Chapter 3 : Frequency Modulation DEKC 3343 Communication Engineering Faculty of Electrical Engineering 1 3.8: FM Modulators • Direct FM is obtained when frequency of the carrier oscillator is modulated by the information signal. • Direct FM modulator 1. Varactor diode modulator 2. FM reactance modulators 3.8.1 : Direct FM Modulators with direct FM, the instantaneous frequency deviation is directly proportional to the amplitude of the modulating signal. schematic diagram of a simple direct FM generator :
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 1
3.8: FM Modulators• Direct FM is obtained when frequency of the carrier oscillator is modulated by
the information signal.
• Direct FM modulator
1. Varactor diode modulator
2. FM reactance modulators
3.8.1 : Direct FM Modulators
� with direct FM, the
instantaneous frequency
deviation is directly
proportional to the amplitude
of the modulating signal.
� schematic diagram of a simple
direct FM generator :
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 2
3.8.1 : Direct FM Modulators
� the tank circuit (L and Cm) is the frequency determining section for a standard LC
oscillator.
� Cm is a capacitor microphone that converts the acoustical energy into a mechanical
energy, which is used to vary the distance between the plates of Cm and
consequently change its capacitance.
� as Cm is varied, the resonant frequency is varied. I.e. the oscillator output frequency
varies directly with the external sound forces (i.e. direct FM).
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 3
3.8.2 : Varactor diode modulator
� Direct FM generator using varactor diode to deviate the frequency of a crystal
oscillator :
� R1 and R2 develop a DC voltage that reverse bias the varactor diode VD1 and
determine the resonant frequency of the oscillator.
� external modulating signal voltage added or subtracted from the DC bias, which
changes the capacitance of the diode and consequently changes the frequency of the
oscillation.
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 4
3.8.2 : Varactor diode modulator
� positive alternations of the modulating signal increase the reverse bias of VD1,
which decrease its capacitance and increase the frequency of the oscillation.
� negative alternations of the modulating signal decrease the reverse bias of VD1,
which increase its capacitance and decrease the frequency of the oscillation.
� simple to use, stable and reliable but limited peak frequency deviation thus limited
use to the low index applications.
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 5
� the use of varactor diode to transform changes in modulating signal amplitude
to changes in frequency :
� the center frequency for the oscillator :
(1)
where fc = carrier frequency
L = inductance of the primary winding of T1
C = varactor diode capacitance
3.8.2: VCO FM Modulator
LCfc
π2
1=
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 6
3.8.2 : VCO FM Modulator
� when a modulating signal is applied, the frequency is
(2)
where f = new frequency
∆C = change in varactor diode capacitance due to modulating signal
� the change in frequency is (3)
where ∆f = peak frequency deviation (hertz)
)(2
1
CCLfc
∆+=
π
fff c −=∆
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 7
3.8.3 : Indirect FM (Direct PM) Modulator
� with indirect FM, the instantaneous phase deviation rather than instantaneous
frequency deviation is directly proportional to the modulating signal.
� I.e. the indirect FM is accomplished by directly changing the phase of the
carrier.
� schematic diagram of an indirect FM modulator using a varactor diode :
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 8
3.8.3 : Indirect FM (Direct PM) Modulator
� varactor diode VD1 placed in series with the inductive network (L1 and R1).
� this combined series-parallel network appears as series resonant circuit to the output
frequency from the crystal oscillator.
� the modulating signal is applied to VD1, which changes its capacitance and
subsequently the phase angle of the impedance seen by the carrier also varies,
which results in a corresponding phase shift in the carrier.
� advantage of using indirect FM modulator is it is more stable than the direct
modulator.
� However, it has more distortion in the modulated waveform compared to direct FM.
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 9
3.9 : Frequency Up-conversion
� after the modulation, the frequency of the modulated-wave is up-converted to
the desired frequency of transmission.
� 2 basic methods of frequency up-conversion :
� heterodyning process
� frequency multiplication
3.9.1 : Heterodyne Method
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 10
3.9.1 : Heterodyne Method
� 2 inputs to the balanced modulator :
angle-modulated carrier and its side
frequencies, an also the
unmodulated RF carrier signal.
