04-12-2012, 02:06 PM
Analog communication systems
ANALOG COMMUNICATION.docx (Size: 516.73 KB / Downloads: 45)
INTRODUCTION
Modulation:
Modulation is the process of varying a carrier signal in order to use that signal to convey information. The three key parameters of a sinusoid are its amplitude, its phase and its frequency, all of which can be modified in accordance with an information signal to obtain the modulated signal. A device that performs modulation is known as a modulator and a device that performs the inverse operation of demodulation is known as a demodulator. A device that can do both operations is a modem (a contraction of the two terms).
In digital modulation, the changes in the signal are chosen from a fixed list (the modulation alphabet) each entry of which conveys a different possible piece of information (a symbol). The alphabet is often conveniently represented on a constellation diagram. In analog modulation, the change is applied continuously in response to the data signal. The modulation may be applied to various aspects of the signal as the lists below indicate.
Amplitude Modulation (AM):
In amplitude modulation, the instantaneous amplitude of a carrier wave is varied in accordance with the instantaneous amplitude of the modulating signal. Main advantages of AM are small bandwidth and simple transmitter and receiver designs. Amplitude modulation is implemented by mixing the carrier wave in a nonlinear device with the modulating signal. This produces upper and lower sidebands, which are the sum and difference frequencies of the carrier wave and modulating signal.
c(t)=Acos(wct)
The carrier signal represented by
The modulating signal is represented by
m(t)=Bsin(wmt)
Then the final modulated signal is
A [1 + m(t)] c(t)
= A [1 + m(t)] cos(wct)
= A [1 + B sin(wmt)] cos(wct)
= A cos(wct) + A m/2 (cos((wc+wm)t)) + A m/2 (cos((wc-wm)t))
Because of demodulation reasons, the magnitude of m(t) is always kept less than 1 and the frequency much smaller than that of the carrier signal. Note that the modulated signal has frequency components at frequencies wc, wc+wm and wc-wm.
1.3. Frequency-Division Multiplexing (FDM):
Frequency-division multiplexing (FDM) is a form of signal multiplexing where multiple baseband signals are modulated on different frequency carrier waves and added together to create a composite signal.
Historically, telephone network used FDM to carry several voice channels on a single physical circuit. In this, 12 voice channels would be modulated onto carriers spaced 4 kHz apart. The composite signal, occupying the frequency range 60 – 108 kHz, was known as a group. In turn, five groups could themselves be multiplexed by a similar method into a supergroup, containing 60 voice channels.
There were even higher levels of multiplexing, and it became possible to send thousands of voice channels down a single circuit. Modern telephone systems employ digital transmission, in which time-division multiplexing (TDM) is used instead of FDM.
Generation of AM Waves:
Multipliers difficult to build in hardware (at least circa 1920)
AM waves typically generated using a nonlinear device to obtain the desired multiplication
Square law modulator sums carrier c (t) and information m (t) signals, and then squares the m using a nonlinear device. Unwanted terms are filtered out with a band pass filter.
Switched modulation sums c (t) and m (t) then passes sum through a switch, which approximately multiplies it by a periodic square wave. This generates the desired signal plus extra terms that are filtered out.
Square Law Diode Modulator:
Methods of amplitude modulation can be put in the two categories namely Linear modulation methods and Square law modulation methods. Linear modulation method utilizes the linear region of the current voltage characteristics of the amplifying device that is transistor or electron tube. Square law modulation method utilizes the square law region of some current voltage characteristics of a diode or transistor or electron tube. A large number of linear modulation methods have been devised and have been used to varying degree. These methods are namely linear shunt plate modulation or anode choke modulation, linear series plate modulation, grid bias modulation, cathode modulation, suppressor grid modulation, screen grid modulation, collector modulation.
Square law modulation circuits make use of non linear current voltage characteristics of diodes or triodes and are in general suited for use at low voltages. Important square law modulation methods are square law diode modulation and balanced modulator.
Applications:
The ‘switching-modulator’ or the ‘square-law’ modulator can be used for generating amplitude modulation (AM/DSB), while the ‘ring-modulator’ or the balanced modulator can be used to generate double-side-band-suppressed-carrier (DSB-SC) in the laboratory. It has been accepted that different circuits are required for generating these two forms of AM, which differ only in the presence or absence of the independent carrier. In this module, the switching-modulator is modified by introducing an additional active device. With this modification, the modulator becomes capable of generating AM with varying depths of modulation, including the DSB-SC. Thus a single circuit can be used to generate both the DSB and the DSB-SC. The simplicity of the proposed method makes it ideally suited for laboratory implementation.
1.6. Detection of AM Waves
o AM detection typically entails tradeoffs between performance and complexity (cost).
o Square law detector squares the received signal followed by a low pass filter. This detection is simple but introduces an unwanted distortion term proportional to m2(t).
o Envelope detector is a simple circuit for AM detection consisting of resistors, a capacitor, and a diode. It only works when |kam(t)| <= 1t (Can’t detect sign change). The RLC circuit must track envelope but not the carrier (f−1c<<).
o Modulation is the process of encoding a message signal orbits into a carrier signal.
o AM modulation modulates the amplitude of the carrier wave form with a message signal.
o A constant term is added to the message signal to simplify demodulation: this is wasteful of power and hurts SNR.
Envelope detector:
There are various ways to measure or detect the amplitude (as opposed to the power) of a waveform. Here we'll consider one of the simplest, used by most portable radios, etc, the Envelope Detector.
