111 lecture 7 basic modulation techniques (iii) principles of communications fall 2008 nctu ee...
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111
Lecture 7Basic Modulation Techniques (IIi)
Principles of Communications
Fall 2008
NCTU EE Tzu-Hsien Sang
2
Outlines
• Linear Modulation
• Angle Modulation
• Interference
• Feedback Demodulators
• Analog Pulse Modulation
• Delta Modulation and PCM
• Multiplexing
2
Vestigial-SideBand Modulation (VSB)
• Remind the advantages of SSB over DSB.
• There are, however, problems related to implementation of practical SSB systems: (1) imperfect Hilbert filter, (2) loss of low-frequency components, and (3) loss of carrier.
• Solutions? To come up with an intermediate scheme between DSB and SSB, i.e., do not cut out half of the spectrum completely. We call the new scheme VSB.
3
4
f fc -fc
M(f-fc) M(f+fc)
f
M(f)
f fc -fc
M(f-fc)H(f) M(f+fc)H(f)
f
Demodulation of DSB and SSB signals
• Q: How to design the vestigial band such that they add up back to the original spectrum?
5
LPF
cos(ct)
d(t) xc(t) yd(t)
.)( from )(recover can we,conditionscertain satisfies )( If
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ffHffHfM
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cccc
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• One solution: H( f ) is mean-shifted conjugate anti-symmetric about fc. Explicitly, let H( f ) be an LP anti-symmetric filter; i.e., H( f ) = -H( -f ) and H( f ) = 0 for | f | > .
6
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H(f)
-1/2
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- f
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1/2 U(f-fc)
fc+ f
fc- fc
• Often, a carrier is added to the transmitted signal. This is similar to SSB with carrier insertion. We only need to use an envelope detector at the receiver.
7
• Example: Color TV; B/W: VSB; Chrominance: I/Q; Audio: FM
8
• Frequency Translation and Mixing: mostly the goal is to move the signal to occupy a band. Why do we want to do that? (If you are an RF/analog designer, this part may be the stuff you ever need to understand in this course.)
9ttmttmte )2cos()(cos)()( 212
10
Sometimes, unwanted signals got mixed in.
• A broadcast receiver should perform the following functions in addition to “demodulation.” 1. Carrier-frequency tuning: select the desired signal (channel) 2. Filtering: separate the desired signal from other modulated signals. 3. Amplification: compensate for transmission loss.
• The super-heterodyne (“superhet”) receiver fulfils the above requirements without using a high-gain tunable bandpass filter.
• Remark: it is difficult to construct a narrow bandpass filter at high frequency
11
12
Get to know the word: what does it exactly mean by superheterodyne?
13
At the antenna: the desired signal with carrier c. (undesired signals: c + 2IF or c - 2IF)After the RF filter: only the desired signal at wc can go through (a wide BPF).(Some near-by channels can go through but will be filtered out at IF )After the mixer: the desired signal at IF.(There are other near-by channels)After the IF filter: only the desired signal at IF can go through. (a narrow BPF).(Other channels are filtered out)
Question: Can you visualize the procedure with proper diagrams inFrequency domain?
• Example:
14
108MHz.~88 :RF 200kHz, IFBW 10.7MHz, IF :FM
1.6MHz.~0.54 :RF 10kHz, IFBW 455kHz, IF:AM
High or low? One way to choose is to look at the tuning range of LO.AM range: 540kHz ~ 1600kHzIF: 455kHzLow-side tuning: LO = c - IF
Range: 85kHz ( 540kHz – 455kHz ) ~ 1145kHz ( 1600kHz – 455kHz )1 : 13.47
High-side tuning: LO = c + IF
Range: 995kHz ( 540kHz + 455kHz ) ~ 2055kHz ( 1600kHz + 455kHz )1 : 2.07
Angle Modulation
15])(2cos[)(
)](cos[)(
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(FM) ModulationFrequency and (PM) Modulation Phase
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16
17
• Angle modulation analysis: Let’s just see it as an exercise for sharpening your analytical claws…
18
General
toneSinglebandWide
bandNarrow
shift. phase degree 90 except the signal, AMan like looksIt
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))}(1(Re{
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)](cos[)( :analysis narrowbandFor
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• Wideband analysis (the case of single tone): I don’t think this material is of significant importance except that it is a good exercise mathematically. Unfortunately, we might still see this stuff popped up in graduate entrance exams from time to time.
20 tone?single of case study the wedo Why :Question
.sin)(Let tt m
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