elective vhf

superheterodyne receiver

converts an incoming radio -frequency (RF) signal to a fixed intermediate frequency (IF) before demodulation.

true

Why don't we simply amplify and demodulate the received RF signal directly?

Different stations operate at different frequencies.

• Filtering becomes more difficult.

• A fixed IF makes filtering and amplification easier.

rf signal, local oscillator

superheterodyne mixer combines two frequencies

wanted signal, image channel

two potential radio frequency signals that can mix with the local oscillator signal in order to provide the required IF.

image channel selection

prevents an unwanted signal from being mistaken for the desired signal. This can be achieved by making the RF tuned circuits as selective as possible (so that the image channel lies well outside their pass - band).

relatively high value of intermediate frequency

The problem of rejecting the image channel is made easier

by selecting a relatively ___ ____ __ __________ _______

excessive output, possible distortion

what happens if the receiver is very close to the transmitter?

very low output, difficult to hear

what happens when the aircraft moves farther away?

automatic gain control

agc meaning

automatic gain control

low level signal benefits from the maximum amount of gain present in a system whilst the larger signal require correspondingly less gain to avoid non linearity and consequent distortion of the signals and modulation

automatic gain control

automatically adjust receiver gain according to signal strength

agc voltage

amplified before being applied to the if and rf stages

image channel rejection, excellent selectivity

high - performance receivers use two separate intermediate frequencies .

digital frequency synthesizer

generates a desired frequency from a stable reference frequency.

digital frequency synthesizer

What replaces manual oscillator tuning in modern radios?

phase locked loop

What is a common technology used in frequency synthesis?

phase detector

senses any error between the vco and reference frequency

voltage controlled amplifier

designed so that its free running frquency is at near the reference frequency

capture range

The frequency range over which the circuit can achieve lock is known as the?

phase detector

detects discrepancy between the vco output and reference frequency

error voltage

if there is any discrepancy between the VCO Output and the reference frequency, this voltage is produced and this is used to correct the VCO frequency.

vhf communication

widely used for line of sight communication including air traffic control

amplitude modulation

used for voice communications as well as several of the VHF data link (VDL) modes .

dual sideband

The system vhf uses this modulation because this has implications for the

bandwidth of modulated signals.

upper sideband, lower sideband, carrier

dual sideband produces 3 separate component when modulating a carrier

channel spacing

is the frequency separation between adjacent communication channels.

guard band

small frequency range intentionally left between adjacent signals or channels to help prevent interference or overlap.

modulation

process of putting information onto a carrier signal by changing one of the carrier's characteristics.

carrier

is a high -frequency signal used to transport that information.

modulating signal, carrier signal, modulator, output signal

4 basic parts of modulation

envelope

an imaginary line connecting the positive peaks and negative peaks of the carrier waveform gives the exact shape of the modulating information signal.

modulating index, distortion

If the modulating voltage ( Vm) is larger than the carrier voltage (Vc), the ____ becomes greater than 1 , causing what?

overmodulation

sine wave information signal is modulating a sine wave carrier, but the modulating voltage is much greater than the carrier voltage. This condition is also known as

audio compression

reduces the difference between loud and soft sounds.

compressor

makes speech clearer and increases the average modulation depth.

3

most aicraft are equiped with how many vhf radio?

air traffic control

Most aircraft are equipped with three VHF radios . Two radios are dedicated to _____ voice communication ,

airline operational control communication

Most aircraft are equipped with three VHF radios . The third radio is used exclusively for the ACARS data link , also known as

data link terminal

installed in the aircraft handles both outgoing and incoming ACARS messages . It generates downlink messages sent to the ground and processes uplink messages received through the VHF data link. These

ACARS

is a digital communication system used to exchange operational and administrative information between an aircraft and ground facilities

aircraft communication addressing and reporting system

ACARS meaning

aircraft communication addressing and reporting system

allows airlines to send and receive information automatically , without human intervention.

management unit, control unit

2 aircraft acars component:

management unit

which deals with the reception and transmission of messages via the VHF radio

control unit

provides the Crew interface and consists of a display screen and printer.

smaller

signal’s envelope moves above and below this peak. Because of this, the modulating signal must have a ___ amplitude than the carrier.

aircraft communication addressing

Modern aircraft VHF communication systems support both voice and data transmission . The data link used on board is called the

acars ground network

comprises the ARINC (Aeronautical Radio Inc.) ACARS remote

transmitting/receiving stations and a network of computers and switching systems.