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PART IVPower Distributions of an Angle- modulated wave
The Use of Preemphasis and Deemphasis
Operations and Types of FM Modulators
Operations and Types of PM Modulators
-Mark Christopher P. Tan
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1. Describe the Power Distribution of an Angle-Modulated wave.
2. Know the Benefits of Using Preemphasis and Deemphasis Networks.
3. Explain How Preemphasis and Deemphasis Affect the Signal- to – Noise Ratio of Angle-Modulated Wave.
OBJECTIVES:
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4. Describe the Operations of Frequency and Phase Modulators.
5. Know the Advantages and Disadvantages of Frequency and Phase Modulators.
6. Distinguish the Difference Between the Frequency and Phase Modulators.
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Unlike AM, The Total Power in an Angle-Modulated Wave is EQUAL to the Average Power of the Unmodulated Carrier.
Power Distribution of an Angle-Modulated Wave
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Power of Unmodulated Carrier in AM:
Total power of Modulated Wave in AM:
Pt = Pc + +
or
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The Average Power of an Angle -
Modulated Wave is
INDEPENDENT of the
Modulating Signal, Modulation
Index, and Frequency Deviation.
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The average power in the unmodulated carrier is:
Pc = Carrier Power (watts)
Vc = Peak Unmodulated Carrier Voltage(volts)
R = Load Resistance(ohms)
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Total Instantaneous Power of Angle –Modulated Wave is:
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Total modulated wave power is:Pt = P0 + P1 + P2 + P3 + Pn
Pt = Total power (Watts)
P0= Modulated Carrier Power (Watts)
P1 = Power in First Set of Sidebands(Watts)
P2 = Power in Second Set of Sidebands(Watts)
P3 = Power in Third Set of Sidebands(Watts)
Pn = Power in nth Set of Sidebands(Watts)
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Why There’s A Need To Find The Average Power of the Angle Modulated Wave?
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For an FM modulator with a peak frequency deviation f = 10 kHz, a modulating-signal frequency fm= 10 kHz, Vc = 10V, and a 500-kHz carrier.
Example
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a. Determine the unmodulated carrier power for the FM modulator (assume a load resistance RL = 50Ω).
b. Determine the total power in the angle-modulated wave.
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Noise at the Higher-Modulating-Signal Frequencies is Inherently Greater in Amplitude than at the Lower Frequencies.
The Use of Preemphasis and Deemphasis
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The Higher Modulating Signal Frequencies Have a lower Signal- to- Noise Ratio than the Lower Frequencies.
-try it on your calculator
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“EMPHASIS”
FM broadcast frequency is 88- 108MHz
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Preemphasis Network allows the High Frequency Modulating Signals to Modulate the Carrier at a Higher Level, and thus Cause More Frequency Deviation than their Original Amplitudes Would have Produced.
Deemphasis Network is the Reciprocal of Preemphasis. Restores the Original Amplitude-Versus- Frequency Characteristics to the Information Signals.
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Preemphasis networks is a High Pass Filter (differentiator),and normally located at the Transmitter.
Frequency Modulator
Pre-emphasized FM output
Pre-emphasis Circuit
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ACTIVE FILTER(preemphasis)
OUTPUT
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While, Deemphasis Networks is a Low Pass Filter(Integrator) and Normally Located at the Receiver.
FM Demodulato
r
FM inAudio Out
Deemphasis Circuit
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Preemphasis Network provides a constant Increase in the amplitude of the modulating signal with an increase of frequency.
Amplitude
Frequency
fb
Preemphasis
Deemphasis
Combined Frequency Response
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Break Frequency:
1fb =
2πRC= Deemphasis
1fb =
2πL/R= Preemphasis
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The Benefit of Using Preemphasis Network and Deemphasis Network is it Improves the Higher Modulating-Signal Frequencies, thus Producing a more uniform Signal-to-Noise Ratio at the Demodulator.
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kHz
42
1
1 2 3
Modulating Signal Spectrum
EXAMPLE:
kHz
0.1
0.25
0.5
1 2 3
Generated Noise
Without Using Preemphasis And Deemphasis
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VCO Direct frequency Modulator
K1 = 1KHz/VkHz
42
1
1 2 3OUTPUT = F x K1
PLL Demodulator Kd = 1V/KHz
Modulating Signal
kHz
42
1
1 2 3
kHz
42
1
1 2 3
OUTPUT = F x Kd
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kHz
42
1
1 2 3
Demodulator Output Signal
40 8 21 2 3 KHz
kHz
0.1
0.25
0.5
1 2 3
Generated Noise
SIGNAL-TO-NOISE RATIO:
Use your calculator
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kHz
42
1
1 2 3
Input Modulating Signal
Preemphasis Network
kHz
4 4 4
1 2 3
VCO Direct frequency Modulator
K1 = 1KHz/V
PLL Demodulator Kd = 1V/KHz
OUTPUT
Using Preemphasis and Deemphasis Network
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kHz
4 4 4
1 2 3
PLL Output Signal Spectrum
PLL Demodulator Kd = 1V/KHz
kHz
0.10.25
0.5
1 2 3
PLL Output Noise Spectrum
40 16 81 2 3 KHz
OUTPUT
SIGNAL-TO-NOISE RATIO:Use your calculator
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kHz
42
1
1 2 3
Deemphasis Output Signal Spectrum
kHz
0.1
0.125 0.125
1 2 3
Deemphasis Output Noise Spectrum
SIGNAL-TO-NOISE RATIO: 40 16 81 2 3 KHz
PLL Demodulator Kd = 1V/KHz
Deemphasis Network
OUTPUT
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Frequency Modulators - is a Circuit in Which the Carrier is Varied in Such a Way that its Instantaneous Phase is Proportional to the Integral of the Modulating Signal
-FM Modulator = Integrator Followed by PM Modulator
FREQUENCY AND PHASE MODULATORS
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Phase Modulators - is a Circuit in Which the Carrier is Varied in Such a Way that its Instantaneous Phase is Proportional to the Modulating Signal
-PM Modulator = Differentiator Followed by FM Modulator
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When the Frequency of the Carrier Oscillator is Modulated by the Information, Direct FM(Indirect PM) Results .
