NAv Topic 4 vhf communication system

AV2220 - Aircraft Communication Systems Chapter 2 1

AVIONICS AVIONICS TECHNOLOGYTECHNOLOGY

Very High Frequency Very High Frequency (VHF) Communication(VHF) Communication

VHF communication systems are employed largely for controlling air traffic.

These systems are installed in all types of aircraft so the pilot may be given information and directions and may request information from air traffic control centers, control towers, and flight service stations.

On the approach to any airport with two-way radio facilities, the pilot of an aircraft calls the tower and requests information and landing instructions.

In airline operations and all instrument flights, the flight of an aircraft is continuously monitored by air traffic control (ATC), and the aircraft's crew is given instructions as necessary to maintain conditions of safe flight.



VHF communication systems operate in the frequency range of 118 to 136.975 MHz. For international operations the frequencies may extend to 151.975 MHz.

The nature of radio-wave propagation at these frequencies is such that communication is limited to line-of-sight distances.

The advantage of VHF communication, however, is that the signals are not often distorted or rendered unintelligible by static and other types of interference.

VHF communication radios are currently available with 720, 760, or 360 channels. The 720- or 760-channel radio is preferred by most pilots owing to

its versatility in frequency selection.



In 1976 the FAA changed the minimum frequency spacing for VHF systems from 50 to 25 kHz between 118 and 135.975 MHz. This change made the 720- channel radio possible.

Recently the FAA and FCC authorized the general use of frequencies up to 136.975 MHz. This change added 40 channels to increase the selection to 760

channels. Some older 360-channel radios are still in use; however, they are

quickly becoming obsolete.



Aircraft VHF communication systems consists of a VHF transceiver, control head, antenna, and an interface to the aircraft audio system for access to the microphone and cockpit speaker.

In light aircraft, the transceiver is mounted in the instrument panel and contains all the necessary controls and displays.

In larger aircraft, the control head, which is used for selecting the receiver and transmitter frequencies, is usually located in the center console between the pilot and copilot, and the transceiver is remotely located in the radio rack aft or below the crew station.

VHF communication transmitters provide AM voice communication transmission between aircraft and ground stations or between aircraft.

VHF System Description



Because of the nature of VHF radio signals, the average communicating - distance from aircraft to ground is: Approximately 30 mi [48 km] when the airplane is flying at 1000 ft [305 m] Approximately 135 mi [217 km] when the airplane is at 10,000 ft [3048 m].

Transmitting frequency is determined by the position of the selector switches on the VHF control panel.

The transmitter is tuned at the same time and to the same frequency as the receiver.

The most modern VHF communication radios incorporate the latest digital design features.

In general, the use of microprocessors and digital circuits has allowed for a 50 percent reduction in parts count and an 80 percent reduction in internal shop adjustments as compared with the use of analog circuits.

VHF System Description (cont’d)



A modular design of a modern digital system reduces maintenance time by providing easy access to all circuit boards and components.

VHF communication equipment for light aircraft is typically combined with a VHF navigation (NAV) radio system.

The VHF transceiver is a solid-state or digital system that can receive or transmit on any one of the 720 channels in the COMM range of frequencies.

The frequencies are spaced at 25-kHz intervals throughout the range.

Frequencies are selected simultaneously for both the receiver and the transmitter by rotating the frequency selector knobs. The large outer knob is used to change the megahertz portion of the frequency

display, and the smaller concentric knob changes the kilohertz portion. The small knob will change the frequency in 50-kHz increments when it is

pushed in and in 25-kHz increments when it is pulled out.





A VHF-700 transceiver



Some control heads employ a universally accepted 2-out-of-5 frequency selection scheme.

Other control heads use a digital serial data bus, such as ARINC 429, to select the desired frequency of a remotely-mounted transceiver.

With 2-out-of-5 tuning, any two out of a maximum of five frequency selection inputs to the transceiver will be grounded by the selector switch in the control unit to correspond with the desired frequency selection.

For example, if the control head displays the frequency 21.5, the "A" and "C" 10-MHz, "A" and "B" 1-MHz, and "C" and "D" 0.1-MHz frequency selection inputs to the transceiver will be grounded by the discrete signals from the control head.

