NEETS Module 17 — Radio-Frequency Communications Principles
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Figure 3-20A.—CW compared to an RFCS teletypewriter signal.
Figure 3-20B.—CW compared to an RFCS teletypewriter signal.
AUDIO FREQUENCY TONE SHIFT.—Tone-modulated (AFTS) systems use amplitude modulation to change
dc mark and space impulses into audio electrical impulses.
A basic tone-modulated system is shown in
figure 3-21. Conversion to audio tones is accomplished by an audio oscillator in the tone converter. Rapid varying
of the tone, according to the characters transmitted from the teletypewriter equipment, amplitude modulates the
carrier wave in the transmitter. The receiver receives the modulated signal and separates the audio signal from
the carrier. This process of separating the modulated signal is known as detection or demodulation.
Figure 3-21.—Basic tone modulated (AFTS) system.
RADIO-FREQUENCY-CARRIER SHIFT.—For frequency-shift (FSK) systems, the transmitter provides a
source of radio-frequency excitation. Figure 3-22 illustrates a basic frequency-shift keyed system. In modern
systems, the keyer is built into the transmitter. The keyer shifts the signal box below or above the assigned
frequency to correspond with the mark or space required to transmit TTY characters. Normally the keyer is adjusted
for an 850-hertz spread, 425 hertz above and 425 hertz below the assigned frequency. A spacing impulse will be 425
hertz above the operating frequency, and a marking impulse will appear 425 hertz below.
Figure 3-22.—Basic radio-frequency-carrier shift system (RFCS).
In both the tone-modulated system and the carrier-frequency shift system, all TTY signals pass through the TTY
panel that controls the looping current in all the circuits. Looping current is the current supplied by the TTY
battery. The TTY panel integrates the tone-modulated and the carrier-frequency shift systems. It provides every
possible interconnection of available TTY equipment. With this configuration maximum operational flexibility is
achieved with the least amount of circuitry and equipment.
Q19. What is the function of a keyer?
Q20. What is the function of a converter?
Q21. Basically describe an AFTS system.
Q22. Basically describe an RFCS system.
RFCS Send System
Figure 3-23 shows an RFCS teletypewriter transmit communications system.
You should refer to this figure frequently while reading the functional descriptions of the equipment shown.
Figure 3-23.—RFCS transmit (send) system.
TELETYPEWRITER SETS.—Most of the teletypewriter sets used by the Navy belong to one family of
TTY equipment. This equipment features various weights and sizes, quiet operation, and high operating speeds. They
present relatively few maintenance problems. Because of this they are well suited for severe shipboard conditions
of roll, vibration, and shock.
These teletypewriters operate at various speeds. Conversion from one speed
to another is usually only a matter of changing the gears that are located within the equipment.
Teletypewriters may be send/receive units or receive only units. They may be designed as floor models, table
models, or rack and wall-mounted sets. The teletypewriter shown is a send/receive floor model.
teletypewriter receives messages and prints them on page-size copy paper. In addition, it can receive and record
messages on perforated tape. You can use the keyboard or perforated tape to send messages. Page print monitoring
is available with both methods. The set shown can prepare perforated and printed tape for separate transmission.
It does this with or without simultaneous transmission and page-print monitoring. The combinations of services
available are extensive.
The TTY set may include a CABINET, KEYBOARD, PAGE PRINTER, TYPING PERFORATOR,
TRANSMITTER DISTRIBUTOR, TYPING REPERFORATOR, power distribution panels, and a power supply.
the components are linked by electrical or mechanical connections. You are given a wide range of possibilities for
sending, receiving, or storing TTY messages. All equipment components are housed within the cabinet. Transmission
signals are initiated through the keyboard (kybd) or through the
transmitter distributor (td). Signals received or local transmissions can be monitored on the page
printer. The typing perforator and typing reperforator are devices for preparing tapes on which locally initiated
or incoming TTY messages can be stored for future transmission through the td.
PATCHING PANELS.—TTYs are provided flexibility by jacks that are used to terminate all TTYs and
associated equipment. The jacks are wired in communications patching panels, usually referred to as TTY patch
panels. You are able to connect any combination of equipment electrically by means of patch cords.
plugs on the patch cords are inserted into the jacks at the front of the panel. These plugs have three different
parts. They are the tip, ring, and sleeve. The tip carries the intelligence signal while the ring carries the
synchronizing (step) or timing signals. The sleeve carries an alarm signal that indicates (both visually and
audibly) a problem to the operator. The problem may be equipment failure, loss of loop current, or improper
patching. Commonly used combinations of equipment are often wired together within the panel (called
normal-through). Individual pieces of equipment are wired on jacks to allow you to use them alone or in
TTY patch panels also furnish a central point for connecting the dc voltage supply into the
TTY circuits. One source of supply can be used for all circuits passing through a particular panel.
