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

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:

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.

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

Power of Unmodulated Carrier in AM:

Total power of Modulated Wave in AM:

Pt = Pc + +

or

The Average Power of an Angle -

Modulated Wave is

INDEPENDENT of the

Modulating Signal, Modulation

Index, and Frequency Deviation.

The average power in the unmodulated carrier is:

Pc = Carrier Power (watts)

Vc = Peak Unmodulated Carrier Voltage(volts)

R = Load Resistance(ohms)

Total Instantaneous Power of Angle –Modulated Wave is:

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)

Why There’s A Need To Find The Average Power of the Angle Modulated Wave?

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

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.

Noise at the Higher-Modulating-Signal Frequencies is Inherently Greater in Amplitude than at the Lower Frequencies.

The Use of Preemphasis and Deemphasis

The Higher Modulating Signal Frequencies Have a lower Signal- to- Noise Ratio than the Lower Frequencies.

-try it on your calculator

“EMPHASIS”

FM broadcast frequency is 88- 108MHz

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.

Preemphasis networks is a High Pass Filter (differentiator),and normally located at the Transmitter.

Frequency Modulator

Pre-emphasized FM output

Pre-emphasis Circuit

ACTIVE FILTER(preemphasis)

OUTPUT

While, Deemphasis Networks is a Low Pass Filter(Integrator) and Normally Located at the Receiver.

FM Demodulato

r

FM inAudio Out

Deemphasis Circuit

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

Break Frequency:

1fb =

2πRC= Deemphasis

1fb =

2πL/R= Preemphasis

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.

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

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

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

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

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

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

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

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

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 .

Direct FM Modulators :

1. Varactor Diode Modulator2. FM Reactance Modulator3. Linear Integrated-Circuit Direct FM Modulator

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

Varactor Diode Schematic Symbol

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.

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

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

REACTANCE MODULATOR

REACTANCE- is the opposition to alternating current due to capacitance (capacitive reactance) or inductance (inductive reactance)

JFET REACTANCE MODULATOR(schematic diagram)

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.

AC Equivalent Circuit of Reactance Modulator

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

Primary Advantage of Reactance Modulator - is that relatively high-frequency deviations and modulation indices are easily obtained because the oscillators are inherently unstable.

Linear Integrated-Circuit Direct FM Modulator

Modulating Signal FM Output

Band Pass Filter

FMoutput = fc + f

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.

Varactor Diode Direct PM Modulator

Transistor Direct PM Modulator

Direct PM Modulators

Varactor Diode Direct PM Modulator

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.

Transistor Direct PM Modulator

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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or Faster Man, But Sooner or Later, The Man Wins is The Man Who Thinks He Can…

-Arnold Palmer