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ANALOG MODULATION
ANGLE MODULATION
A.Sanyasi Rao Assoc. Prof & HoD
Dept. of ECEBalaji Institute of Engineering & Sciences
Narsampet, Warangal
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What is Angle Modulation?
In angle modulation, information is embedded in the angle of the carrier.
We define the angle of a modulated carrier by the argument of...
s t Ac cos t
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Phasor Form
In the complex plane we have
t=1
t=0
t=3
Phasor rotates with nonuniform speed
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Angular Velocity
Since phase changes nonuniformly vs. time, we can define a rate of change
This is what we know as frequency
i di (t)
dt
s t Ac cos 2fct c
i t
di
dt2fc
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Instantaneous Frequency
We are used to signals with constant carrier frequency. There are cases where carrier frequency itself changes with time.
We can therefor talk about instantaneous frequency defined as
fi t 1
2di t
dt
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Examples of Inst. Freq.
Consider an AM signal
Here, the instantaneous frequency is the frequency itself, which is constant
s t 1 km(t) cos 2fct c
i t
di
dt2fc
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Impressing a message on the angle of carrier
There are two ways to form a an angle modulated signal.– Embed it in the phase of the carrier
Phase Modulation(PM)– Embed it in the frequency of the carrier
Frequency Modulation(FM)
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Phase Modulation(PM)
In PM, carrier angle changes linearly with the message
Where – 2πfc=angle of unmodulated carrier
– kp=phase sensitivity in radians/volt
s t Ac cos i t Ac cos 2fct kpm t
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Frequency Modulation
In FM, it is the instantaneous frequency that varies linearly with message amplitude, i.e.
fi(t)=fc+kfm(t)
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FM Signal
We saw that I.F. is the derivative of the phase
Therefore,
fi t 1
2di t
dt
i t 2fct 2k f m t 0
t
s t Ac cos 2fct 2k f m(t)dt0
t
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FM for Tone Signals
Consider a sinusoidal message The instantaneous frequency
corresponding to its FM version is
m(t) Am cos 2fmt
fi t fc k f m(t)
fcresting frequency
k f Am cos 2fmt
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Illustrating FM
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
FM message
Inst.frequencyMoves with theMessage amplitude
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Frequency Deviation
Inst. frequency has upper and lower bounds given by
fi t fc f cos 2fmt where
f frequency deviation k f Am
then
fi max fc f
fi min fc f
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FM Modulation index
The equivalent of AM modulation index is which is also called deviation ratio. It quantifies how much carrier frequency swings relative to message bandwidth
fW
baseband
orffm
tone
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Example:carrier swing
A 100 MHz FM carrier is modulated by an audio tone causing 20 KHz frequency deviation. Determine the carrier siwng and highest and lowest carrier frequencies
f 20KHz
frequency swing 2f 40KHz
frequency range :
fhigh 100MHz 20KHz 100.02MHz
flow 100MHz 20KHz 99.98MHz
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Example: deviation ratio
What is the modulation index (or deviation ratio) of an FM signal with carrier swing of 150 KHz when the modulating signal is 15 KHz?
f 150
275KHz
ffm
7515
5
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Myth of FM
Deriving FM bandwidth is a lot more involved than AM
FM was initially thought to be a bandwidth efficient communication because it was thought that FM bandwidth is simply 2f
By keeping frequency deviation low, we can use arbitrary small bandwidth
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FM bandwidth
Deriving FM bandwidth is a lot more involved than AM and it can barely be derived for sinusoidal message
There is a graphical way to illustrate FM bandwidth
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Piece-wise approximation of baseband
Look at the following representation
1/2W
Baseband bandwidth=W
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Corresponding FM signal
FM version of the above is an RF pulse for each square pulse.
The frequency of the kth RF pulse at t=tk is given by the height of the pulse. i.e.
fi fc k f m tk
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Range of frequencies?
We have a bunch of RF pulses each at a different frequency.
