Chapter - 2 Analog Modulation Systems Flashcards
It is the process of impressing low frequency information signals into a high frequency carrier signals
Also defined as the process of changing one or more properties of the analog carrier in proportion with the infromation signal
MODULATION
It is the reverse process of modulation
Demodulation
It is a High Frequency signal ( the one that is modulated ) used to carry information from source to destination
Carrier
It is also known as Intelligence,Modulating Signal or Baseband Signal, which is understandable information (one that is modulating) such as voice , audio ,video ,documents etc
Information
Advantages of Modulation
- practical size of antenna
* there will be no interferrence since every transmitter will use different frequencies
Signals have longer wavelength thus they require longer antennas for transmission
Low Frequency
Signals having shorter wavelength thus may require shorter antenna
High Frequency
Types of Modulation
Analog Modulation and Digital Modulation
Types of Analog Modulation
•Amplitude Modulation
•Angle Modulation
-Frequency Modulation
-Phase Modulation
Modulation technique wherein both of the carrier and information are in analog forms
Analog Modulation
It is a modulation technique invented by reginald fessenden .
Wherein the information signal is analog and the amplitude of the carrier is varied proportional to the information signal
AMPLITUDE MODULATION
Standard AM range
535-1605khz
Standard AM Intermediate Frequency
455khz
AM Equation
V(t)=vcsin(wct) + (mVc/2)cos(wc-wm)t - (mVc/2)cos(wc+wm)t
It is also known as Coefficient of Modulation or Depth of Modulation . It is the term used to describe the amount of amplitude change(modulation) present
AM Modulation Index
Ideally 1
Typically Less than 1
Greater than 1 will caused distortion to the signal or Overmodulation
Modulation Index Formula
M=Vm/Vc
M=Vmax-Vmin/Vmax+Vmin
It is the coefficient of modulation expressed as percentage
%m=m x 100%
AM Current and Voltage
IT=IC(sqrt 1+m^2/2)
VT=VC(sqrt 1+m^2/2)
AM Power Relationship
PT = Pc+2Psb
Psb = Pc (m^2/4)
Psbt = Pc (m^2/2)
Pt = Pc (1+m^2/2)
AM Bandwidth Requirements
B(Am)=2 x fmax
Modulation by Several Signals
VT = sqrt v1^2 + v2^2 ... IT = sqrt i1^2 + i2^2 ... M(eff) = sqrt m1^2 + m2^2 ...
Is the curved produced by joining the tips of the individual RF cycles of a modulated wave . It contains the information
Envelope
Useful Power in AM , Increases with Modulation becoming 1/3 of the total signal power for 100% Modulation
Sideband Power
Used for observing the Modulation Characteristics of AM Transmitters
Trapezoidal Patterns
AM Transmitter
C BIM S DAPL
Crystal Oscillator Buffer Amplifier Intermediate Power Amplifier Modulated Power Amplifier: -speech amplifier -driver amplifier -push pull modulator Linear Power Amplifier
It provides a stable carrier frequency at low power. It is an RF oscillator which can be a hartley , colpitts , clapp etc
Crystal Oscillator
It is a CLASS A RF Amplifier that isolates the crystal oscillator to improve its stability
High input impedance
And Low Output Impedance
To match the high output crystal oscilator and low input IF amplifier
Buffer Amplifier
Class C RF amplifier that Raises the output of the buffer to a level sufficient to drive the modulated RF amplifier
It Amplifies the signal from the oscillator
Intermediate Power Amplifier
Class C RF amplifier that supplies the energy which is required to drive the antenna system at the rated RF power for high level modulation.
it multiplies the signals from the IF power amplifier and push pull modulator
Modulated Power Amplifier
It is a Class A AF pre-amplifier that raises the level of the input AF after being subjected to processing and filtering.
it amplifies the weak audio frequency so that it can be detected by the driver amplifier. it improves the Signal to Noise Ratio before mixing
Speech Amplifier
It is Class A/B/AB that supplies the necessary audio power to drive class B modulator.
It Amplifies the output of the speech amplifier so that it can be detected by push pull modulator
Driver Amplifier
It is Class B AF Output Amplifier that varies the plate voltage of the class C RF amplifier in accordance with the frequency and the amplitude of the AF signal
Push Pull Modulator
It is Class A/B/AB amplifier which provides linear power amplification of the amplitude-modulated output signal from the class C modulated power amplifier.
it is used for low-level modulation
Linear Power Amplifier
Two Types Of Amplitude Modulation
Low-Level And High-Level Modulation
The Modulation takes place prior to the output element of the final stage of the transmitter
Low-Level Modulation
The Modulation takes place in final element of the final stage where the carrier signal is at its maximum amplitude
High-Level Modulation
Effiency of the transmitter is calculated :
Power output / Power input
is used in AM amplifiers since it can be operate in high frequencies.
