L5 Modulation Flashcards
Communication systems
1- Analog Modulation: AM and FM 2-Digital Modulation ASK and; FSK 3- Modems
Communication systems
1- Input Transducer 2- Transmitter 3- Transmission channel 4- Receiver 5- Output transducer
Input transducer
The device that converts a physical signal from source to an electrical, mechanical or electromagnetic signal more suitable for communicating
Transmitter:
The device that sends the transduced signal
Transmission channel:
The physical medium on which the signal is carried
Receiver:
The device that recovers the transmitted signal from the channel
Output transducer
The device that converts the received signal back into a useful quantity
The purpose of a communication system
transmit information signals (baseband signals) through a communication channel
baseband
a signal that has a very narrow frequency range.
the band of frequencies representing the original signal as delivered by the input transducer
Example of baseband signal
the voice signal from a microphone
The voice signal from a microphone contains frequencies in the range of
0-3000 Hz
modulation
a procedure that shifts the range of baseband frequencies to other frequency ranges suitable for transmission
demodulation
shift back to the original frequency range after reception.
AM radio system transmits electromagnetic waves with frequencies of
around a few hundred kHz (MF band)
The FM radio system must operate with frequencies in the range of
88-108 MHz (VHF band)
baseband signal contains frequencies in the audio frequency range
3 kHz
The transmitter block in any communications system contains the …………….. device
modulator
The receiver block in any communications system contains the ………………… device
demodulator
The modulator modulates a …………………………….
carrier wave
…………………….. is the process of impressing a low-frequency information signal (baseband signal )onto a higher frequency carrier signal
Modulation
……………………….is done to bring information signals up to the Radio Frequency (or higher) signal
Modulation
Amplitude Modulation (AM)
Amplitude modulation is the process of varying the amplitude of a carrier wave in proportion to the amplitude of a baseband signal. The frequency of the carrier remains constant
Frequency Modulation (FM)
Frequency modulation is the process of varying the frequency of a carrier wave in proportion to the amplitude of a baseband signal. The amplitude of the carrier remains constant
Types of analog modulation
Amplitude modulation(AM) Frequency modulation(FM) Phase Modulation (PM)
Digital communication systems also employ modulation techniques, some of which include:
Amplitude Shift Keying
Frequency Shift Keying
Phase Shift Keying
Amplitude Shift Keying (ASK)
involves the process of switching the carrier either on or off, in correspondence to a sequence of digital pulses that constitute the information signal.
Frequency remains fixed .
Frequency Shift Keying (FSK)
involves the process of varying the frequency of a carrier wave by choosing one of two frequencies (binary FSK) in correspondence to a sequence of digital pulses that constitute the information signal.
Amplitude remains fixed
how is ASK represented?
One binary digit is represented by the presence of a carrier, the other binary digit is represented by the absence of a carrier.
how is FSK represented?
Two binary digits are represented by two frequencies around the carrier frequency.
Modems
devices used to enable the transfer of data over the public switched telephone network (PSTN)
The name modem comes from the name
MOulator- DEModulator
MOulator- DEModulator describes
describes the function the modem performs to transfer digital information over an analog network
The goal of modems is to
produce a signal that can be transmitted easily and decoded to reproduce the original digital data.
what are modems used for?
communicate via telephone lines, modems can be used over any means of transmitting analog signals
kinds of modems available
1- Internal modem
2- external modem
3- DSL (Digital Subscriber line)
4- cable modem
Internal modem
A modem card in your computer that is integrated within the system
Less expensive than external modems
Disadvantage is that you need to access inside the computer to replace the modem
External modem
A device that connects externally to your computer through a serial port
External power supply does not drain power from the computer
Modem activity can easily be observed
More expensive than an internal modem
DSL (Digital Subscriber Line)
A high-speed data service that works over conventional telephone lines and is typically offered by telephone companies
It does not occupy the phone line-you can still talk on the phone
Speed is much higher than regular modem
Cable modem
A device that connects to the existing cable feed and to an Ethernet network card in the PC (also called a NIC for Network Interface Card)
Is different than a common dial up modem
Supports higher speeds
Typically offered by cable companies