Inst.freq corresponding to square pulses lie in the following range
fi max fc k f mmax
fi min fc k f mmin
mmin
mmax
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A look at the spectrum
We will have a series of RF pulses each at a different frequency. The collective spectrum is a bunch of sincs
f
highestlowest
4W
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So what is the bandwidth?
Measure the width from the first upper zero crossing of the highest term to the first lower zero crossing of the lowest term
f
highestlowest
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Closer look
The highest sinc is located at fc+kfmp
Each sinc is 1/2W wide. Therefore, their zero crossing point is always 2W above the center of the sinc.
f2W
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Range of frequenices
Above range lies
<fc-kfmp-2W,fc+kfmp+2W>
f
highestlowest
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FM bandwidth
The range just defined is one expression for FM bandwidth. There are many more!
BFM=4W+2kfmp
Using =∆f/W with ∆f=kfmp
BFM=2(+2)W
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Carson’s Rule
A popular expression for FM bandwidth is Carson’s rule. It is a bit smaller than what we just saw
BFM=2(+1)W
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Commercial FM
Commercial FM broadcasting uses the following parameters– Baseband;15KHz– Deviation ratio:5– Peak freq. Deviation=75KHz
BFM=2(+1)W=2x6x15=180KHz
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Wideband vs. narrowband FM
NBFM is defined by the condition– ∆f<<W BFM=2W
– This is just like AM. No advantage here WBFM is defined by the condition
– ∆f>>W BFM=2 ∆f
– This is what we have for a true FM signal
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Boundary between narrowband and wideband FM
This distinction is controlled by – If >1 --> WBFM– If <1-->NBFM
Needless to say there is no point for going with NBFM because the signal looks and sounds more like AM
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Commercial FM spectrum
The FM landscape looks like this
FM station BFM station A FM station C
25KHz guardband
150 KHz
200 KHz
carrier
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FM stereo:multiplexing
First, two channels are created; (left+right) and (left-right)
Left+right is useable by monaural receivers
-
Left channel
Right channel
+
+
+
mono
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Subcarrier modulation
The mono signal is left alone but the difference channel is amplitude modulated with a 38 KHz carrier
Left channel
Right channel
+
+
+
mono
DSB-SCfsc=38 kHz
+
fsc=38KHz
freqdivider
Composite baseband
-
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Stereo signal
Composite baseband signal is then frequency modulated
Left channel
Right channel
+
+
+
mono
DSB-SCfsc=38 kHz
+
fsc=38KHz
freqdivider
Composite baseband
FM transmitter
-
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Stereo spectrum
Baseband spectrum holds all the information. It consists of composite baseband, pilot tone and DSB-SC spectrum
38 KHz19 KHz
15 KHz
Left+rightDSB-SC
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Stereo receiver
First, FM is stripped, i.e. demodulated Second, composite baseband is lowpass
filtered to recover the left+right and in parallel amplitude demodulated to recover the left-right signal
38 KHz19 KHz
15 KHz
Left+rightDSB-SC
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Receiver diagram
FMreceiver
lowpass filter(15K)
bandpassat 38KHz
X lowepass
VCODivide 2
X lowpass
+
+-
+
++
Left+right left
right
PLL
coherent detector
38 KHz19 KHz
15 KHz
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Subsidiary communication authorization(SCA)
It is possible to transmit “special programming” ,e.g. commercial-free music for banks, department stores etc. embedded in the regular FM programming
Such programming is frequency multiplexed on the FM signal with a 67 KHz carrier and 7.5 KHz deviation
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SCA spectrum
38 KHz19 KHz15 KHz
Left+rightDSB-SC
59.5 67 74.5 f(KHz)
SCA signal
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FM receiver
FM receiver is similar to the superhet layout
RF
mixer
LO
limiterDiscrimi-
natordeemphasis
AF poweramp
IF
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Frequency demodulation
Remember that message in an FM signal is in the instantaneous frequency or equivalently derivative of carrier angle
s t Ac cos 2fct 2k f m(t)dt0
t
s t Ac 2fc 2k f m t sin 2fct 2k f m(t)dt
t
Do envelope detection on s’(t)
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Receiver components:RF amplifier
AM may skip RF amp but FM requires it FM receivers are called upon to work with
weak signals (~1V or less as compared to 30 V for AM)
An RF section is needed to bring up the signal to at least 10 to 20 V before mixing
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Limiter
A limiter is a circuit whose output is constant for all input amplitudes above a threshold
Limiter’s function in an FM receiver is to remove unwanted amplitude variations of the FM signal
Limiter
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Limiting and sensitivity
A limiter needs about 1V of signal, called quieting or threshold voltage, to begin limiting
When enough signal arrives at the receiver to start limiting action, the set quiets, i.e. background noise disappears
Sensitivity is the min. RF signal to produce a specified level of quieting, normally
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Sensitivity example
An FM receiver provides a voltage gain of 200,000(106dB) prior to its limiter. The limiter’s quieting voltage is 200 mV. What is the receiver’s sensitivity?
