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Linear CW Modulation
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Roadmap1. Bandpass Signals and Systems2. Double-Sideband Amplitude Modulation3. Modulators and Transmitters4. Suppressed-Sideband Amplitude Modulation5. Frequency Conversion and Demodulation
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BANDPASS SIGNALS AND SYSTEMS
• Analog Message Conventions• Bandpass Signals• Bandpass Transmission• Bandwidth
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Analog Message Conventions
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sinusoidal or tone modulation
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Bandpass Signals
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where A(t) is the envelope and φ(t) is the phase, both functions of time
The envelope is defined as nonnegative, so that A(t) ≥ 0 . Negative “amplitudes,” when they occur, are absorbed in the phase by adding ±180o .
envelope-and-phase description
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quadrature-carrier description
quadraturecomponent
in-phase component
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DOUBLE-SIDEBAND AMPLITUDE MODULATION
• AM Signals and Spectra• DSB Signals and Spectra• Tone Modulation and Phasor Analysis
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AM Signals and Spectra
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If Ac denotes the unmodulated carrier amplitude, modulation by x(t) produces the AM signal
The signal’s envelope is
modulation index
xc(t) has no time-varying phase, its in-phase and quadrature components are
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The condition fc >> W ensures that the carrier oscillates rapidly compared to thetime variation of x(t); otherwise, an envelope could not be visualized.
The condition μ ≤ 1 ensures that Ac[ 1 + μx(t) ] does not go negative.
With 100 percent modulation (μ = 1), the envelope varies between Amin = 0 and Amax = 2Ac .
Overmodulation ( μ > 1), causes phase reversals and envelope distortion
The envelope clearly reproduces the shape of if
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12AM transmission bandwidth
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Another important consideration is the average transmitted power
Upon expanding
averages to zero under the condition fc >> W
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The term Pc represents the unmodulated carrier power, since ST = Pc when μ = 0
the term Psb represents the power per sideband since, when μ ≠ 0, ST consists of the power in the carrier plus two symmetric sidebands.
The modulation constraint
requires that
Consequently, at least 50 percent (and often close to 2/3) of the total transmittedpower resides in a carrier term that’s independent of and thus conveys nomessage information.
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DSB Signals and Spectra
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The “wasted” carrier power in amplitude modulation can be eliminated bysetting and suppressing the unmodulated carrier-frequency component. Theresulting modulated wave becomes
which is called double-sideband–suppressed-carrier modulation—or DSB forshort. (The abbreviations DSB–SC and DSSC are also used.)
the DSB spectrum looks like an AM spectrum without the unmodulated carrierimpulses. The transmission bandwidth thus remains unchanged .
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the DSB envelope and phase are
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The envelope here takes the shape of |x(t)|, rather than x(t), and the modulatedwave undergoes a phase reversal whenever x(t) crosses zero.
Full recovery of the message requires knowledge of these phase reversals, and could not be accomplished by an envelope detector.
Carrier suppression does put all of the average transmitted power into theinformation-bearing sidebands.
Practical transmitters also impose a limit on the peak envelope power
We’ll take account of this peak-power limitation by examining the ratio
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DSB conserves power but requires complicated demodulation circuitry,
whereas AM requires increased power to permit simple envelope detection.
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EXAMPLE Consider a radio transmitter rated for
Let the modulating signal be a tone with
If the modulation is DSB,
the maximum possible power per sideband equals the lesser of the two values determined from
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If the modulation is AM with μ = 1, then
To check on the average-power limitation,
Hence, the peak power limit again dominates and the maximum sideband power is
Since transmission range is proportional to Psb , the AM path length would be only 25 percent of the DSB path length with the same transmitter.
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Tone Modulation and Phasor Analysis
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Setting
the tone-modulated DSB waveform
tone-modulated AM wave
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tone-modulated AM with
EXAMPLE: AM and Phasor Analysis
the phasor sum equals the envelope
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Suppose a transmission channel completely removes the lower sideband,
Now the envelope becomes
from which the envelope distortion can be determined.
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SUPPRESSED-SIDEBAND AMPLITUDEMODULATION
• SSB Signals and Spectra• SSB Generation• VSB Signals and Spectra
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The in-phase and quadrature functions must be lowpass signals
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the lowpass equivalent spectrum
lowpass equivalent signal
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Rectangular-to-Polar conversion yields
The lowpass-to-bandpass transformation in the time domain.
The corresponding frequency-domain transformation is
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Since we’ll deal only with real bandpass signals, we can keep the hermitiansymmetry, in mind and use the simpler expression
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It’s usually easier to work with the lowpass equivalent spectra related by
which is the lowpass equivalent transfer function.
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In particular, after finding , you can take its inverse Fourier transform
The lowpass-to-bandpass transformation then yields the output signal
Or you can get the output quadrature components or envelope and phase immediately from
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SSB Signals and Spectra
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The resulting signal in either case has
Removing one sideband line leaves only the other line. Hence,
Note that the frequency of a tone-modulated SSB wave is offset from fc by ±fmand the envelope is a constant proportional to Am.
Obviously, envelope detection won’t work for SSB.
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To analyze SSB with an arbitrary message x(t),
we’ll draw upon the fact that the sideband filter is a bandpass system with a bandpass DSB input
and a bandpass SSB output
applying the equivalent lowpass method.
Since xbp(t) has no quadrature component, the lowpass equivalent input is simply
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The bandpass filter transfer function for USSB along with the equivalent lowpassfunction
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The corresponding transfer functions for LSSB are
Both lowpass transfer functions can be represented by
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yields the lowpass equivalent spectrum for either USSB or LSSB, namely
Now recall that
Finally, we perform the lowpass-to-bandpass transformation
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SSB Generation
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VSB Signals and Spectra
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Review Questions
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Problems to Ponder
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Review Questions
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Problems to Ponder
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MODULATORS AND TRANSMITTERS
• Product Modulators• Square-Law and Balanced Modulators• Switching Modulators
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Product Modulators
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Square-Law and Balanced Modulators
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Switching Modulators
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Review Questions
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Problems to Ponder
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SUPPRESSED-SIDEBAND AMPLITUDEMODULATION
• SSB Signals and Spectra• SSB Generation• VSB Signals and Spectra
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SSB Signals and Spectra
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SSB Generation
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VSB Signals and Spectra
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Review Questions
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Problems to Ponder
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FREQUENCY CONVERSION AND DEMODULATION
• Frequency Conversion• Synchronous Detection• Envelope Detection
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Frequency Conversion
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Synchronous Detection
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Envelope Detection
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Review Questions
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Problems to Ponder
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