NEETS Module 9 − Introduction to Wave− Generation and Wave−Shaping
and Direct Current
||Alternating Current and Transformers
||Circuit Protection, Control, and Measurement
||Electrical Conductors, Wiring Techniques,
and Schematic Reading
||Generators and Motors
||Electronic Emission, Tubes, and Power Supplies
||Solid-State Devices and Power Supplies
||Wave-Generation and Wave-Shaping Circuits
||Wave Propagation, Transmission Lines, and
||Introduction to Number Systems and Logic Circuits
||- Introduction to Microelectronics
||Principles of Synchros, Servos, and Gyros
||Introduction to Test Equipment
||Radio-Frequency Communications Principles
||The Technician's Handbook, Master Glossary
||Test Methods and Practices
||Introduction to Digital Computers
||Introduction to Fiber Optics
|Note: Navy Electricity and Electronics Training
Series (NEETS) content is U.S. Navy property in the public domain.
Q-16. What is the filter called in which the low frequencies do not
produce a useful voltage?
Q-17. What is the filter called that passes low frequencies but rejects
or attenuates high frequencies? Q-18. How does a capacitor and an inductor
react to (a) low frequency and (b) high frequency?
Q-19. 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?
Q-20. 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
Q-21. 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?
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 p-type filters,
depending upon which symbol they resemble. Two or more T- or p-type filters may
be connected together to produce a still sharper cutoff point.
Figure 1-23, (view A) (view B) and (view C), and figure 1-24, (view A) (view
B) and (view C) depict some of the common configurations of the T- and p-type 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 0967-LP-000-0120.
Figure 1-23A. - Formation of a T-type filter.
Figure 1-23B. - Formation of a T-type filter.
Figure 1-23C. - Formation of a T-type filter.
Figure 1-24A. - Formation of a p-type filter.
Figure 1-24B. - Formation of a p-type filter.
Figure 1-24C. - Formation of a p-type filter.
When working with resonant circuits, or electrical circuits, you must be aware
of the potentially high voltages. Look at figure 1-25. With the series circuit at
resonance, the total impedance of the circuit is 5 ohms.
Figure 1-25. - Series RLC circuit at resonance.
Remember, the impedance of a series-RLC 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:
EC = IT x XC
EC = 2 x 20
EC = 40 volts AC
EL = IT x XL
EL = 2 x 20
EL = 40 volts AC
ER = IT x R
ER = 2 x 5
ER = 10 volts AC
You can see that there is a voltage gain across the reactive components at resonance.
If the frequency was such that XL and XC 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 series-resonant circuits.
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 XL = 2πfL; therefore, XL
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
therefore, the relationship between XC and frequency is that
XC varies inversely with frequency.
RESULTANT REACTANCE X = (XL - XC) or X
= (XC - XL). XL is usually plotted above
the reference line and XC below the reference line. Inductance
and capacitance have opposite effects on the current in respect to the voltage in
AC circuits. Below resonance, XC is larger than XL,
and the series circuit appears capacitive. Above resonance, XL is larger
than XC, and the series circuit appears inductive. At resonance, XL
= XC, 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 Series-LC 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.
IN a PARALLEL-LC 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.
The "Q" OR FIGURE of MERIT of a circuit is
the ratio of XL 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 half-power 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.
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 Low-PASS FILTER passes low frequencies and attenuates high
A High-PASS FILTER passes high frequencies and attenuates low
A Bandpass FILTER will permit a certain band of frequencies
to be passed.
A Band-REJECT 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.
Answers to Questions Q1. Through Q21.
a. XL varies directly with frequency.
XL = 2πfL
b. XC varies inversely with frequency.
c. Frequency has no affect on resistance.
A-2. Resultant reactance.
A-5. Impedance low Current high.
A-6. Nonresonant (circuit is either above or below resonance).
A-7. Inductor magnetic field.
A-9. Natural frequency or resonant frequency (fr).
A-10. Maximum impedance, minimum current.
A-11. At the resonant frequency.
A-13. Bandwidth of the circuit.
A-14. a filter.
A-16. High-pass filter, low-frequency discriminator, or low-frequency
A-17. Low-pass filter, high-frequency discriminator or high-frequency
A-18. At low-frequency, a capacitor acts as an open and an inductor
acts as a short. At high-frequency, a capacitor acts as a short and an inductor
acts as an open.
A-19. Frequency cutoff (fco).