NEETS Module 9 — Introduction to Wave Generation and WaveShapingPages
i  ix, 11 to 110,
111 to 120, 121 to 130,
131 to 140, 141 to 152,
21 to 210, 211 to 220,
221 to 230, 231 to 238,
31 to 310, 311 to 320,
321 to 330, 331 to 340,
341 to 350, 351 to 356,
41 to 410, 411 to 420,
421 to 430, 431 to 440,
441 to 450, 451 to 461, Index
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.
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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.
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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}
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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.
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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.
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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.
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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.
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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.
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A HIGHPASS FILTER passes high frequencies and attenuates low frequencies.
A BANDPASS FILTER will permit a certain band of frequencies to be passed.
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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.
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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.
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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.
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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
