Comms Part 1 Flashcards
What is Squelch?
When no signal is being RX, the AF output is attenuated or decreased so background noise is not annoying. as soon as signal is received, attenuation is removed. Also it supresses weak communication signals
What is AGC (Automatic Gain Control)?
to hold the output constant as the input signal voltage changes, some output from the detector goes to AGC. Its fed back to the IF amp so it increases amplification when signal is weak and attenuates when its too strong.
What are the basic components of an oscillator?
Coil and capacitor
What are the characteristics of a Quarz oscillator?
Crystal resonators are small chips or sticks put in a special was to for a crystal. Pressure or pull on crystal surface cause electrical charge displacement and thus electrical voltages (piezo-effect)
Today most transmitters and receivers use PLL why is this?
QUARZ OSCILLATOR PRODUCES ONLY ONE FREQUENCY ACCORDING TO ITS DIMENSIONS. To produce different selectable frequencies, a phase locksed loop (PLL) is used.
Name the advantage of a superheterodyne receiver?
Picks up all radio signals in the area and its taken into a tuneable preamplifier. An electronic filter circuit passes only the frequency that the receiver is tuned to.
What are the characteristics of AM?
Amplitude modulation is a modulation where the amplitude of the signal is changed by the audio or data signal.
What does SSB mean?
Where the lower sideband and the carrier wave is removed.
What are the advantages of SSB?
Higher range with less transmitter power, More radio stations can share a frequency band. Less reception noise, Less Fading.
What are the disadvantages of SSB?
More complex it requires a very accurate local-oscillator in the receiver so it makes it more expensive.
What is PCM?
Instead of transmitting the information continuously, the data is sent periodically sampled and converted to different codes. Short sequence of pulses are used to carry information.
What is multiplexing?
To send different information on the same line TDM samples each channel for a short period of time. A mixed sequence of pulses are transmitted over the line and the receiver reconstructs a continuous signal from the incoming pulses.
An antenna works like?
A special conductor connected to a radio transmitter to radiate electromagnetic energy into space.
What are the 3 characteristics of an antenna?
Length, Polarization and directivity.
What are the various types of antenna?
Dipole, Vertical Whip, Loop
What is the definition of wavelength?
One Cycle, the distance between a peak or trough of an electromagnetic wave and the corresponding peak or trough in the next cycle of the wave.
What are the different types of transmission lines?
Coax, Open wire lines and waveguides.
The power in a matched transmission line is dissipated by?
The Load
In a mismatched transmission line there is a?
Standing Wave
What is a standing wave?
A stationary wave from the combination of 2 waves moving in opposite directions.
What are the frequency’s in the radio frequency band?
AF, VLF, LF, MF, HF, VHF, UFH, SHF, UHF
Name some common characteristics of radio wave propagation?
Field Strength, Reflection, Refraction, Costal Effect, Reflected Sky Wave, Diffraction
What is the propagation of VHF, UHF and SHF?
Line of sight
What is the propagation and coverage of MF?
300Khz - 3000Khz Space and ground waves
The propagation and coverage of LF is?
30Khz - 300Khz Space and ground waves
What does fading mean?
Sudden disappearance of radio waves as a result of absorption of radio energy in the ionosphere
What is the purpose of a static discharger?
To eliminate radio interference by lowering the electrical current that dissipates into the air
