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    Modulation (AM/DSB/SSB)

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    Paul Malcolm
    doc describing modulation of signals using AM, DSB/SC, SSB/SC, outlining their similarities and differences

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    © All Rights Reserved
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    Modulation (AM/DSB/SSB)

    Uploaded by

    Paul Malcolm
    430 views20 pages
    doc describing modulation of signals using AM, DSB/SC, SSB/SC, outlining their similarities and differences

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    © © All Rights Reserved

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    doc describing modulation of signals using AM, DSB/SC, SSB/SC, outlining their similarities and differences

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    430 views20 pages

    Modulation (AM/DSB/SSB)

    Uploaded by

    Paul Malcolm
    doc describing modulation of signals using AM, DSB/SC, SSB/SC, outlining their similarities and differences

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
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    EE 179, Lecture 9, Handout #14

    AM Modulators: Review

    Multiplier modulator (perhaps using variable gain amplifier)

    Nonlinear modulator, using square-law device

    Switching modulator effectively multiplies signal by square wave

    Multiplying a signal by a sinusoid shifts the frequency band.

    Super-heterodyning: mix = c + I .
    Sub-heterodyning: mix = c I .
    EE 179, April 18, 2014

    Lecture 9, Page 1

    Demodulation of DSB-SC Signals


    Both modulator and demodulator use a multiplier by carrier signal.

    Modulator uses bandpass filter

    Demodulator uses lowpass filter

    The carrier used by the demodulator must be in phase with the transmitter
    carrier (taking into account transmission delay).
    Such a receiver is called synchronous, coherent, homodyne.
    e2(t) = x(t) * sin(2*pi*fc*t)
    0.5

    0.5

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    e2(t) lowpass filtered


    0.1

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    2.5

    homo = same, dyne = power (irregular suffix)


    EE 179, April 18, 2014

    Lecture 9, Page 2

    Demodulation of DSB-SC Signals (cont.)


    The phase of the carrier in the received signal must be extracted.
    Suppose that the signal is not ideal (frequency is shifted):
    
    r(t) = Ac m(t t0 ) cos (c + )(t t0 )
    
    = Ac m(t t0 ) cos (c + )t d
    where d = (c + )t0 .
    The receiver has a local oscillator that must be adjusted to stay in phase
    with the received signal.
    A voltage-controlled oscillator (VCO) that is controlled by a phase-locked
    loop (PLL) is commonly used.
    If the goal is cheap receivers, then we can eliminate the PLL by
    transmitting the carrier signal along with the modulated message.
    AM (t) = A cos c t + m(t) cos c t = (A + m(t)) cos c t
    The tone A cos c t contains the desired carrier in correct phase.
    EE 179, April 18, 2014

    Lecture 9, Page 3

    AM Modulation
    AM modulation is a form of amplitude modulation. For ka > 0,
    s(t) = (Ac + m(t)) cos(c t) = Ac (1 + ka m(t)) cos(2fc t)
    We need bandwidth of m(t) fc and modulation index ka < 1.
    Spectrum of modulated signal:
    
    S(f ) = 12 Ac (f + fc ) + (f fc ) +
    1
    2 ka Ac

    EE 179, April 18, 2014

    
    M (f + fc ) + M (f fc )

    Lecture 9, Page 4

    DSB-SC vs. AM
    DSB-SC modulated signals undergo phase reversal when m(t) changes sign.
    It is difficult to extract carrier from received signal.
    1
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    EE 179, April 18, 2014

    Lecture 9, Page 5

    DSB-SC vs. AM (cont.)


    In AM, the carrier signal is modulated by Ac + m(t) = Ac (1 + ka m(t)).
    Examples: ka = 1 and ka = 0.5.
    k =1
    a

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    k = 0.5
    a

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    EE 179, April 18, 2014

    0.2

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    Lecture 9, Page 6

    AM Signal and Its Envelope

    EE 179, April 18, 2014

    Lecture 9, Page 7

    Envelope Detection of AM Signals


    The term detection means extracting signal from received data. In some
    cases it means demodulation.
    Suppose that a signal x(t) can be written as
    x(t) = E(t) cos c t ,
    where E(t) varies slowly compared to the carrier cos c t.
    Then |E(t)| is called the envelope of x(t).
    For envelope detection to work, we need

    fc bandwidth of m(t)
    Otherwise positive and negative spectral components overlap.

