Transcript Document

Angle Demodulator using AM

FM demodulators first generate an AM signal and then use an AM
demodulator to recover the message signal.

To transform the FM signal into an AM signal, Pass the FM signal through
an LTI system, whose frequency response is approximately a straight line in
the frequency band of the FM signal.
 If the frequency response of such a system is given by
H ( f )  V0  k  f  f c 

for
f  fc 
Bc
2
And if the input to the system is u (t )  Ac cos  2f c t  2k f  m( )d 

t


Then the output will be the signal
t
v0 (t )  Ac V0  kk f m(t ) cos  2f c t  2k f  m( )d 



The next step is to demodulate this AM signal to obtain Ac(Vo+kkfm(t)),
from which the message m(t) can be recovered.


1
Angle Demodulators using AM

Many circuits can be used to implement the first stage of an
FM demodulator, i.e., FM to AM conversion.
 One candidate is a simple differentiator with H ( f )  2f

Another candidate is the rising half of the frequency characteristics
of a tuned circuit, as shown in below
 Such a circuit can be easily implemented, but usually the linear
region of the frequency characteristic may not be wide enough.
 To obtain linear characteristics over a wide range of frequencies,
usually two circuits tuned at two frequencies f1 and f2 are connected
in a configuration, which is known as a balanced discriminator.
2
Angle Demodulator using AM
A balanced discriminator and the corresponding frequency response.
3
Angle Demodulator using PLL

A different approach to FM-signal demodulation is to use a
phase-locked loop (PLL) => PLL-FM demodulator
e(t )
u(t )
v(t )
yv (t )

The input to the PLL is the angle-modulated signal
u(t )  Ac cos2f ct   (t )
t
(where, for FM,  (t )  2k f  m( )d )


The VCO generates a sinusoid of a fixed frequency; in this
case, it generates the carrier frequency fc, in the absence of an
input control voltage.
4
Angle Demodulator using PLL


Now, suppose that the control voltage to the VCO is the loop
filter's output, denoted as v(t).
Then, the instantaneous frequency of the VCO is
f v (t )  f c  kv v(t )


Consequently, the VCO output may be expressed as
yv (t )  Ac sin2f ct  v (t )



t
where v (t )  2kv  v( )d
0
The phase comparator is a multiplier and a filter that rejects
the signal component centered at 2fc.
Hence, its output may be expressed as


where kv is a deviation constant
e(t )  12 Av Ac sin (t )  v (t )
where the difference (t) - v(t)e(t) constitutes the phase error.
The signal e(t) is the input to the loop filter.
5
Angle Demodulators using PLL

Let us assume that the PLL is in lock position, so the phase
error is small.
Then, sin (t )  v (t )   (t )  v (t )  e (t )
 under this condition, so we may deal with the linearized model of the
PLL, shown in below
t
 We may express the phase error as e (t )   (t )  2kv 0 v( )d
d
d
e (t )  2kv v(t )   (t )
 Or equivalently, either as
dt
dt

d
d

(
t
)

2

k

(

)
g
(
t


)
d


 (t )
 Or as
(Eq. 1)
e
v
e
0
dt
dt

Linearized PLL:
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Angle Demodulators using PLL


The Fourier transform of (Eq. 1) is
( j 2f )e ( f )  2kve ( f )G( f )  ( j 2f )( f )
Hence
e ( f ) 


V ( f )   e ( f )G( f ) 
Now, suppose that we design G(f) such that
Then, from (Eq.2),


1
( f )
 kv 
1   G( f )
 jf 
Or equivalently,
V( f ) 
G( f )
( f )
 kv 
1   G( f )
(Eq.2)
 jf 
G( f )
kv
 1
jf
j 2f
1
( f ) 
{ j 2f ( f )}
2kv
2kv
kf
1 d
v(t ) 
 (t )  m(t )
2kv dt
kv
Since the control voltage of the VCO is proportional to the message
signal, v(t) is the demodulated signal.
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FM-Radio Broadcasting





Commercial FM-radio broadcasting utilizes the frequency
band 88-108 MHz for the transmission of voice and music
signals.
The carrier frequencies are separated by 200 kHz and the peak
frequency deviation is fixed at 75 kHz.
Preemphasis is generally used, as described in Chapter 6, to
improve the demodulator performance in the presence of noise
in the received signal.
The receiver most commonly used in FM-radio broadcast is a
superheterodyne type.
The block diagram of such a receiver is shown in below
8
FM-Radio Broadcasting
Block diagram of a superheterodyne FM-radio receiver.
9
FM-Radio Broadcasting