VHF Transceiver Frequency Selection



VHF Transceiver Frequency Selection (cont’d)

XX9

X = Ground

XX8

XX7

XX6

XX5

XX4

XX3

XX2

XX1

EDCBANo.

2 of 5 Code Table

Table: Two-out-of-five frequency selection



The ARINC 429 serial data bus provides a balanced differential signal using nominally zero to 5-volt switching levels from the control head.

ARINC 429 is also used to send digital data from the avionics equipment to the cockpit displays.

ARINC 429 messages are comprised of 32-bit data words.

Each bit in the data word is set at either V if no voltage is present, or "1” if +5 volts DC is present.

This serial data stream runs across the two-wire bus at speeds of up to 100 kilobits per second using a command-response protocol.

The message format requires that a record, consisting of up to 126 data words, begin with an initial word that notifies the receiving unit that a message is being sent, and ends with a final word that is used to test for errors in the record.




Initial words and final words do not contain data in bits 11 through 29. The first 8 bits in the initial word may contain one of the following messages: Request to Send, Clear to Send, Data Follows, Data Received OK, Data Received Not OK, or Sync Lost. The first 8 bits in the final word is the file label, and bits 9 through 29 is the error control checksum, which is the addition of bits 9 through 29 in all the intermediate words in the record.ARINC 429 does not provide for error correction, but only error detection within the serial data stream. Typically, manufacturers of avionics systems will provide not only ARINC 429, but a variation of this format, such as the Collins Commercial Standard Digital Bus (CSDB), as their own unique data bus to be used for sending and receiving data between only their brands of equipment.





ARINC 429 data bus message formats



To tune the transceiver to the desired operating frequency, it is necessary to first enter the selected frequency into the STANDBY display.

The frequency is then activated by pushing the transfer button, and the word USE will be displayed.

Another frequency may then be entered into the STANDBY mode.

The STANDBY mode will store the selected frequency to allow for a "quick switch" of the frequency being used by the receiver.

This becomes very helpful while operating an aircraft in crowded airspace in which several communication frequencies are used for air traffic control. Control panels for VHF communication systems vary in design, depending on the manufacturer of the equipment and the requirements of the aircraft manufacturer. Typically, the control panel located in the flight deck contains the frequency selectors and the digital displays for the main and standby frequencies.

VHF Control Panel



Most VHF systems for corporate and transport-category aircraft use a separate radio control panel, and the receiver transmitter (r-t) is located in the electric equipment center.

Also on these aircraft, the VHF communication radio system is often independent of the VHF navigation system.

On light aircraft the r-t and control panel are often one unit mounted in the instrument panel.

VHF Control Panel (cont’d)

A VHF instrument panel

Interior of a VHF communication radio



Antennas for VHF systems are low-drag stub units extending from the top and bottom centerline of the airplane.

These antennas are matched to their respective transmission lines by means of carefully measured lengths of tuning line.

The antennas are used for both transmitting and receiving.

VHF Antenna

VHF antenna configuration



The receiver portion of a VHF communication system is typically the superheterodyne type.

The antenna receives an induced signal from the electromagnetic fields passing the antenna.

This signal is sent through a band-pass filter to an RF amplifier.

Once amplified, the signal passes through a low-pass filter and into the first-stage mixer.

The mixer converts the RF into an intermediate frequency (IF).

The IF is a lower frequency and is easier to control through the receiver.

The IF is amplified to produce a stronger signal, which is sent to the second-stage mixer where again a lower frequency is produced.

This signal is amplified and sent to the detector, where the audio wave is separated from the carrier wave.

The audio signal is then amplified by the buffer and broadcast into the aircraft by the speaker.

VHF System Operation



The buffer amplifier receives inputs from the AGC (automatic gain control) circuit, which ensures correct signal amplification at varied input signal strengths.

The transmitter receives an input signal from the microphone or data inputs.

This signal is amplified by the audio buffer and sent to the modulator (synthesizer).

The modulator produces an AM signal, which is filtered, amplified, and sent to an ALC (automatic level control) circuit.

Similar to the AGC in the receiver, the ALC ensures that a consistent output signal is sent to the antenna, even at varying input signal strengths.

