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118 Cards in this Set

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Components of a typical communications system

Transmitter, receiver, modulator

Superimposition of low frequency information on a higher frequency carrier signal

Modulation

"Intelligence" extracted from the carried

Demodulation

Conversion of audio frequency to sound frequency accomplished by...

Loud speaker

Most important region of the earth's atmosphere for long distance, point to point communications

Ionosphere

F-layer

Highest of ionospheric layers, also has longest propagation capability

The lower the frequency of a radio wave....

The more rapidly the wave is refracted by a given degree of ionization

Critical frequency

Maximum frequency just below the "escape point"

As wavelength decreases and frequency of a radio wave increases, the diffraction ability...

Decreases

D-layer

Disappears at night

Skip zone

Area of silence between the end of the Ground Wave Coverage and the beginning of the Sky Wave Coverage

Critical angle

Radio waves depend on the layer density and wavelength of the signal

Factors that influence ionospheric refraction

Ionospheric density, transmitter operating frequency, and angle at which radio waves strike the ionosphere

Critical frequency

Each layer of the ionosphere has a maximum frequency that it will refract

Purpose of the HF system

Provide long range high frequency surface to surface, surface to air, and air to air radio comms

What enables the HF system to communicate over long ranges

High power transmitter, sensitive receivers, and ionospheric conditions

Frequency range for HF systems

3MHz to 30MHz

Most common mode of HF systems

USB

Receiver transmitter

HF system component that contains all the circuitry required to generate transmission signals and process received RF signals into audio signals

Antenna coupler

Electrically couples the antenna to the receiver transmitter

Phase lock loop circuit

Demodulates FM signals according to frequency variations produced by the difference between the FM input frequency and VcO frequency

If the VCO frequency is lagging the reference frequency...

The DC error voltage increases

Phase lock loop

Commonly used for band pass filter

The PLL would be erratic or not work at all if...

The low pass filter is missing

Amplitude of DC error voltage

Directly proportional to the phase difference between the comparator's input signals

When VCOs free running frequency matches the reference signal phase and frequency...

DC error voltage is 0VDC

When the error voltage from the low pass filter increases...

VCO output frequency increases

Component of PLL that has a center frequench

VCO

Component of PLL that has a center frequench

VCO

PLLs output of the low pass filter

Error voltage

PLL system make-up

Phase detector, frequency counter/divider, VCO and loop filter

Two categories of frequency synthesis

Direct and indirect

Sequential circuits employ

Digital gates and components

Typical AM receiver includes:

RF section, mixer, LO, IF amp, AM detector, AGC and AF amp

Frequency at the output of the IF amp should be

455 kHz

Local oscillator in the AM receiver

Tuned to 455kHz above the tuned RF

Purpose of AM detector circuit

Extracts the intelligence from the IF

Purpose of AGC circuitry

Eliminate fluctuations in the output when RF input varies

Bandwidth of IF filter

Approx 10kHz

Two basic categories of AGC

Carriers derived and audio derived

Of the two AGC types

Carrier derived is most common

Low pass filter following the detector diode

Removes remaining IF signal

FM side bands that contain the majority of the power

Significant side bands

Advantages of FM

Less interference between stations, greater noise suppression, and reduced transmitter power to cover an assigned area

FM receiver's ability to select the stronger of two signals with the same frequency

Capture effect

Standard FM broadcast IF

10.7MHz

100 channels spaced 200kHz apart

fM broadcast band of 88 to 108 MHz

VCO output

10.7MHz below the incoming tuned RF

Incident traveling waves

RF signal traveling through a transmission line

Squelch circuit

Quiets the transceiver when a signal is not present

CH radio limited to...

Transmissions at 5W

Common SSB used by military

ISB

Balanced modulators

Produce only the sum and difference

Demodulation in SSB receivers accomplished by

Product detectors and balanced modulators

Balanced modulator

Suppresses the carrier

SSB receivers require that the carrier

Be recreated in order for the intelligence to be detected

Bandwidth for NBFM

10kHz to 30kHz

Advantages of NBFM Comms

RF spectrum conservation, greater noise suppression and reduced transmitter power to cover assigned area

Part of comms troubleshooting phase

Repair phase, assessment phase, and reassessment phase

RF energy reflected from an object provides

Range, bearing, elevation and velocity

Video detector

Converts IF pulses to video pulses

Immediately after transmitter fires..