RED and BLACK are used on patch panels to identify whether that panel is used for passing secure or nonsecure
information. Red indicates that secure (encrypted) information is being passed through the panel. Black indicates
that nonsecure (unencrypted) information is being passed. Patch panels through which secure information is passed
are indicated by a red sign on the front that has inch high white block letters that say "RED PATCH PANEL." Panels
through which nonsecure information is passed are indicated by two black signs on the front with inch high white
block letters. One sign says "BLACK PATCH PANEL" and the other "UNCLAS ONLY."
Each panel contains six
channels. Each channel has its own series circuit of looping jacks, set jacks, and a rheostat for adjusting line
current. The number of looping and set jacks in each channel varies with the panel model. Each panel includes a
meter and rotary selector switch for measuring the line current in any channel. There are six miscellaneous jacks.
Any TTY equipment not regularly assigned to a channel, may be connected to one of these jacks.
desired TTY equipment is wired in the same looping channel as the radio adapter used, no patching is required.
But, if the desired TTY is not wired in the same looping channel as the keyer or converter, it must be patched.
For example, let's put a TTY on channel 1 and a converter on channel 3. If you want to receive, you must insert
one end of the patch cord in the set jack for channel 1 and the other end in either one of the two looping jacks
of channel 3.
In any switching operation between the plugs and jacks of a TTY panel, the cord plug must be pulled from the
looping jack before you remove the other plug from the set (machine) jack. Pulling the plug from the set jack
first opens the circuits to the channel, causing all TTY messages in the channel to be interrupted.
Removing the set (machine) jack before the looping jack exposes a dangerous dc voltage
on the exposed plug.
Q23. Most Navy TTY sets operate at what speeds?
Q24. A receive TTY set provides
outputs in what formats?
Q25. What does the color red indicate on a TTY patch panel?
EQUIPMENT.—Cryptographic equipment is used to ENCRYPT and DECRYPT TTY messages that require security
handling. (Encrypting is the method used to code a transmitted message; decrypting is used to decode a received
message.) To code or decode any message, the send and receive cryptographic equipment must be compatible.
REMOTE TRANSMITTER CONTROL UNIT.—The remote transmitter-control unit is mounted close to the KYBD and
permits remote control of the transmitter (XMTR). It has a transmitter power on-off switch, a power-on indicator
lamp, a carrier-on indicator lamp, and a three-position rotary selector switch. For RFCS operation you set the
switch to CFS SEND to transmit and to CFS REC to receive. Use the TONE S/R position for both transmitting and
receiving AFTS signals.
An audio frequency tone-shift system will be discussed later in this chapter.
SWITCHBOARD.—The transmitter transfer switchboard is used in this system to connect the remote
transmitter control unit to the radio transmitter.
RADIO TRANSMITTER.—The radio
transmitter transmits the TTY signal. You should be careful when tuning the transmitter for RFCS operation. The
carrier frequency setting is critical and must be properly set to ensure a correct output from the transmitter.
Q26. What are the functions of cryptographic equipment?
RFCS Receive System
Figure 3-24 shows the RFCS receive system used to receive the transmitted signal and translate it back to a usable
output. You should look at this figure while studying the units in this section.
Figure 3-24.—RFCS receive system.
ANTENNA FILTER ASSEMBLY.—The antenna filter assembly is connected to the antenna and receives
the RF signal from the antenna. It filters out any unwanted RF signals and allows the desired band of frequencies
RADIO RECEIVER.—The radio receiver takes the RF signal passed on by the antenna
filter and translates it to an audio signal.
RECEIVER TRANSFER SWITCHBOARD.—The receiver
transfer switchboard is used to tie the receiver to any converter unit connected to it. This allows you a wide
selection of equipment for connection to the same receiver.
converter-comparator group is used with receivers in either space or frequency diversity operation. When diversity
operation is not required, each converter can be used separately with a single receiver.
has its own COMPARATOR circuitry. This built-in design feature results in a considerable reduction in size from
older units. The comparator was located in a separate chassis in the older units. Size has been further reduced
through the use of microelectronics.
Figure 3-25 shows the basic method we use to convert a frequency-shift RF signal into a signal that controls the
dc loop of a TTY. The frequency shifts of the AF output from the receiver are converted into dc pulses by the AF
discriminator. The dc pulses are then fed into the keyer. The keyer opens and closes the dc loop of the TTY
according to the mark and space characters received.