What we are really asking is the required signal at RF’s input to produce 200 mV at the output
200 mV/200,000= 1V->sensitivity
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Discriminator
The heart of FM is this relationship
What we need is a device that linearly follows inst. frequency
fi(t)=fc+kfm(t)
Disc.output
f
Deviation limits
+75 KHz-75 KHz
fcarrier
fcarrier is at the IF frequencyOf 10.7 MHz
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Examples of discriminators
Slope detector - simple LC tank circuit operated at its most linear response curve
This setup turns an FM signalinto an AM
fc fo
output
f
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Phase-Locked Loop
PLL’s are increasingly used as FM demodulators and appear at IF output
Phase
comparator
Lowpass
filter
VCO
fin Error signal
fvcoVCO input
Control signal:constantWhen fin=fvco
Output proportional toDifference between fin and fvco
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PLL states
Free-running– If the input and VCO frequency are too far apart,
PLL free-runs Capture
– Once VCO closes in on the input frequency, PLL is said to be in the tracking or capture mode
Locked or tracking– Can stay locked over a wider range than was
necessary for capture
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PLL example
VCO free-runs at 10 MHZ. VCO does not change frequency until the input is within 50 KHZ.
In the tracking mode, VCO follows the input to ±200 KHz of 10 MHz before losing lock. What is the lock and capture range?– Capture range= 2x50KHz=100 KHz– Lock range=2x200 KHz=400 KHz
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Advantages of PLL
If there is a carrier center frequency or LO frequency drift, conventional detectors will be untuned
PLL, on the other hand, can correct itself. PLL’s need no tuned circuits
fc fo
output
f
If fc drifts detector has no way of correcting itselfSlope detector
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Zero crossing detector
Hard limiter
Zero Crossingdetector
Multi-vibrator
Averagingcircuot
FM Output
FM input
Hard limiter
ZC detector
multiV
more frequentZC’s meanshigher inst freqin turn meansLarger messageamplitudes
Averaging circuit
NOISE IN ANALOG MODULATION
AMPLITUDE MODULATION
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Receiver Model
The objective here is to establish a relationship between input and and output SNR of an AM receiver
BPF detector
Noise n(t)
Modulated signal s(t)l
output
filter
fc-fc
BT=2W
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Establishing a reference SNR
Define “channel” SNR measured at receiver input
(SNR)c=avg. power of modulated signal/
avg. noise power in the message bandwidth
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Noise in DSB-SC Receiver
Tuner plus coherent detection
BPF LPFDSB-SC
n(t) Cos(2πfct)
x(t) v(t)
s(t)
s t Acm(t)cos 2fct s2 t avg.power Ac
2 m2(t) / 2 Ac2P / 2
P avg. message power
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Receiver input SNR
Also defined as channel SNR:
(SNR)c Ac
2P / 2WNo
noise power in the message bandwidth
Ac
2P2WNo
W-W
No/2Flat noise spectrum:white noise
Noise power=hatched area
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Output SNR
Carrying signal and noise through the rest of the receiver, it can be shown that output SNR comes out to be equal to the input. Hence
Therefore, any reduction in input SNR is linearly reflected in the output
SNR o
SNR c
1
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(SNR)o for DSB-AM
Following a similar approach,
Best case is achieved for 100% modulation index which, for tone modulation, is only 1/3
SNR o
SNR c
k2P
1 k2P1
k : AM modulation index
P : avg. message power
