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

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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

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Demodulation of DSB and SSB signals

• Q: How to design the vestigial band such that they add up back to the original spectrum?

5

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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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• 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.)

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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?

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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:

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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

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• Angle modulation analysis: Let’s just see it as an exercise for sharpening your analytical claws…

18

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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.

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