ANALOG COMMUNICATION.docx (Size: 516.73 KB / Downloads: 45)
INTRODUCTION
Modulation:
Modulation is the process of varying a carrier signal in order to use that signal to convey information. The three key parameters of a sinusoid are its amplitude, its phase and its frequency, all of which can be modified in accordance with an information signal to obtain the modulated signal. A device that performs modulation is known as a modulator and a device that performs the inverse operation of demodulation is known as a demodulator. A device that can do both operations is a modem (a contraction of the two terms).
In digital modulation, the changes in the signal are chosen from a fixed list (the modulation alphabet) each entry of which conveys a different possible piece of information (a symbol). The alphabet is often conveniently represented on a constellation diagram. In analog modulation, the change is applied continuously in response to the data signal. The modulation may be applied to various aspects of the signal as the lists below indicate.
Amplitude Modulation (AM):
In amplitude modulation, the instantaneous amplitude of a carrier wave is varied in accordance with the instantaneous amplitude of the modulating signal. Main advantages of AM are small bandwidth and simple transmitter and receiver designs. Amplitude modulation is implemented by mixing the carrier wave in a nonlinear device with the modulating signal. This produces upper and lower sidebands, which are the sum and difference frequencies of the carrier wave and modulating signal.
c(t)=Acos(wct)
The carrier signal represented by
The modulating signal is represented by
m(t)=Bsin(wmt)
Then the final modulated signal is
A [1 + m(t)] c(t)
= A [1 + m(t)] cos(wct)
= A [1 + B sin(wmt)] cos(wct)
= A cos(wct) + A m/2 (cos((wc+wm)t)) + A m/2 (cos((wc-wm)t))
Because of demodulation reasons, the magnitude of m(t) is always kept less than 1 and the frequency much smaller than that of the carrier signal. Note that the modulated signal has frequency components at frequencies wc, wc+wm and wc-wm.
1.3. Frequency-Division Multiplexing (FDM):
Frequency-division multiplexing (FDM) is a form of signal multiplexing where multiple baseband signals are modulated on different frequency carrier waves and added together to create a composite signal.
Historically, telephone network used FDM to carry several voice channels on a single physical circuit. In this, 12 voice channels would be modulated onto carriers spaced 4 kHz apart. The composite signal, occupying the frequency range 60 – 108 kHz, was known as a group. In turn, five groups could themselves be multiplexed by a similar method into a supergroup, containing 60 voice channels.
There were even higher levels of multiplexing, and it became possible to send thousands of voice channels down a single circuit. Modern telephone systems employ digital transmission, in which time-division multiplexing (TDM) is used instead of FDM.
Generation of AM Waves:
Multipliers difficult to build in hardware (at least circa 1920)
AM waves typically generated using a nonlinear device to obtain the desired multiplication
Square law modulator sums carrier c (t) and information m (t) signals, and then squares the m using a nonlinear device. Unwanted terms are filtered out with a band pass filter.
Switched modulation sums c (t) and m (t) then passes sum through a switch, which approximately multiplies it by a periodic square wave. This generates the desired signal plus extra terms that are filtered out.
Square Law Diode Modulator:
Methods of amplitude modulation can be put in the two categories namely Linear modulation methods and Square law modulation methods. Linear modulation method utilizes the linear region of the current voltage characteristics of the amplifying device that is transistor or electron tube. Square law modulation method utilizes the square law region of some current voltage characteristics of a diode or transistor or electron tube. A large number of linear modulation methods have been devised and have been used to varying degree. These methods are namely linear shunt plate modulation or anode choke modulation, linear series plate modulation, grid bias modulation, cathode modulation, suppressor grid modulation, screen grid modulation, collector modulation.
Square law modulation circuits make use of non linear current voltage characteristics of diodes or triodes and are in general suited for use at low voltages. Important square law modulation methods are square law diode modulation and balanced modulator.
Applications:
The ‘switching-modulator’ or the ‘square-law’ modulator can be used for generating amplitude modulation (AM/DSB), while the ‘ring-modulator’ or the balanced modulator can be used to generate double-side-band-suppressed-carrier (DSB-SC) in the laboratory. It has been accepted that different circuits are required for generating these two forms of AM, which differ only in the presence or absence of the independent carrier. In this module, the switching-modulator is modified by introducing an additional active device. With this modification, the modulator becomes capable of generating AM with varying depths of modulation, including the DSB-SC. Thus a single circuit can be used to generate both the DSB and the DSB-SC. The simplicity of the proposed method makes it ideally suited for laboratory implementation.
1.6. Detection of AM Waves
o AM detection typically entails tradeoffs between performance and complexity (cost).
o Square law detector squares the received signal followed by a low pass filter. This detection is simple but introduces an unwanted distortion term proportional to m2(t).
o Envelope detector is a simple circuit for AM detection consisting of resistors, a capacitor, and a diode. It only works when |kam(t)| <= 1t (Can’t detect sign change). The RLC circuit must track envelope but not the carrier (f−1c<<).
o Modulation is the process of encoding a message signal orbits into a carrier signal.
o AM modulation modulates the amplitude of the carrier wave form with a message signal.
o A constant term is added to the message signal to simplify demodulation: this is wasteful of power and hurts SNR.
Envelope detector:
There are various ways to measure or detect the amplitude (as opposed to the power) of a waveform. Here we'll consider one of the simplest, used by most portable radios, etc, the Envelope Detector.