Principles of Communications

COMMUNICATIONS

• Refers to the sending reception and processing of information by electrical means

• Transferring information from one point to another

BLOCK DIAGRAM OF COMMUNICATION SYSTEMS

ANALOG SIGNAL

• Continuous in both time and amplitude

• Electrical properties used are voltage, followed closely by frequency, current and charge.

DIGITAL SIGNAL

• Discrete in both time and amplitude

• Sampling: Selecting values of an analog signal at discrete time instants.

MODULATION

• The process of altering a characteristic of the carrier in accordance with the instantaneous value of the intelligence/modulating signal.

• SUPERIMPOSING AN INFORMATION SIGNAL ONTO A CARRIER SIGNAL

Characteristics of a Modulating Signal

• Low frequency (fc > fm)

• AF signal

• Information

• Intelligence

• Signal that causes the variation

Characteristics of a Carrier Signal

• High frequency signal (fc > fm)

• RF signal

• No information

• Unmodulated

Carrier characteristics that can be varied

Linear types of Modulation in Analog (Continuous) Signals

Amplitude modulation

Angular types of Modulation in Analog (Continuous) Signals

Frequency and Phase modulation

Digital types of Modulation in Digital (Discrete) Signals

• Pulse Amplitude

• Pulse Position

• Pulse Width

• Pulse Code

• To reach far distance

• Practical size of antenna

• Frequency selection

• Security

Why do we need to modulate

• Directly proportional

Relationship of Length of antenna to Wavelength

Marconi Antenna

• for AM broadcasting

• Quarter-wavelength

Amplitude Modulation

• Is a modulation technique which changes the amplitude of a relatively high frequency in proportion with a low frequency modulating signal

• Vc = [ Vc + ΔV ]

• ΔV = m(t)

Operation used in modulation

Multiplication

Equation of Amplitude Modulation

MODULATION INDEX

• also called the Modulation Depth

• Describes by how much the modulated variable of the carrier signal varies around its unmodulated level.

• It is defined differently in each modulation scheme.

Modulation Index by several modulating signals

• m=1

• Vm = Vc

PERFECT/IDEAL MODULATION in AM

• m<1

• Vm < Vc

UNDERMODULATION in AM

• m > 1

• Vm > Vc

• Creates crossover distortion

Overmodulation in AM

The frequency spectrum of AM

Frequency of side bands

= 2fm

= USF - LSF

Bandwidth (BW)

Side-band amplitude

Double Side Band Frequency Carrier (DSBFC)

• Conventional AM system

Power of Carrier Signal

P = Vc²/2R

Power of Modulating Signal

P = m²Pc

Power of Side Band

P = m²Pc/4

Power of Total Side Band

P = m²Pc/2

Power of the Amplitude Modulated signal

P = Pc(1+m²/2)

Power in relation to Voltage and Current

Transmit Voltage and Current in AM

A3E

[ITU designations for types of AM ]

• Double-sideband full-carrier - the basic Amplitude Scheme
  DSBFC

International Telecommunication Union

ITU means

R3E

[ITU designations for types of AM ]

• Single-sideband reduced-carrier SSBRC

H3E

[ITU designations for types of AM ]

• Single-sideband full-carrier SSBFC

J3E

[ITU designations for types of AM ]

• Single-sideband suppressed-carrier SSBSC

B8E

[ITU designations for types of AM ]

• Independent-Sideband Emission

C3E

[ITU designations for types of AM ]

• Vestigial Sideband

Lincompex

[ITU designations for types of AM ]

• Linked compressor and expander

Equation of DSBSC

Powers in DSBSC

Equation in SSBFC, H3E

Powers in SSBFC, H3E

Equation in SSBSC, J3E

Power in SSBSC, J3E

Power in SSBRC, R3E

(90% Supression)

Percentage Power Saving

PEAK ENVELOPE POWER

• The PEP input power is simply the input power of the transmitter's final amplifier stage at the instant of the voice envelope peak

AVERAGE POWER

• The power is typically only one-fourth to one-third of the PEP value with typical human speech

Power efficiency

• The percentage of total power that contains useful information

Percentage power saving of a J3E system

Power of carrier, Pc
PT = Pc

Power radiated, PT at m=0 is equal to

AM transmitter Block Diagram

Class A amplifier used

RECEIVER

• The electronic unit that selects the desired AM signal transmitted and extracts the original information signal

• Tunes to accept the desired carrier signal

• Detects the intelligence from the radio frequency

• Sufficient amplification is provided

SENSITIVITY

• A measure of a receiver's ability to receive and amplify weak signal

SELECTIVITY

• A measure of the receiver's ability to select one signal while rejecting all others at nearby frequencies

STABILITY

• It is the ability of the receiver to be fixed or tuned to a desired frequency

TUNED RADIO FREQUENCY RECEIVER (ARDAS) Block Diagram

• Sensitive

• Not good in being selective

Q factor equations

fr = resonant frequency

Resonant frequency formula

more prone to noise

effect

Highly selective

effect

SUPERHETERODYNE RECEIVER (ARMIDAS) Block Diagram

• Sensitive

• Selective

“mixing”

Heterodyning =

High-Side Injection in ARMIDAS

• Tuning the local oscillator frequency ABOVE THE RF SIGNAL by an amount equal to the intermediate frequency

• Recommended to use

Frequency Standard in AM

535kHz-1605kHz

Frequency Standard in FM

88MHz - 108MHz

Standard Channel BW for AM

10kHz

Standard Channel BW for FM

200kHz

IF for AM

455kHz

IF for FM

10.7MHz

Low-Side Injection in ARMIDAS

• Tuning the local oscillator frequency BELOW THE RF SIGNAL by an amount equal to the intermediate frequency

Image Frequency

• Any frequency other than the selected radio frequency carrier that, if allowed to enter a receiver and mix with the local oscillator will produce a cross-product frequency that is equal to the intermediate frequency

Image Frequency Rejection Ration (IFRR)

• Q - quality factor; describes the selectivity of the tuned circuit (usually the RF preselector or tuned circuit) in the receiver

• ρ - tells you how far apart the image frequency and desired signal frequency are in a normalized way.

ANGULAR MODULATION

• defined as the process in which either the frequency or the phase of the carrier signal varies with respect to the message signal.

PHASE MODULATION

• also called Phase Deviation

FREQUENCY MODULATION

• also called Frequency Modulation

Modulating Index in FM

how much the carrier frequency is varying relative to the frequency of the modulating signal.

Deviation Ratio in FM

similar to the modulation index, but it uses the maximum allowed frequency deviation and the maximum allowed modulating frequency of the FM system.

Bessel Function

FM BANDWIDTH

• Theoretically: INFINITE because of the Bessel FUnction

CARSON'S RULE

• States that "the bandwidth of an FM system is double the sum of the maximum frequency deviation (75kHz) and the highest modulating frequency (15kHz)."

Standard maximum frequency deviation

Standard highest modulating frequency

Standard deviation ratio and BW for FM

Guard Bands

The remaining 10kHz on either side of the BW in FM

Wideband

• m ≥ 1

• BW = 2(m+1)fm

Narrowband

• m < 1

• BW = 2fm

Comparison of PM and FM

FM transmitter Block Diagram

FM Receiver Block Diagram

Better noise immunity

Primary Advantage of FM over AM

integrated

Frequency modulated signal is regarded as the phase modulated signal in which the modulating wave is _____ before modulation.

Bessel functions

The collection of sideband pairs generated in Wideband FM follows the coefficients of: