Bandpass Signaling

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Chapter 4

• Bandpass Signaling

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In this chapter, we consider the situations where
the information from a source is transmitted at
its non-natural frequency (i.e., shifted
frequency). This process is called the
modulation.

1. Representation of modulated signals
2. Spectra
3. Distortions (linear and non-linear)
4. Functional blocks in bandpass communication
   systems

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Basic Model for Bandpass Communication
Source
Destination
Source can be analog or digital. The use of
channel is restricted around certain frequency,
fc (gtgt 0). For example, a radio station may be
given this frequency range for commercial
broadcasting. The goal is to recover the original
information, m, exactly or in the minimum, as
closely as possible.
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Definition. A baseband waveform has a spectral
magnitude (and thus its power) concentrated
around f0 and zero elsewhere.Definition. A
bandpass waveform has a spectral magnitude
concentrated around ffc (fc gtgt 0) and zero
elsewhere. (fc carrier frequency)Definit
ion. Modulation translates the baseband waveform
from a source to a bandpass waveform with carrier
frequency, fc. baseband waveform
modulating signal bandpass waveform
modulated signal
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Examples of Frequency Spectrum

• 300 Hz 20K Hz human voice / sound
• 50 kHz navigation (ships, submarines, etc)
• 1 MHz AM radio (20 k Hz channels)
• 10 MHz CB, short wave
• 100 MHz FM radio, TV
• 1 GHz UHF TV, mobile telephony
• 10 GHz amateur satellite
• 100 GHz upper microwave
• 10 T Hz Infrared
• 1015 Hz Visible light
• 1018 Hz X-rays

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Bandpass Signals over Bandpass Channel
Channel
Out of Transmitter
Into Receiver
Can we translate this into a baseband model?
YES!
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Equivalent Baseband Model for Bandpass Signals
Channel
Out of Transmitter
Into Receiver
Equivalent baseband impulse response
We can now decouple the complexity of shifted
frequency.
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Distortionless Bandpass Channel
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Types of (Analog) Filter
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analog x(t)
analog y(t)
Digital Filter
A/D
D/A
manipulate digital data
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Example of Non-Linear Distortion by Output
Saturation
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Harmonic Distortion
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Intermodulation Distortion (IMD)
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IMD Analysis for Filter Output
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Cross Modulation (Distortion)
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Limiter
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Mixer
input1(t)
output(t) input1(t) x input2(t)
input2(t)
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The nonlinear device generates undesired
effects of product term between vin(t) and vLO(t).
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Mixer Implementation through Switching
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Double-Balanced Mixer
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More on Frequency Multiplier
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Detector Circuits
Source
Destination
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Envelop Detector
low pass filter
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Product Detector
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Frequency Modulation Detector
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Slope Detector (FM-to AM Conversion)
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Slope Detector Circuit
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Balanced Discriminator
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Balanced Zero-Crossing Detector
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Different Phase Detector Characteristics
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Linearized PLL Model
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Hold-in Range and Pull-in Range
Hysteresis indicates stored energy (or
inertia) in the PLL.
Hysteresis is useful against noises or unexpected
interruptions in received signals. This
is called the anti ping-pong characteristic.
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Direct Digital Synthesis (DDS)
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Generalized Transmitter (Type 1)
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Generalized Transmitter (Type 2)
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Generalized Receiver
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Example of Image Frequency
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Zero IF Receiver
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Note. If the receivers were made in digital
circuit, the incoming signal must be sampled at
the bandpass frequency. It is not easy to do so.