� the 2 inputs mix nonlinearly in the
balanced modulator producing the
sum and difference frequencies at its
output.
� the BPF (bandpass filter) is tuned to
the sum frequency with a passband
wide enough to pass carrier plus the
upper and lower side frequencies
while the difference frequencies are
blocked.
RFincoutc fff += )()(
� the frequency deviation, rate of
change, modulation index, phase
deviation and bandwidth are
unaffected by the heterodyne process.
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 11
3.9.2 : Multiplication method
� with multiplication method, the frequency of the modulated carrier is multiplied by
a factor of N in the frequency multiplier.
� frequency deviation, phase deviation and modulation index are also multiplied.
� However, the rate of the deviation is unaffected (i.e. the separation between
adjacent side frequencies remains unchanged).
� as frequency deviation and modulation index are multiplied, the number of side
frequency also increases. Thus, the bandwidth also increases.
� For modulation index higher than 10, Carson’s Rule can be applied
inout NBfNB =∆= )2(
3.10 FM Transmitter
� DIRECT FM TRANSMITTER
Direct FM transmitters produce an output waveform in which the
frequency deviation is directly proportional to the modulating
signal.
1. Crosby Direct FM transmitter
2. Phase-Locked-Loop Direct FM transmitter
� INDIRECT FM TRANSMITTERS
1. Armstrong Indirect FM transmitter
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 12
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 13
3.10 : FM Transmitter
3.10.1 : Direct FM Transmitter
� Block diagram for
a commercial broadcast-band
transmitter :
� also known as Crosby direct FM transmitter (includes an automatic frequency
control –AFC loop)
� the carrier frequency is basically the center frequency of the master oscillator fc =
5.1 MHz, which is multiplied by 18 to produce a final transmission carrier
frequency ft = 91.8 MHz.
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 14
3.10.1 : Direct FM Transmitter
� the frequency and frequency deviations at the output of the modulator are also
multiplied by 18.
To achieve maximum deviation allowed for FM stations at antenna (75 kHz), the
deviation at the output of the modulator is
HzN
kHzf 7.4166
18
7500075===∆
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 15
3.10.1 : Direct FM Transmitter
The modulation index at the output of the modulator,
For maximum modulating signal frequency allowed for FM (15 kHz)
Thus, modulation index at antenna is m= 0.2778 (18) = 5
mfm
7.4166=
2778.015000
7.4166==m
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 16
3.10.2 : AFC Loop
� for medium and high index FM systems, the oscillator cannot be a crystal type
because the frequency at which the crystal oscillates cannot be significantly
deviated.
� as a result, the stability of the oscillator in the direct FM is low.
� to overcome this problem, AFC loop is used.
� with AFC, the carrier signal is mixed in a nonlinear device with the signal from a
crystal reference oscillator (the output is down-converted in frequency).
� the output is then fed back to the input of a frequency discriminator. It is a
frequency-selective device whose output voltage is proportional to the difference
between the input frequency and its resonant frequency.
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 17
3.10.2 : AFC Loop
� if there is a master oscillator frequency drift (resulting in a change of carrier center
frequency), the discriminator responds by producing a DC correction voltage.
� this voltage is added to the modulating signal to automatically adjust the master
oscillator’s center frequency.
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 18
3.11 : Indirect FM Transmitter
� Indirect FM transmitters produce an output
waveform in which the phase deviation is
directly proportional to the modulating
signal.
� Consequently, the carrier oscillator is not
directly deviated – crystal can be used
without use of AFC loop.
� Block diagram for wideband Armstrong
indirect FM transmitter :
� low frequency sub-carrier fc is phase
shifted 90˚ and fed to a balanced
modulator. It is mixed with the
modulating signal fm.
� the output from the balanced modulator
is DSBSC wave that is combined with
the original carrier in a combining
network to produce a low-index, phase-
modulated waveform.
Chapter 3 : Frequency Modulation
DEKC 3343 Communication Engineering
Faculty of Electrical Engineering 19
3.11 : Indirect FM Transmitter
� Proof :
By using trigonometric function : cos (A+B) =cos A cos B – sin A sin B