Figure 3-25.—Frequency shift receiving system simplified block diagram.
In diversity operation the comparator section of the converter-comparator group (shown in figure 3- 24)
compares the strength of the signals from two receivers. Signals from each converter are fed into a comparator
circuit that compares the signals. This comparison is displayed on a CRT on the front of the equipment. The
comparison is in the form of LISSAJOUS PATTERNS. A lissajous pattern is a combined, simultaneous display of the
amplitude and phase relationships of two input signals. One signal is applied to the vertical and the other to the
horizontal deflection circuits. Lissajous patterns have many applications in electronics. They have operational
uses as well as uses in corrective and preventive maintenance. Further coverage on lissajous patterns can be found
in NEETS, Module 19, The Technician's Handbook. Figure 3-26 shows several typical lissajous monitoring patterns
for the converter- comparator group. Once we have a correctly tuned signal, the comparator feeds it to the
communication patching panel for patching to the TTY. Now let's refer back to figure 3-24 while we discuss the
rest of the units in the system.
Figure 3-26.—Typical lissajous monitoring patterns.
COMMUNICATION PATCH PANEL.—The communication patch panel serves the same functions on the
receive side of the RFCS system as it did on the transmit side. It routes the dc signal to the
proper cryptographic equipment. It also routes the decoded teletypewriter signal from the cryptographic
equipment to the selected TTY.
CRYPTOGRAPHIC EQUIPMENT.—The cryptographic equipment
converts the transmitted coded signal to a decoded signal that can be printed out in its original state.
TELETYPEWRITER.—The TTY equipment is used to convert the dc signal received from the
communication patch panel to a printed copy of the original transmitted message. The TTY shown is used only for
receive and does not have the ability to transmit.
Q27. What are the functions of a
Figure 3-27 is a simplified block diagram of a HALF-DUPLEX (send or receive)
uhf, audio- frequency-tone shift system. A half-duplex communications circuit permits two-way communications
between stations. Communications can be in either direction but not simultaneously. The term half-duplex is
qualified by adding send only, receive only, or send or receive. Let's use the block diagram to trace a signal
through the system.
Figure 3-27.—Half-duplex AFTS teletypewriter system.
SIGNAL FLOW.—On the transmit side, dc signals from the TTY set are fed to the communication
patching panel. From the panel they are patched to the tone terminal set. The tone terminal set converts
the dc signals into audio tone-shift signals. These signals are then patched to the transmitter section
of the transceiver through the transmitter transfer switchboard. The audio tone-shift signals modulate the RF
carrier generated by the transmitter (XMTR). The RF tone-modulated signals are then radiated by the antenna.
On the receive side, the RF tone-modulated signals are received at the antenna. You then patch the signal via the
multicoupler to the receiver section of the transceiver. Demodulation takes place at this point. The resulting
audio tone-shift signals are then patched through the receiver transfer switchboard. The signals now go from the
switchboard to the tone terminal set, where they are converted back to dc signals. The dc signals are then patched
through the communication patching panel to the TTY for printing.
TONE TERMINAL SET.—In
tone modulation transmission, the TTY pulses are converted into corresponding audio tones. These tones amplitude
modulate the RF carrier in the transmitter. Conversion to audio tones is accomplished by an audio oscillator in
the tone converter.
An internal relay in the tone converter closes the control line to the transmitter. This keys the transmitter on
the air when the operator begins typing a message. The transmitter remains keyed until after the message has been
On the receive side, the tone converter accepts the mark and space tones coming in from a receiver and converts
them into signals suitable to operate a relay in the converter. The make and break contacts of the relay are
connected in the local TTY dc loop circuit. This causes the teletypewriter to print in unison with the mark and
space signals from the distant TTY.
The number of
communications networks in operation throughout any given area is increasing. As a result, all areas of the RF
spectrum have become highly congested.
The maximum number of intelligible transmissions taking place in
the radio spectrum is being increased through the use of MULTIPLEXING. Multiplexing is the simultaneous
transmission of a number of intelligible signals (messages) in either or both directions using only a single RF
carrier. You may use two methods of multiplexing. These are TIME-DIVISION and FREQUENCY-DIVISION.
TIME-DIVISION.—With AM voice and tone communications, we want to transmit and receive for 360 degrees of
each sine wave. However, an audio signal may be transmitted and received satisfactorily by periodically sampling
the signal. The sampling process yields a received signal like the one shown in figure 3-28. There is no limit to
the maximum number of samples that may be made, but you must sample at least twice per cycle of audio to get
satisfactory results. In practical systems, 2.4 samples per cycle are usually taken. This concept of sampling
forms the basis for time-division multiplex (tdm) operation.
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