111 Lecture 7 Basic 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 Demodulation of DSB and SSB signals Q: How to design the vestigial band such that they add up back to the original spectrum?

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One solution: H( f ) is mean-shifted conjugate anti-symmetric about f c. Explicitly, let H  ( f ) be an LP anti-symmetric filter; i.e., H  ( f ) = - H  ( -f ) and H  ( f ) = 0 for | f | > . 6

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.) 9

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 + 2  IF or  c - 2  IF ) After the RF filter: only the desired signal at w c 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 in Frequency domain?

Example: 14 High or low? One way to choose is to look at the tuning range of LO. AM range: 540kHz ~ 1600kHz IF: 455kHz Low-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

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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. 20