VHF System Operation (cont’d)



VHF System Operation (cont’d)

Block diagram of a typical VHF communication system for a large aircraft



The Collins VHF-20A, a typical remotely-mounted VHF transceiver, provides AM voice communication in the frequency range from 117.00 MHz through 135.975 MHz, in 25-kHz increments.

The VHF-20A consists of a power supply, frequency synthesizer, receiver, modulator, and transmitter.

The VHF-21/22 is an advanced microprocessor-based version of the VHF-20A that employs the ARINC 429 and Collins CSDB.

Rockwell Collins VHF-20A Transceiver

Collins VHF-20/21/22 transceiver

Collins VHF-20A block diagram



The VHF frequency synthesizer, having only one crystal controlled oscillator, derives accurate RF output frequencies through the use of a phase-lock-loop and solid-state switching circuits.

The synthesizer interprets 2-out-of-5 frequency information from the VHF control head and provides all internal RF signals required by the VHF receiver and transmitter.

Rockwell Collins VHF-20A Transceiver (cont’d)

Collins VHF-20/21/22 transceiver

Collins VHF-20A block diagram




Collins VHF-20A synthesizer block diagram



In the receive mode, the synthesizer outputs a DC tuning voltage to the variable-voltage capacitors in the preselector to eliminate mechanical tuning.

The synthesizer also applies an injection frequency to the mixer to output a 20-MHz IF.

The 20-MHz IF amplifier, which is AGC controlled, provides the required selectivity and signal amplification.

The detected audio is amplitude and bandpass limited and applied to the audio output amplifier.

Squelch circuits disable the output amplifier if proper signal-to-noise ratio or carrier level is not present.

When the push-to-talk switch on the microphone is applied, the synthesizer removes the receiver injection and provides transmitter excitation at the selected frequency.




Power is applied to the transmitter by a +16-volt DC transmit series regulator, and the broadband RF amplifiers raise the synthesizer excitation to 20 wafts minimum output.

The RF output is low-pass filtered and applied through the transmit/receive switch to the antenna.

The AM modulator is a variable voltage power supply that varies the transmitter drive voltage consistent with the microphone inputs.

Carrier modulation is detected by a sidetone detector and applied through the receiver audio amplifier so the pilot can monitor his or her voice transmissions through the aircraft's audio system.




The Bendix/King KX-170A/KX-175 is a combination panel-mounted VHF communications transceiver and navigation receiver that operates on either 28-volt or 14-volt DC power. The communications section is: Dual-conversion Superheterodyne receiver with a

9.0-MHz IF 861.25-kHz second IF frequency 360 channels are synthesized at

the first mixer Low-side injection is used for

channels 127.00 MHz to 135.95 MHz

High-side injection for 118.00 MHz to 126.95 MHz

Characteristics of Bendix/King KX-170A/KX-175 are: The received antenna signal is

coupled to the preselector through a diode transmit/receive (T/R) switch.

A two-pole, varactor-tuned RF filter couples the antenna to the RF stage.

A second varactor-tuned filter couples the amplified RF signal to the first mixer and supplies additional image and ½ IF spurious rejection.

Bendix/King KX-170A/KX-175 VHF Transceiver



Characteristics of Bendix/King KX-170A/KX-175 are (cont’d): The amplifier RF signal is mixed

with the synthesized injection frequency in a balanced mixer.

A two-pole crystal filter couples the difference frequency to the second mixer and provides image and ½ IF selectivity.

The 8.13875-MHz crystal controlled second local oscillator develops injection for the second mixer.

The second IF contains two integrated circuit (I.C.) amplifiers with three double-tuned interstage networks for additional receiver selectivity.

An active detector/noise limiter provides audio gain, rate noise limiting, and 90% AM clipping of noise spikes.

A two-stage AGC amplifier is used to control the gain of the RF stage and the first IC amplifier in the second IF strips.

Bendix/King KX-170A/KX-175 VHF Transceiver (cont’d)




Bendix/King KK-170/175 VHF communications transceiver block diagram



Characteristics of Bendix/King KX-170A/KX-175 are (cont’d): The receiver outputs 6 dB into

the AGC with no input signal. This eliminates conventional gain

threshold effects and establishes a constant "signal plus noise" at the detector output.

The detector noise bandwidth is approximately 15 kHz.

A noise filter passes "white noise" containing frequency components above 7 kHz.

The filtered noise is amplified and used to operate a squelch gate.

