more T or ptype filters may be connected together to produce a still sharper cutoff point. Figure 123, (view A) (view B) and (view C), and figure 124, (view A) (view B) and (view C) depict some of the common configurations of the T and ptype filters. Further discussion about the theory of operation of these circuits is beyond the intended scope of this module. If you are interested in learning more about filters, a good source of information to study is the Electronics Installation and Maintenance Handbook (EIMB), section 4 (Electronics Circuits), NAVSEA 0967LP0000120. Figure 123A.  Formation of a Ttype filter. 141
Figure 123B.  Formation of a Ttype filter. Figure 123C.  Formation of a Ttype filter. Figure 124A.  Formation of a ptype filter. Figure 124B.  Formation of a ptype filter. 142
Figure 124C.  Formation of a ptype filter. SAFETY PRECAUTIONS When working with resonant circuits, or electrical circuits, you must be aware of the potentially high voltages. Look at figure 125. With the series circuit at resonance, the total impedance of the circuit is 5 ohms. Figure 125.  Series RLC circuit at resonance. Remember, the impedance of a seriesRLC circuit at resonance depends on the resistive element. At resonance, the impedance (Z) equals the resistance (R). Resistance is minimum and current is maximum. Therefore, the current at resonance is: The voltage drops around the circuit with 2 amperes of current flow are: E_{C} = I_{T} x X_{C} E_{C} = 2 x 20 E_{C} = 40 volts AC E_{L} = I_{T} x X_{L} 143
E_{L} = 2 x 20 E_{L} = 40 volts AC E_{R} = I_{T} x R E_{R} = 2 x 5 E_{R} = 10 volts AC You can see that there is a voltage gain across the reactive components at resonance. If the frequency was such that X_{L} and X_{C} were equal to 1000 ohms at the resonant frequency, the reactance voltage across the inductor or capacitor would increase to 2000 volts AC with 10 volts AC applied. Be aware that potentially high voltage can exist in seriesresonant circuits. SUMMARY This chapter introduced you to the principles of tuned circuits. The following is a summary of the major subjects of this chapter. THE EFFECT OF FREQUENCY on an INDUCTOR is such that an increase in frequency will cause an increase in inductive reactance. Remember that X_{L} = 2πfL; therefore, X_{L} varies directly with frequency. THE EFFECT OF FREQUENCY on a CAPACITOR is such that an increase in frequency will cause a decrease in capacitive reactance. Remember that therefore, the relationship between X_{C} and frequency is that X_{C} varies inversely with frequency. 144
RESULTANT REACTANCE X = (X_{L}  X_{C}) or X = (X_{C}  X_{L}). X_{L} is usually plotted above the reference line and X_{C} below the reference line. Inductance and capacitance have opposite effects on the current in respect to the voltage in AC circuits. Below resonance, X_{C} is larger than X_{L}, and the series circuit appears capacitive. Above resonance, X_{L} is larger than X_{C}, and the series circuit appears inductive. At resonance, X_{L} = X_{C}, and the total impedance of the circuit is resistive. A RESONANT CIRCUIT is often called a TANK CIRCUIT. It has the ability to take energy fed from a power source, store the energy alternately in the inductor and capacitor, and produce an output which is a continuous AC wave. The number of times this set of events occurs per second is called the resonant frequency of the circuit. The actual frequency at which a tank circuit will oscillate is determined by the formula: IN A SERIESLC CIRCUIT impedance is minimum and current is maximum. Voltage is the variable, and voltage across the inductor and capacitor will be equal but of opposite phases at resonance. Above resonance it acts inductively, and below resonance it acts capacitively. 145
IN A PARALLELLC CIRCUIT impedance is maximum and current is minimum. Current is the variable and at resonance the two currents are 180 degrees out of phase with each other. Above resonance the current acts capacitively, and below resonance the current acts inductively. 146
THE "Q" OR FIGURE OF MERIT of a circuit is the ratio of X_{L} to R. Since the capacitor has negligible losses, the circuit Q becomes equivalent to the Q of the coil. THE BANDWIDTH of a circuit is the range of frequencies between the halfpower points. The limiting frequencies are those at either side of resonance at which the curve falls to .707 of the maximum value. If circuit Q is low, you will have a wide bandpass. If circuit Q is high, you will have a narrow bandpass. 147
A FILTER CIRCUIT consists of a combination of capacitors, inductors, and resistors connected so that the filter will either permit or prevent passage of a certain band of frequencies. A LOWPASS FILTER passes low frequencies and attenuates high frequencies. 148
A HIGHPASS FILTER passes high frequencies and attenuates low frequencies. A BANDPASS FILTER will permit a certain band of frequencies to be passed. 149
A BANDREJECT FILTER will reject a certain band of frequencies and pass all others. A SAFETY PRECAUTION concerning series resonance: Very high reactive voltage can appear across L and C. Care must be taken against possible shock hazard. 150
ANSWERS TO QUESTIONS Q1. THROUGH Q21. A1. a. X_{L} varies directly with frequency. X_{L} = 2πfL b. X_{C} varies inversely with frequency. c. Frequency has no affect on resistance. A2. Resultant reactance. A3. A4. Decreases. A5. Impedance low Current high. A6. Nonresonant (circuit is either above or below resonance). A7. Inductor magnetic field. A8. Capacitor. A9. Natural frequency or resonant frequency (f_{r}). A10. Maximum impedance, minimum current. A11. At the resonant frequency. A12. A13. Bandwidth of the circuit. A14. A filter. 151
A15. a. Lowpass. b. Highpass c. Bandpass. d. Bandreject. A16. Highpass filter, lowfrequency discriminator, or lowfrequency attenuator. A17. Lowpass filter, highfrequency discriminator or highfrequency attenuator. A18. At lowfrequency, a capacitor acts as an open and an inductor acts as a short. At highfrequency, a capacitor acts as a short and an inductor acts as an open. A19. Frequency cutoff (f_{co}). A20. Bandpass. A21. Bandreject. 152
NEETS Table of Contents
 Introduction to Matter, Energy,
and Direct Current
 Introduction 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 SolidState Devices and
Power Supplies
 Introduction to Amplifiers
 Introduction to WaveGeneration and WaveShaping
Circuits
 Introduction to Wave Propagation, Transmission
Lines, and Antennas
 Microwave Principles
 Modulation Principles
 Introduction to Number Systems and Logic Circuits
 Introduction to Microelectronics
 Principles of Synchros, Servos, and Gyros
 Introduction to Test Equipment
 RadioFrequency Communications Principles
 Radar Principles
 The Technician's Handbook, Master Glossary
 Test Methods and Practices
 Introduction to Digital Computers
 Magnetic Recording
 Introduction to Fiber Optics
