Chapter 4 Bandpass Signaling

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

Basic Model for Bandpass Communication SourceDestination Source can be analog or digital. The use of channel is restricted around certain frequency, f c (>> 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.

Definition. A baseband waveform has a spectral magnitude (and thus its power) concentrated around f=0 and zero elsewhere. Definition. A bandpass waveform has a spectral magnitude concentrated around f=±f c (f c >> 0) and zero elsewhere. (f c : carrier frequency) Definition. Modulation translates the baseband waveform from a source to a bandpass waveform with carrier frequency, f c. baseband waveform: modulating signal bandpass waveform: modulated signal

Examples of Frequency Spectrum 300 Hz – 20K Hzhuman voice / sound 50 kHz navigation (ships, submarines, etc) 1 MHzAM radio (20 k Hz channels) 10 MHzCB, short wave 100 MHzFM radio, TV 1 GHzUHF TV, mobile telephony 10 GHzamateur satellite 100 GHzupper microwave 10 T HzInfrared Hz Visible light HzX-rays

Bandpass Signals over Bandpass Channel Out of TransmitterInto Receiver Channel Can we translate this into a baseband model? YES!

Equivalent Baseband Model for Bandpass Signals Out of TransmitterInto Receiver Channel Equivalent baseband impulse response We can now decouple the complexity of shifted frequency.

Distortionless Bandpass Channel

Types of (Analog) Filter

A/D Digital Filter D/A analog x(t) analog y(t) manipulate digital data

Example of Non-Linear Distortion by Output Saturation

Harmonic Distortion

Intermodulation Distortion (IMD)

IMD Analysis for Filter Output

Cross Modulation (Distortion)

Limiter

Mixer input1(t) input2(t) output(t) = input1(t) x input2(t)

The nonlinear device generates “undesired” effects of product term between v in (t) and v LO (t).

Mixer Implementation through Switching

Double-Balanced Mixer

More on Frequency Multiplier

Detector Circuits SourceDestinatio n

Envelop Detector low pass filter

Product Detector

Frequency Modulation Detector

Slope Detector (FM-to AM Conversion)

Slope Detector Circuit

Balanced Discriminator

Balanced Zero-Crossing Detector

Different Phase Detector Characteristics

Linearized PLL Model

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.

Direct Digital Synthesis (DDS)

Generalized Transmitter (Type 1)

Generalized Transmitter (Type 2)

Generalized Receiver

Example of Image Frequency

Zero IF Receiver

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.