As in AM-radio reception, common tuning between the RF
amplifier and the local oscillator allows the mixer to bring all
FM-radio signals to a common IF bandwidth of 200 kHz,
centered at fIF = 10.7 MHz.
Since the message signal m(t) is embedded in the frequency of
the carrier, any amplitude variations in the received signal are
a result of additive noise and interference.
The amplitude limiter removes any amplitude variations in the
received signal at the output of the IF amplifier by
bandlimiting the signal.
A bandpass filter, which is centered at fIF = 10.7 MHz with a
bandwidth of 200 kHz, is included in the limiter to remove
higher-order frequency components introduced by the
nonlinearity inherent in the hard limiter.
10
FM-Radio Broadcasting



A balanced frequency discriminator is used for frequency
demodulation.
The resulting message signal is then passed to the audiofrequency amplifier, which performs the functions of
deemphasis and amplification.
The output of the audio amplifier is further filtered by a
lowpass filter to remove out-of-band noise, and this output is
used to drive a loudspeaker.
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FM-Stereo Broadcasting
<Transmitter>
 Many FM-radio stations transmit music programs in stereo by
using the outputs of two microphones
FM-stereo transmitter and
signal spacing.
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FM-Stereo Broadcasting






The signals from the left and right microphones, ml(t) and
mr(t), are added and subtracted .
The sum signal ml(t)+mr(t) is left unchanged and occupies the
frequency band 0-15 kHz.
The difference signal ml(t)-mr(t) is used to AM modulate
(DSB-SC) a 38 kHz carrier that is generated from a 19-kHz
oscillator.
A pilot tone at the frequency of 19 kHz is added to the signal
for the purpose of demodulating the DSB-SC AM signal.
We place the pilot tone at 19 kHz instead of 38 kHz because
the pilot is more easily separated from the composite signal at
the receiver.
The combined signal is used to frequency modulate a carrier.
13
FM-Stereo Broadcasting

By configuring the baseband signal as an FDM signal, a
monophonic FM receiver can recover the sum signal
ml(t)+mr(t) by using a conventional FM demodulator.


Hence, FM-stereo broadcasting is compatible with conventional FM.
In addition, the resulting FM signal does not exceed the allocated 200kHz bandwidth.
<Receiver>
 The FM demodulator for FM stereo is basically the same as a
conventional FM demodulator down to limiter/discriminator.


Thus, the received signal is converted to baseband.
Following the discriminator, the baseband message signal is
separated into the two signals, ml(t)+mr(t) and ml(t)-mr(t), and
passed through deemphasis filters, as shown in Figure 4.18.
14
FM-Stereo Broadcasting





The difference signal is obtained from the DSB-SC signal via
a synchronous demodulator using the pilot tone.
By taking the sum and difference of the two composite signals,
we recover the two signals, ml(t) and mr(t).
These audio signals are amplified by audio-band amplifiers,
and the two outputs drive dual loudspeakers.
As indicated, an FM receiver that is not configured to receive
the FM stereo sees only the baseband signal ml(t)+mr(t) in the
frequency range 0-15 kHz.
Thus, it produces a monophonic output signal that consists of
the sum of the signals at the two microphones.
15
FM-Stereo Broadcasting
Figure 4.18 FM-stereo receiver.
16
Television Broadcasting

Commercial TV broadcasting began as black-and-white picture
transmission in London in 1936 by the British Broadcasting
Corporation (BBC).

Color TV was demonstrated a few years later, but commercial TV
stations were slow to develop the transmission of color-TV signals.


This was due to the high cost of color-TV receivers.
With the development of the transistor, the cost of color TV decreased significantly.
 By the middle 1960s, color TV broadcasting was widely used by the industry.

The frequencies allocated for TV broadcasting fall in the VHF and
UHF bands.




Table 4.2 lists the TV channels allocated in the United States.
The channel bandwidth allocated for the transmission of TV signals is 6 MHz.
In contrast to radio broadcasting, standards for television-signal transmission vary
from country to country.
The US standard was set by the National Television Systems Committee (NTSC).
17
Television Broadcasting
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