    A + m(t) 0
    Otherwise phase reversals occur when A + m(t) < 0.

    EE 179, April 18, 2014

    Lecture 9, Page 8

    Message Signal Peak and Offset


    The requirement for envelope detection,
    A m(t)
    is equivalent to
    A mmin = min m(t)
    Typically m(t) has zero offset, i.e., min m(t) = max m(t) = mp .
    The modulation index of the modulated signal is defined by
    mp
    =
    A
    Larger modulation index reduces power but makes demodulation harder.
    Broadcast AM stations use modulation index close to 1. Input signals are
    controlled using automatic gain control (AGC).
    Zero offset is not the same as zero DC value.
    EE 179, April 18, 2014

    Lecture 9, Page 9

    Modulation Index Example


    For tone modulation,
    m(t) = b cos m t
    the message peak is b and = b/A.
    AM (t) = (A + b cos m t) cos c t = A(1 + cos m t) cos c t

    EE 179, April 18, 2014

    Lecture 9, Page 10

    Modulation and Demodulation of AM Signals

    Modulation: use DSB-SC modulator with A cos c t added to signal.

    Demodulation: we could use coherent DSB-SC demodulator.

    But this approach fails to use the carrier component.


    EE 179, April 18, 2014

    Lecture 9, Page 11

    Envelope Detector for AM

    EE 179, April 18, 2014

    Lecture 9, Page 12

    AM Demodulation: Rectifier + Lowpass Filter

    Diodes were the first semiconductor electronic devices. The discovery of crystals rectifying abilities was made by German physicist
    Ferdinand Braun in 1874. The first semiconductor diodes, called cats whisker diodes, developed around 1906, were made of
    mineral crystals such as galena. Today most diodes are made of silicon, but other semiconductors such as germanium are
    sometimes used.
    EE 179, April 18, 2014

    Lecture 9, Page 13

    AM Demodulation Experiment
    m(t) = cos 2t , fc = 10 , h(t) =
    2

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    EE 179, April 18, 2014

    0.5

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    1 t/RC
    e
    u(t)
    RC

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    Lecture 9, Page 14

    Cats Whiskers (Crystal) Radio


    This radio was powered only by received radio energy.

    The point-contact semiconductor detector was subsequently resurrected around World


    War II because of the military requirement for microwave radar detectors.
    EE 179, April 18, 2014

    Lecture 9, Page 15

    Corncob Pipe Radio

    EE 179, April 18, 2014

    Lecture 9, Page 16

    Single Sideband (SSB)


    DSB-SC is spectrally inefficient. It uses twice the bandwidth of the
    message.

    The signal can be reconstructed from either the upper sideband (USB) or
    lower sideband.
    SSB transmits a bandpass filtered version of the modulated signal.

    EE 179, April 18, 2014

    Lecture 9, Page 17

    Single Sideband (cont.)

    EE 179, April 18, 2014

    Lecture 9, Page 18

    Single Sideband (cont.)

    EE 179, April 18, 2014

    Lecture 9, Page 19

    Single Sideband Modulation and Demodulation

    SSB can be transmitted using a DSB-SC modulator with a narrower


    bandpass filter. For USB, center frequency is
    fc = fc + 12 B
    and cutoff frequency is B/2.
    The bandfilter must roll off quickly to eliminate unwanted contributions
    from the other sideband.
    Message frequencies near 0 will be affected by the nonideal filter.

    SSB demodulation can use a DSB-SC demodulator with no change.


    The input to the lowpass filter is different from that of DSB-SC.

    EE 179, April 18, 2014

    Lecture 9, Page 20

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