The transmitter is a solid-state, four-stage, broadband, 30-dB gain, RF power amplifier.

Modulation is applied to the driver and final stages.

The low-pass filter provides harmonic spurious rejection.

A series regulator supplies 8.5 volts to RF and audio circuitry.

A zener regulator maintains 5.0 volts to digital circuitry used in the frequency synthesizers.




The AN/ARC-197 VHF communication system provides an aircraft with two-way, plain voice radio communications in the very high-frequency range.

Its actual frequency range is from 116.000 to 151.975 MHz.

In the P-3C aircraft (maritime patrol aircraft), this system interfaces with four of the intercommunication stations in the transmit/receive functions.

The pilot, copilot, TACCO, and NAV/COMM stations can transmit and receive over this radio.

The other stations in the aircraft have received function only.

AN/ARC-197 VHF Transceiver



Major Components

There are three components to the AN/ARC-197 system: The RT-1397/ARC-197 transceiver The C-11067/ARC-197 VHF-AM control panel The 949880 VHF antenna

The RT-1397/ARC-197 transceiver is a solid-state unit, consisting of:

a power supply frequency synthesizer receiver modulator transmitter one indicator one push button, one microphone jack one headphone jack on the unit

AN/ARC-197 VHF Transceiver (cont’d)

RT-1397/ARC-197 transceiver



The RT-1397/ARC-197 transceiver (cont’d): The indicator is labeled TRANSMIT

POWER, which illuminates when output power is greater than 10 watts.

The push button is labeled SQUELCH DISABLE, which will disable the squelch for low signal levels.

The microphone and headphone jacks are used for maintenance and emergency VHF communication in case of ICS failure in-flight.


RT-1397/ARC-197 transceiver



C-11067/ARC-197 VHF-AM Control Box The control box controls the

operation of the system. There are two dual function knobs

and a display window on the control panel.

The display window shows the selected frequency of the system.

The outer ring of the dual function knob on the left applies system power and selects the test function.

The inner knob changes the frequency of operation in 1-MHz steps over the range of control.


VHF-AM control box



C-11067/ARC-197 VHF-AM Control Box (cont’d):

The outer ring of the dual function switch on the right is labeled VOL, and it is not used in the P-3 aircraft.

Volume is controlled by the ICS system.

The inner knob of this control is used to change the frequency of operation in 25-kHz steps over the range of control.


VHF-AM control box



949880 VHF Antenna: The 949880 antenna is located in the

tailcap on top of the vertical stabilizer of the P-3 aircraft.

This antenna radiates and receives the VHF radio frequency signals.

Signals routed to and from the antenna go through a VHF bandpass filter, which reduces the crosstalk between the VHF and UHF systems.




Functional Description

There are two modes of operation with the AN/ARC- 197 radio. The receive mode The transmit mode

Receive Mode: The received RF signals from the

antenna are routed through the filter, and applied to the receiver circuits in the transceiver.

The frequency selected on the control box is applied to the frequency synthesizer.

The synthesizer uses a single phase-locked loop to generate RF injection frequencies, in 25-kHz steps, from 116.000 to 155.975 MHz.

The RF injection frequencies, along with DC tuning voltages, electronically tune the receiver to the selected frequency.

The AM detected audio is applied to the audio amplifier circuit.

Squelch circuits disable the output amplifier if the required signal-to-noise ratio or carrier level is not present.

The output audio is then applied to the ICS interconnection box for distribution to the various stations.




Functional Description (cont’d)

Transmit Mode: The VHF XMTR control signal from

any one of the four ICS master control panels applies a ground to the transceiver as the VHF key signal.

This VHF key signal provides the push-to-talk command to the transceiver to switch it from the receive to the transmit mode of operation.

The synthesizer generates transmitter drive frequencies from 116.000 to 155.975 MHz in 25 kHz steps.

Audio from any of the four ICS master control boxes are applied to the modulator circuit.

The modulator circuit provides 90-percent amplitude modulation.

The transmitter uses five stages of amplifiers to raise the output to 20 watts.

The RF output is routed to the antenna, through the filter, for radiation.

The transceiver also produces a sidetone output, which is provided to the ICS system in the same manner as the receiver audio.


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NAv Topic 4 vhf communication system