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DSB-AM and DSB-SC noise performance
An AM system using envelope detection needs 3 times as much power to achieve the same output SNR as a suppressed carrier AM with coherent detection
This is a result similar to power efficiency of the two schemes
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Threshold effect-AM
In DSB-AM (not DSB-SC) there is a phenomenon called threshold effect
This means that there is a massive drop in output SNR if input SNR drops below a threshold
For DSB-AM with envelope detection, this threshold is about 6.6 dB
NOISE IN ANALOG MODULATION
FREQUENCY MODULATION
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Receiver model
Noisy FM signal at BPF’s output is
BFP LimiterFM
detectorLPF(W)
n(t)
FMs(t)
x t s t n(t)
Ac cos 2fct t r(t)cos 2fct t noise
where
t m(t)dt
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Phasor model
We can see the effect of noise graphically
(t)-(t)
reference
(t) (t)
FM signal
nois
e
Received signal
The angle FM detector will extract
(t)
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Small noise
For small noise, it can be approximated that the noise inflicted phase error is
=[r⁄Ac]Sin( So the angle available to the FM detector
is + FM Detector computes the derivative of
this angle. It will then follow that...
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FM SNR for tone modulation
Skipping further detail, we can show that for tone modulation, we have the following ratio
SNR rises as power of 2 of bandwidth; e.g. doubling deviation ratio quadruples the SNR
SNR o
SNR c
32
2
Bandwidth-SNR exchange
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Comparison with AM
In DSB-SC the ratio was 1 regardless. For commercial FM, =5. Therefore,
(SNR)o/(SNR)c=(1.5)x25=37.5 Compare this with just 1 for AM
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Capture effect in FM
An FM receiver locks on to the stronger of two received signals of the same frequency and suppresses the weaker one
Capture ratio is the necessary difference(in dB) between the two signals for capture effect to go into action
Typical number for capture ratio is 1 dB
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Normalized transmission bandwidth
With all these bandwidths numbers, it is good to have a normalized quantity.
Define
normalized bandwidth=Bn=BT/W
Where W is the baseband bandwidth
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Examples of Bn
For AM:
Bn=BT/W=2W/W=2 For FM
Bn=BT/W~2 to 3 For =5 in commercial FM, this is a very
large expenditure in bandwidth which is rewarded in increased SNR
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Noise/bandwidth summary
AM-envelope detection
SNR o 2
2 2 SNR c
Bn 2
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Noise/bandwidth summary
DSB-SC/coherent detection
(SNR)o=(SNR)c
Bn=2 SSB
(SNR)o=(SNR)c
Bn=1
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Noise/bandwidth summary
FM-tone modulation and =5
(SNR)o=1.5 2(SNR)c=37.5 (SNR)c
Bn~16 for =5
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Preemphasis and deemphasis
High pitched sounds are generally of lower amplitude than bass. In FM lower amplitudes means lower frequency deviation hence lower SNR.
Preemphasis is a technique where high frequency components are amplified before modulation
Deemphasis network returns the baseband to its original form
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Pre/deemphasis response
Flat up to ~500Hz, rises from 500-15000 Hz
500 Hz 2120 Hz 15KHz
-17dB
17dB
+3dB
-3dB
preemphasis
deemphasis
Deemphasis circuitIs between the detectorAnd the audio amplifier
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Suggested homework
3.41 5.3 5.7