Post on 03-Oct-2021
EELE 3370
Communications I
Angle Modulation and Demodulation
Dr. Talal Skaik 2016
Islamic University of Gaza
Electrical Engineering Department
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Generating FM Waves Two Methods: [1] Indirect method using NBFM Generation
[2] Direct method
NBFM Generation
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Dr. Talal Skaik IUG 2016
Generating FM Waves
Bandpass Limiter
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With the NBFM generation, the
amplitude of the NBFM modulator
will have some amplitude variation.
Dr. Talal Skaik IUG 2016
Generating FM Waves Circuit Realization of Hard Limiter The circuit consists of a high gain amplifier, a current limiting resistor R
and two zener diodes arranged in a back-to-back configuration.
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Generating FM Waves Indirect Method of Armstrong
We start with the generation of a NBFM with frequency deviation ∆f
as described previously.
Then we use a frequency multiplier ( x N ) to obtain a WBFM.
After filtering using a bandpass filter centered at Nfc, we get an FM
signal with N ∆f.
Sometimes the frequency increase of the carrier is not needed.
Solution: after the multiplier we insert a mixer to down convert the
carrier to the wanted one.
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Generating FM Waves Indirect Method
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Generating FM Waves
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A frequency multiplier can be realized by a nonlinear device:
Generally, a nonlinear device may have the characteristic:
If , then y(t) is of the form:
c fx ( t ) A cos t k m( )d
0 1 2 2 2c f c f
n c f
y ( t ) c c cos t k m( )d c cos t k m( )d
+ +c cos n t nk m( )d
Use Bandpass filter to recover
the desired FM signal
Generating FM Waves
Example: The final output is desired to have a carrier frequency of
91.2 MHz and ∆f=75 KHz. We begin with carrier frequency
fc1=200KHz. The baseband signal has frequency range 50Hz -15
KHz. ∆f is initially chosen 25 Hz [gives β= ∆f / fm = 0.5 for worst
case fm=50 Hz].
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Generating FM Waves Example solution
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Generating FM Waves
Example: Design an Armstrong indirect FM modulator to generate
an FM signal with carrier frequency 97.3 MHz and Δf = 10.24 kHz.
A NBFM generator of fc1 = 20 kHz and Δf = 5 Hz is available. Only
frequency doublers can be used as multipliers. Additionally, a local
oscillator (LO) with adjustable frequency between 400 and 500 kHz
is readily available for frequency mixing.
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Generating FM Waves Example solution
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Generating FM Waves Example solution
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Generating FM Waves Frequency Doubler Circuit Frequency multipliers are special class C amplifiers that are used to
generate a frequency that is a multiple (harmonic) of a lower frequency. The shown circuit is second harmonic generator.
The LC tank circuit is tuned to the second harmonic 2MHz.
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Generating FM Waves Direct Generation
The frequency of a voltage-controlled oscillator (VCO) is controlled
by the voltage m(t).
Use an operational amplifier to build VCO.
Another way is to vary one of the reactive parameters (C and L) of
the resonant circuit of an oscillator.
A reversed biased semiconductor diode acts as a capacitor whose
capacitance varies with the bias voltage.
These diodes are called Varicaps or Varactors.
The frequency of oscillation is given by:
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0 1/ LC
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Generating FM Waves If the capacitance C is varied by the modulating signal m(t),
C=C0 - k m(t) Then
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1 1 1n
Here we use the approximation x nx x
Dr. Talal Skaik IUG 2016
Generating FM Waves Because C=C0 – k m(t) , the maximum capacitance deviation is:
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Varactor symbols and capacitance curve.
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The varactor diode is a semiconductor diode whose junction capacitance
changes voltage.
This diode is shunted with the tuned circuit (tank circuit) of the carrier
oscillator as shown in the figure.
V0 is the polarizing voltage to maintain a reverse bias across the varactor
diode. 17
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Demodulation of FM Signals A frequency-selective network with a transfer function of the form
|H(f)|=2af + b over the FM band would yield an output proportional to
the instantaneous frequency.
A possible circuit is an ideal differentiator with transfer function j2πf.
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Demodulation of FM Signals 19
Differentiator
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Demodulation of FM Signals
Practical Frequency Demodulators Differentiation: Operational Amplifier (OPAMP) differentiator can
be used to convert frequency variation to amplitude variation that can
be detected using a simple envelop detector.
Slope detection: Any tuned circuit which has a linear segment of
positive slope in the frequency response under or above the resonance
can be used instead of the OPAMP differentiator.
Examples: high pass RC filter
Tuned RLC filter:
Limitation: narrow bandwidth. 20
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Demodulation of FM Signals High Pass Filter
The slope is linear over small band, so distortion occurs if the signal band is
larger than the linear band.
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Demodulation of FM Signals Tuned LC circuit The transfer characteristic of a tuned circuit in a small region off
resonance is approximately linear.
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Superheterodyne Analog AM/FM Receivers The radio receiver used in broadcast AM and FM systems is called the
superheterodyne receiver.
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fIF = 455KHz (AM radio); 10.7 MHz (FM); 38 MHz (TV)
AM: 530 KHz -1710 KHz FM: 88MHz – 108 MHz
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Superheterodyne Analog AM/FM Receivers
AM stations that are 2fIF apart are called image stations and both
would appear simultaneously at the IF output.
Example: the stations at 1000 KHz and 1910 KHz are called image
stations. They are separated by (2* 455 KHz=910 KHz).
If we pick 1000 KHz station, the LO frequency is K1455 Hz.
Another station 1910 KHz also appears at the IF output
(1910 KHz-1455 KHz=455 KHz)
But the 1910 KHz station is filtered out using the RF filter.
The tunable RF filter has poor selectivity by can filter out the image
station.
The IF section filter has good selectivity and it is fixed at 455 KHz
and it filters out undesired neighboring stations (adjacent stations are
separated by 10 KHz). 24
Dr. Talal Skaik IUG 2016