Common Base Configuration
at 100% modulation
the power of modulating amplifier is one-half of the supply power
AM Detection :
- Diode Envelop Detector
- Crystal Detector
- Power Detector
- Grid Leak Detector
- Regenerative Autodyne Detector
it is also known as peak detector. it consist of a diode in series with an RC Low pass filter. the diode acts as a rectifier that allows only one-half of the modulated AM signal to pass and the other half being clipped.
Diode Envelop Detector
it is the first rectifying detector. A thin,pointed wire known as CAT WHISKERS is pressed against the surface of the crystal, when a sensitive spot is found, more current flows in one direction that in the opposite thus rectification occurs
Crystal Detector
it is the type of detector using amplifying devices like transistors in place of a diode to provide rectification and amplification at the same time.
Power Detector
it consist of a tuned circuit a rectifier and an RC low pass filter for the recovery of the modulating signal. it has better sensitivity than diode detector
Grid Leak Detector
it employs tickler coil which is genarates energy from the plate circuit into the grid circuit of the triode.
when the regenerative detector oscillates it becomes autodyne detector (self heterodyning) or product detector and can be used to detech CW, SSB or FSK emission.
Regenerative and Autodyne Detector
means the extraction of the information signal from the AM signal. it is only part of the Demodulator System
Detection
is any frequency,phase or amplitude variations that are present in the demodulated waveform that were not in the original information signal. when noise is added to the AM signal, the Output at the detector is always Distorted
Distortion
it is the measure of the ability of a communication system to produce , at the output of the receiver , an exact replica of the original source information
Fidelity
Two Types Of AM Receiver :
- TRF (Tuned Radio Frequency) Receiver
- SuperHeterodyne Receiver
one of the earliest type of AM receiver inveted by Reginald Fessenden, and is probably the simplest designed radio receivers available today.
Tuned Radio Frequency: consist of (ARDAS)
- Antenna
- RF Amplifier
- Detector
- AF Amplifier
- Speaker
Disadvantages Of TRF:
- Inconsistent Bandwidth
- Unstable due to large number of amplifier all tuned to the center frequency
- Gains are not Uniform over wide range of frequencies
it amplifies the weak RF signal. it has a variable resistor that controls the RF Gain and Sensitivity. It also Contains the pre-selector , which is used to tune to the desired frequency
RF Amplifier
it makes use of regenerative detector that provides rectification and detection for modulated signals. it extracts the information
Detector
it amplfies the recovered information (weak AF signal). it is a volume controlled amplifier that raises the power level of the AF (audio) signal to a value sufficient to drive the loudspeaker of the receiver.
AF Amplifier
means to mix two frequencies together in a non linear device or to translate one frequency to another using non linear mixing
Heterodyne
was invented by Edwin Armstrong , its gain , selectivity and sensitivity is far superior to the other receivers
Superheterodyne Receiver : consist of
- Antenna
- RF Amplifier
- Mixer and Local Oscillator
- IF Section (1st and 2nd IF amplifier)
- 2nd Detector
- AF Amplifier
- Speaker
it is stable crystal oscillator whose frequency beats with the incoming signal to produce the correct intermediate frequency
Local Oscillator
it down-convert the received RF frequencies to IFs (Intermediate Frequencies). it is the first detector which operates in a non-linear fashion and provides the action which produces the desired intermediate frequency. its outputs includes the original , sum , and difference of the local oscillator frequency and the RF input frequency.
Mixer
its primary functions are amplification and selectivity.
it consist of a series of IF amplifiers and bandpass filter and is often known as IF Strip. it is tuned in most cases to 455khz. it amplifies the 455khz output of the mixer and rejects the remaining output.
IF Section
it demodulates the 455khz IF signals and recovers the original modulating signal or audio signal
Second Detector
it is a receiver parameter that is used to measure the ability of the receiver to accept a given band of frequencies and reject all others
Selectivity
it is also known as a receiver threshold. it is the minimum RF signal level that can be detected at the input to the receiver and still produce a usable demodulated information signal
Sensitivity
measures just how close to perfect a tuner or tuner component can be , and its affects the bandwidth and the selectivity of a certain receiver.
Quality Factor:
Q=Fr/Bw Fr = 1/2pi(sqrt LC)
Q= XL/R