frequency modulation in data transmission
DESCRIPTION
This power point presentation presents The FM transmission system, stereo FM, receiver, transmitter, generation, transmission, reception.TRANSCRIPT
FREQUENCYMODULATION
INDATA TRANSMISSION
PRESENTED ON THE BASIS OF TECHNICAL REPORT BY :
RAJIB MODAKBISWAJIT MONDAL
&PRANAB KUMAR BANDYOPADHYAY
What is MODULATION ??
What is FREQUENCY ??
What is DATA ??
What is TRANSMISSION ??
Modulation
Continuous Wave Modulation
Pulse Wave Modulation
Linear Modulation
Non-linear Modulation
Amplitude Modulation
Frequency Modulation
Phase Modulation
What is Frequency Modulation ?? Carrie
r Signal
Baseband Signal
Modulated Signal
There is two types of Frequency Modulation with respect to bandwidth:---
i> Wide Band Frequency Modulation
ii> Narrow Band Frequency Modulation
GENARATION OF NARROW-BAND FM SIGNAL :
Integrator Product Modulator
Phase Shifter
_
+
NBFMem(t)
∑
Accos(2Πfct)
Narrow – Band Phase Modulator
Bandwidth Of FM:Theoretically the Bandwidth Of FM is infinite (for Sinusoidal Modulation). But practically FM wave is effectively limited to a number of significant side frequencies (almost 200 kHz).
From Carson’s rule approximation Bandwidth of FM - -
BT= 2 f + 2fm
BT= 2 f (1 + 1/β)
Difference Between AM & FM :1>In case of AM instantaneous phase contains baseband
Signal. But in case of FM that contains baseband as wellas higher order odd harmonics.
2>AM signal is amplitude modulated , but FM is amplitudeAs well as frequency modulated.
3>
The bandwidth of FM wave is much larger than AM wave.
How we create FM Signal ?
i>Indirect Method
ii>Direct Method
Indirect Method of FM Generation :-----
IntegratorNarrowband
Phase Modulator
Crystal Oscillator
FrequencyMultiplier
Base Band Signal
FM Signal
Indirect Method to Generate WBFM for Practical use:-
IntegratorNarrow
Band Phase Modulator
Frequency Multiplier
(*100)
Frequency Multiplier
(*75)
Crystal Oscillator
Mixer FM signal
Basebandsignal
Crystal Oscillator
f1 =0.1 MHz f2 =8.5 MHz
fc=100MHzΔf=75KHz
Direct Method of FM generation:----
VCOFrequency Multiplier
MixerFrequency Multiplier
Fixed Oscillator
Bandpass Filter
WBFMBase band Signal
DISADVANTAGE: very poor frequency stability due to unstable basic oscillator unlike crystal oscillator.
Feedback Scheme For Frequency Stabilization of a Direct Frequency Modulator:-
VCO
LPF & Amplifier
Frequency Discriminator
MixerCrystal
Oscillator
Base Band Signal
Frequency stabilized FM wave
Demodulation of Frequency Modulated Wave:-
Frequency Demodulation is the process that enables us to extract the original modulating signal.
There are two basic methods for demodulating the FM waves:
1. Based on Frequency Discrimination 2. Based on Phase Locked Loop
Frequency discriminator:-
A frequency discriminator is an FM demodulator which must produce an output voltage linearly dependent on input frequency.
Frequency Discrimination(Slope Detection) technique:
Envelope Detector
H(f)
s(t)sc(t) sd(t)
Sc(t)S(t)
Sd(t)
t
t
t
|H(f)|
f0 fcf
Problems:-
1>The detector also responds to spurious amplitude variation of the input FM.
2> The range of linear slope is quite small.
Frequency Demodulation using Phase Locked Loop:-
PLL is useful in FM demodulation in presence of large noise and low signal power. Thus it finds application in space vehicle to earth data links.
Recently it is being used in Commercial FM receiver.
Phase Locked Loop:-A PLL is basically a negative feedback system. It consists of three major components:
1. A multiplier2. A loop filter3. A voltage controlled oscillator(VCO)
These are connected together in the form of a feedback loop.
X Loop Filter
VCO
FM wave s(t)
e(t)v(t)
r(t)
Circuit for Direct FM generation:-
OSCILLATORSound Wave
Condenser Microphone
CL
FM
Varactor Diode Modulator:-
FM
-Vcc
Cc
C LVaractor Diode
em(t)
Practical Reactance Modulator:-
+Vcc
RFCR1RFCR1
OscilatorReactance Modulator
C
R2
R
Modulating signal
CB
CE
CC
R2
CE
RE RE
C1
C2 C
FM output
FM Receiving System:-
Frequency changer circuit
FM receiver circuit:-
The application circuit to generate FM(88-108 MHz) with stereo headphone amplifier
The
STEREO FM
What Stereo Means :-
How then comes the virtualization ??
What do we need for the transmission?• We need two different signals L(left) & R(right) to be modulated, transmitted, received and demodulated duly without getting distorted
We need :----
a.An input signalb.Two preemphasizersc.A DSB-SC modulatord.A frequency doublere.A pilot signalf.A summerg.An FM modulator
Why do we need L + R & L
– R??Compatibility issue
A mono phonic receiver will work with only the L+R component and there will no loss of AUDIO(so the signal will have the backward compatibility)
We need :----
a. An input signal (through a LIMITER DISCRIMINATOR)
b. A Low Pass Filter(0-15 KHz)
c. A Bandpass Filter(19 KHz)d. A Bandpass Filter( 23 – 53
KHz)e. A pilot signalf. A Frequency Doublerg. A Synchronous Detectorh. Two Deephasizers
WHAT IS A LIMITER DISCRIMINATOR ??
What is a preemphasizer
??
Preemphasizer refers to a system process designed to increase, within a band of frequencies, the magnitude of some (usually higher) frequencies with respect to the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as attenuation distortion or saturation of recording media in subsequent parts of the system.
A preemphasizer Network
What is a deemphasizer
??
Deemphasizer refers to a system process designed to decrease, within a band of frequencies, the magnitude of some (usually higher) frequencies with respect to the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as attenuation distortion or saturation of recording media in subsequent parts of the system.
A Deemphasizer Network
QUESTIONS ARE WELCOME