When the Phase of the Carrier Oscillator is Modulated by the Information, Direct PM(Indirect FM) Results .
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Direct FM Modulators :
1. Varactor Diode Modulator2. FM Reactance Modulator3. Linear Integrated-Circuit Direct FM Modulator
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Varactor Diode- also Known as Variable Capacitance Diode (VariCap).
-This Device is Basically a Semiconductor Junction Diode Operated in a Reverse-bias Mode.
-is Used to Transform Changes in the Modulating Signal Amplitude to Changes in Frequency.
VARACTOR DIODE MODULATOR
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Varactor Diode Schematic Symbol
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Crystal Oscillators-provide highly accurate frequencies and their stability is superior to LC oscillator.
Since Direct FM Varactor Diode Modulator uses Crystal Oscillator the Peak Deviation is Limited to Relatively Small Values.
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Since Peak Deviation is Limited To Relatively Small, Therefore it Only Requires Less Bandwidth to Use, Since it Only Uses Less Bandwidth it has Only Limited Information to Transmit
Therefore, it’s Primary Used for Low index application such as Two-Way Mobile Radio
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Center frequency of oscillator:
1fc=
2π (LC)1/2 Hz
When Modulating Signal is Applied:
f = New Frequency of OscillationC = Change of Frequency Due to Modulating Signal
Peak frequency Deviation
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REACTANCE MODULATOR
REACTANCE- is the opposition to alternating current due to capacitance (capacitive reactance) or inductance (inductive reactance)
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JFET REACTANCE MODULATOR(schematic diagram)
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LC circuit(Tank Circuit) - is a resonant circuit or tuned circuit that consists of an inductor, and a capacitor. When connected together, an electric current can alternate between them at the circuit's resonant frequency.
LC circuits - are used either for generating signals at a particular frequency, or picking out a signal at a particular frequency from a more complex signal. They are key components in many applications such as oscillators, filters, tuners and frequency mixers. An LC circuit is an idealized model since it assumes there is no dissipation of energy due to resistance.
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AC Equivalent Circuit of Reactance Modulator
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Interchanging the values of R and C or L causes the variable reactance to be inductive or capacitive.
Types of Reactance Modulators:Name Condition
RC Capacitive XC > R
RC Inductive R > XC
RL Inductive XL > R
RL Capacitive R > XL
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Primary Advantage of Reactance Modulator - is that relatively high-frequency deviations and modulation indices are easily obtained because the oscillators are inherently unstable.
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Linear Integrated-Circuit Direct FM Modulator
Modulating Signal FM Output
Band Pass Filter
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FMoutput = fc + f
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Disadvantage is their lower Output Power and Need for Several Additional External Components, such as Timing Capacitors and Resistors.
Advantage is it Generates A Direct FM Output Waveform that is Relatively Stable, Accurate, and Directly Proportional to the Input Modulating Signal.
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Varactor Diode Direct PM Modulator
Transistor Direct PM Modulator
Direct PM Modulators
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Varactor Diode Direct PM Modulator
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Advantages of Direct PM – is that a Buffered Crystal Oscillator is Used for the Source of the Carrier Signal, Therefore it is More Frequency Stable.
Disadvantage – since it uses Crystal Oscillator, it is more Difficult for them to Achieve High Phase Deviations and Modulation Indices.
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Transistor Direct PM Modulator
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Electronic Communication System(5th Edition)
-by: Wayne Tomasi Electronic Communication System
- by: Kennedy and Davis Electronic Communication System
-by : Louis Frenzel
www.wikipedia.com
REFERENCES:
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If You Think You are Beaten, You are.If You Think You Dare not , You Don’t.If You’d Like to Win But Think You Can’t,It’s Almost Certain You Won’t.Life’s Battle Don’t Always Go to The Stronger
or Faster Man, But Sooner or Later, The Man Wins is The Man Who Thinks He Can…
-Arnold Palmer