STC circuit reduces the receiver gain to zero

FTC

No effect on receiver gain

Characteristic impedencd

Distributed induction and capacitance of the line

Skin effect

Loss due to high frequency currents flowing near the outer surface of the conductor

Two wire transmission line

Allows greatest RF velocity (97% of the speed of light)

Twisted pair transmission line

Allows the least velocity (60% speed of light)

In a waveguide as the frequency increases

Metallic insulator smaller in size

In a waveguide the bus bar increases when

The frequency increases

Duplexer

RF switch that permits radar system to use a single antenna to both transmit and receive

Physical handling and installation of waveguides

Plumbing

Dummy load

Resistive load that absorbs all the energy from the transmitter so it doesn't leave the antenna

Balanced hybrid crystal (magic-t) found at the input of the radar receiver

Used as a mixer

Couples the maximum strength of the E-field

Locate a probe in the center of the "a" wall, 1/4 wavelength from the shorted end of the waveguide

Efficiency

Ability of an antenna to couple or radiate energy into space

Directivity

Ability of an antenna to direct most of its power in a concentrated area

An antenna's power gain is measured in relation to

Reference antenna

Height finding radar

Antenna has a narrow vertical beam width

Adding a reflector to a horn antenna

Increases power gain and Directivity

Advantages of cavity resonator

High Q, high power, small size and ruggedness

If the CW RADAR RF energy reflects off an inbound object

Frequency increases

Typical RC circuit cannot be used in microwave frequencies because

Components cannot be manufactured with small enough values

Causes oscillations

At high frequencies, the Cpg value of the interelectrode capacitance

At high frequencies, the Lg value of lead inductance can cause

Attenuation of the input signal

Electron transit tims

Time it takes for electron to travel from cathode to the plate

Resonant cavities of a magnetron begin oscillating as a result of

Electrons passing by the opening to the cavity resonators

Bipolar and FET

Used for amplifiers/oscillators in microwave circuits

Gallium Arsenide is used in FETs to reduce transit time by reducing

Ion mobility

Principle of Varactor operation

PN junction capacitance

Tunnel diode operation

Involves electrons flowing across PN junction before the barrier voltage is reached

Gunn diode operation

Transfer of electrons

Purpose of AFC circuit

Compensate for drift and age of components

Dro operation

Involves the dielectric resonator

Semiconductors that exhibit negative resistance

Gunn and tunnel diodes

Dry air in a waveguide

Prevents arcing

Element in telltale oil filter should be changed

Just before red element indicator reaches red line

Type 1 dehydrator

Uses refrigerant to remove liquid and oil

Purpose of dust filter in dry air system

Prevents desiccant and dust from contaminating dry air system

Dew point in a waveguide should be kept below

-40 degrees F

Dehydrator of dry air system

Contains two desiccant chambers

Hermetically sealed

Prevents entrance of moisture

Primary system

Provides initial source of cooling water that can be either seawater or chilled water

Scan indicator

Common in a weapons control RADAR

Secondary system

Transfer heat load from electronic equipment

Cylindrical parabolic antennas

3D air search and ground control precision

Radar uses one antenna to both transmit and receive RF energy

Duplexer

Function of the PFN in Line-type modulator

Stores energy to create High Voltage pulse

PFN discharge time determines

Pulse width of radar. It controls the time the transmitter tube generates an RF energy output

Intermediate Frequencies normally used in RADAR receivers

30MHz to 60MHz

What type of transmission line uses the outer conductor to shield the center conductor

Flexible coaxial cable

The strength of the H-field is indicated by the number of H-lines in a given area and varies directly with the amount of

Current

What characteristic of a vacuum tube can attenuate an input signal at high frequencies?

Lead inductance

What characteristic of a vacuum tube can attenuate an input signal at high frequencies?

Lead inductance

What characteristic of vacuum tubes can attenuate an output signal at high frequencies?

Interelectrode capacitance

What characteristic of a vacuum tube can attenuate an input signal at high frequencies?

Lead inductance

What characteristic of vacuum tubes can attenuate an output signal at high frequencies?

Interelectrode capacitance

Klystron

Overcomes the problems of conventional tubes. Uses a bunched cavity and grid to modulate the electron flow. Produces amplified oscillations.

Magnetron

Overcomes the problems of conventional tubes at microwave freqs. Uses resonant cavities to produce oscillations.