NEETS Module 17 — Radio-Frequency Communications Principles
i - ix
, 1-1 to 1-10
1-11 to 1-20
, 2-1 to 2-10
2-11 to 2-20
,2-21 to 2-30
2-31 to 2-37
, 3-1 to 3-10
3-11 to 3-20
, 3-21 to 3-30
3-31 to 3-40
, 3-41 to 3-47
4-1- to 4-10
, 4-11 to 4-21
5-1 to 5-10
,5-11 to 5-20
Most speakers of the above type receive their input by means of transformer coupling. This is necessary because
of the normal, low impedance of the voice coil. You will find the standard impedance values for this type speaker
are 4, 8, 16, and 32 ohms. Other impedance values may be obtained, but those listed are the most common.
While permanent magnet speakers perform reasonably well in the audio range, they have limitations. Most Navy
speakers reproduce low audio frequencies quite well, mid-band frequencies fairly well, and high frequencies quite
poorly. Let’s see why. When the speaker is constructed, only a limited number of turns may be built into the voice
coil. This gives us a fixed inductance. At low frequencies, the inductive reactance of the voice coil is
relatively low, and large audio currents flow. This provides a strong magnetic field around the voice coil and a
strong interaction with the field of the permanent magnet. Low frequency response is excellent. At midband
frequencies, inductive reactance increases and less current flows in the voice coil. This produces less magnetic
field and less interaction. Midband response is still acceptable in a properly designed speaker. At high audio
frequencies inductive reactance is quite high, and little current flows in the voice coil. This results in a
greatly reduced voice coil field and little interaction with the permanent magnetic field. Also at high
frequencies the interwinding capacitance of the voice coil tends to shunt some of the high audio frequencies,
which further reduces the high frequency response.
Frequency response of most permanent magnet speakers
falls off at the higher audio frequencies. This problem is normally overcome either by the use of an expensive,
specially designed speaker, or through the use of two speakers, one of which is designed to operate well at the
higher audio frequencies (tweeter) and one at the lower frequencies (woofer).
As shown in figure 2-31, an electromagnet may be used in place of a permanent magnet to form an electromagnetic
dynamic speaker. When we do this, sufficient dc power must be available to energize the field electromagnet. The
operation otherwise is much the same as that of the permanent magnet type. This type of speaker is seldom used in
Figure 2-31.—Electromagnetic speaker.
Figure 2-32 shows a diagram of typical headphones used with Navy equipment. The device consists of a
permanent magnet and two small electromagnets through which the signal currents pass. A soft iron diaphragm is
used to convert the electrical effects of the device into sound power. When no signal currents are present, the
permanent magnet exerts a steady pull on the soft iron diaphragm. Signal current flowing through the coils mounted
on the soft iron pole pieces develops a magnetomotive force that either adds to or subtracts from the field of the
permanent magnet. The diaphragm thus moves in or out according to the resultant field. Sound waves are then
reproduced that have an amplitude and frequency (within the mechanical capability of the reproducer) similar to
the amplitude and frequency of the signal currents.
As compared to permanent magnet speakers, standard headphones are considered to be high- impedance devices.
Headphone electromagnets are normally wound with many turns of small wire, which provide the larger impedance.
Because of the physically small size and inflexibility of the metal diaphragm, the headphones often give poor
response to the lower audio frequencies. In the voice range of audio, most standard issue headphones are adequate.
In this chapter you learned transmitter and receiver fundamentals. We also discussed modes of operation and
special controls circuits. Let’s review some of these areas.
A HARMONIC is an exact
multiple of the fundamental frequency. Even harmonics are 2, 4, and so on, times the fundamental. Odd are 3, 5,
and so on, times the fundamental frequency.
A SUBHARMONIC is an exact submultiple of the
fundamental frequency. Even subharmonics are one-half, one-quarter, and so on. Odd subharmonics are one-third,
one-fifth, and so on, of the fundamental frequency.
SUPPRESSION is the process of
eliminating an undesired portion of a signal.
MULTIPLEXING is a method for simultaneous transmission of two or more signals over a
common carrier wave.
An ORDER-WIRE CIRCUIT is a circuit between operators used for
operations control and coordination.
is when an electromagnetic wave passes through a receiver antenna and induces a
voltage in that antenna. DETECTION
is the separation of low-frequency (audio)
intelligence from the high (radio)
frequency carrier. REPRODUCTION
is the process of
converting electrical signals to sound waves. This sound is speech, music, and so on. SENSITIVITY
of a receiver is the ability to reproduce weak signals. The greater the receiver sensitivity, the weaker the
signal that will be reproduced.
is the ability to select the desired
signal and reject unwanted signals. NOISE SILENCER, NOISE SUPPRESSOR
, or NOISE
, are circuits that clip the peaks of the noise spikes in a receiver. SQUELCH
is a circuit that cuts off the output of a receiver when there is no input.
A BALANCED-PHASE DETECTOR or PHASE-DISCRIMINATOR is a circuit that controls
the oscillator frequency (AFC).
FREQUENCY SYNTHESIS is a signal-producing process through heterodyning and frequency
A PERMANENT MAGNET SPEAKER is one with a permanent magnet mounted on soft iron pole pieces.
The FIDELITY of a receiver is the ability to accurately reproduce at its output the
signal at its input.
GANGED TUNING is the process used to tune two or more circuits with
a single control.
HETERODYNING is the mixing of the incoming signal with the local
oscillator frequency. This produces the two fundamentals and the sum and difference frequencies.
IMAGE FREQUENCY is an undesired frequency capable of producing the desired frequency through
AUTOMATIC VOLUME/GAIN CONTROL is a circuit used to limit variations in the output signal strength
of a receiver.
FADING is the variations in signal strength at the antenna of a receiver.
REVERSE AGC is when an amplifier is driven toward cutoff.
FORWARD AGC is when an amplifier is driven toward saturation.
OSCILLATOR is an additional oscillator used in a receiver when the receiver is receiving a CW signal and
provides an audible tone.
ANSWERS TO QUESTIONS Q1. THROUGH Q26.
A1. AM, FM, CW, SSB.
A2. It generates an RF carrier at a given frequency within
A3. Power amplifier.
A4. It converts audio (sound) into
A5. When no modulation is present.
A6. It is an exact
multiple of the basic or fundamental frequency.
A7. 600 megahertz.
A8. To obtain higher carrier frequencies.
A9. It saves power and frequency
A10. For operator-to-operator service messages and frequency changes.
Reception, selection, detection, and reproduction.
A12. Sensitivity, noise, selectivity, and
A14. To extract the modulating audio signal.
A15. Wide bandpass.
A16. A special type of detector and a carrier reinsertion
A17. Attenuates the strong and amplifies the weak.
limit unwanted variations in the output.
A19. Weak signals produce bias, which could result in
no usable receiver output.
A20. DAGC does not attenuate weak signals.
It is heterodyned with the RF to produce an audio frequency.
A22. It eliminates noise when no
signal is being received.
A23. It controls the amount of bass and treble response.
A24. It is used to achieve maximum selectivity.
A25. It is used to accurately
control the frequency of the oscillator.
A26. The process of selecting and/or heterodyning
frequencies to produce a signal frequency.
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