ECE 441 Digital Modulation Flashcards

Flashcards for ECE 441 Communication Principles Part B - Digital Modulation

Modulation

The process of converting digital information to an analog waveform for transmission.

Demodulation/Detection

The process of converting analog signals back to a digital format at the receiving end.

Modulation

Varying one or more properties of a periodic waveform (carrier signal) with a modulating signal.

Modulator

A device that performs modulation

Amplitude Shift Keying (ASK)

Varying amplitude of carrier signal to create signal elements

Frequency Shift Keying (FSK)

Varying the frequency proportional to the information signal.

Phase Shift Keying (PSK)

Varying the phase of the carrier proportional to the information signal

Quadrature Amplitude Modulation (QAM)

Varying both amplitude and phase proportional to the information signal.

Digital Modulation

Translates digital data (0 and 1) into an analog signal (baseband signal)

Modem (Modulator-Demodulator)

Converts digital data to analog signal and vice versa.

Antenna Size and Frequency

High-frequency transmission allows for smaller antenna size because antenna size is roughly one-quarter the wavelength of the propagating wave.

Frequency Division Multiplexing

Sharing a communication medium among many concurrently active users.

Transmission Medium Characteristics and Modulation

Path-loss, penetration of obstacles, reflection, scattering, and diffraction depend heavily on the wavelength of the signal.

Advantages of Digital Modulation over Analog Modulation

Greater noise immunity, robustness to channel impairments, easier multiplexing, enhanced security, error control, equalization, and complex signal processing.

Information Capacity

Information capacity is proportional to the product of the bandwidth and the transmission time.

I = B log2(1 + S/N)

Shannon Limit Formula

M-ary

A digit that corresponds to the number of conditions, levels, or combinations possible for a given number of binary variables.

N

log2 M

Baud

The rate of change of a signal on the transmission medium after encoding and modulation have occurred.

Baud Equation

1/ts

Ideal Nyquist Bandwidth

fb = 2B

Nyquist Formulation for Channel Capacity

B log2 M

Amplitude Shift Keying (ASK)

The simplest digital modulation technique where a binary information signal directly modulates the amplitude of an analog carrier.

ASK Modulator

The output will be the carrier signal at high input. When there is low input, the switch opens, allowing no voltage to appear. Hence, the output will be low.

Asynchronous ASK Demodulator Components

A half-wave rectifier, a low pass filter, and a comparator.

Synchronous ASK Demodulator Components

A square law detector, low pass filter, a comparator, and a voltage limiter.

ASK Equation

(1 + vm(t)) * (A/2) * cos(ωct)

Frequency Shift Keying (FSK)

A form of constant-amplitude angle modulation where the modulating signal is a binary signal that varies between two discrete voltage levels.

FSK Modulator

A logic data bit shifts the VCO output to the mark or space frequency.

Frequency Deviation Equation

∆f = vm(t) * kl

Asynchronous FSK Detector

Two band pass filters, two envelope detectors, and a decision circuit.

Synchronous FSK Detector

Two mixers with local oscillator circuits, two band pass filters and a decision circuit

PLL-FSK Demodulator

Employs a phase-locked loop to demodulate signals

Continuous-Phase Frequency-Shift Keying (CP-FSK)

Mark and space frequencies are selected such that they are separated from the center frequency by an exact multiple of one-half the bit rate.

FSK General Expression

Vc * cos[2π(fc + vm(t)∆f)t]

Frequency Deviation (∆f) Equation

|fm - fs| / 2

Minimum Bandwidth for FSK

2∆f + fb

Phase Shift Keying (PSK)

The phase of the carrier signal is changed by varying the sine and cosine inputs at a particular time.

Binary Phase Shift Keying (BPSK)

Sine wave carrier takes two phase reversals such as 0° and 180°.

BPSK Modulator

A balance modulator which has the carrier sine wave as one input and the binary sequence as the other input.

BPSK Output

sin(2πfat) * sin(2πfct)

BPSK Demodulator

Contains a mixer with local oscillator circuit, a bandpass filter, and a two-input detector circuit.

Quadrature (or Quaternary) Phase Shift Keying (QPSK)

The sine wave carrier takes four phase reversals such as 0°, 90°, 180°, and 270°.

QPSK Modulator

Uses a bit-splitter, two multipliers with local oscillator, a 2-bit serial to parallel converter, and a summer circuit.

QPSK Modulator

Divides digital bits into bit pairs.

QPSK Demodulator

Two product demodulator circuits with local oscillator, two band pass filters, two integrator circuits, and a 2-bit parallel to serial converter

Bandwidth Efficiency Equation

transmission bit rate (in bps) / minimum bandwidth (in Hz)

Bandwidth Efficiency

A measure used to compare the performance of one digital modulation technique to another.