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ANALOGUE TELECOMMUNICATIONS. MAIN TOPICS (Part I). Introduction to Communication Systems Filter Circuits Signal Generation Amplitude Modulation AM Receivers AM Transmitters. MAIN TOPICS (Part II). Single-Sideband Communications Systems Angle Modulation Transmission

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main topics part i
MAIN TOPICS (Part I)
  • Introduction to Communication Systems
  • Filter Circuits
  • Signal Generation
  • Amplitude Modulation
  • AM Receivers
  • AM Transmitters
main topics part ii
MAIN TOPICS (Part II)
  • Single-Sideband Communications Systems
  • Angle Modulation Transmission
  • Angle Modulated Receivers & Systems
  • Introduction To Transmission Lines & Antennas
  • Mobile Telecommunications
elements of a communication system
Elements of a Communication System
  • Communication involves the transfer of information or intelligence from a source to a recipient via a channel or medium.
  • Basic block diagram of a communication system:

Channel

Source

Transmitter

Receiver

Recipient

brief description
Brief Description
  • Source: analogue or digital
  • Transmitter: transducer, amplifier, modulator, oscillator, power amp., antenna
  • Channel: e.g. cable, optical fibre, free space
  • Receiver: antenna, amplifier, demodulator, oscillator, power amplifier, transducer
  • Recipient: e.g. person, speaker, computer
modulation
Modulation
  • Modulation is the process of impressing information onto a high-frequency carrier for transmission.
  • Reasons for modulation:
    • to prevent mutual interference between stations
    • to reduce the size of the antenna required
  • Types of analogue modulation: AM, FM, and PM
  • Types of digital modulation: ASK, FSK, PSK, and QAM
frequency bands
BANDHz

ELF 30 - 300

AF 300 - 3 k

VLF 3 k - 30 k

LF 30 k - 300 k

MF 300 k - 3 M

HF 3 M - 30 M

BANDHz

VHF 30M-300M

UHF 300M - 3 G

SHF 3 G - 30 G

EHF 30 G - 300G

Frequency Bands
  • Wavelength, l = c/f
information and bandwidth
Information and Bandwidth
  • Bandwidth required by a modulated signal depends on the baseband frequency range (or data rate) and the modulation scheme.
  • Hartley’s Law: I = k t B

where I = amount of information; k = system constant; t = time available; B = channel bandwidth

  • Shannon’s Formula: I = B log2 (1+ S/N) in bps

where S/N = signal-to-noise power ratio

transmission modes
Transmission Modes
  • Simplex (SX) – one direction only, e.g. TV
  • Half Duplex (HDX) – both directions but not at the same time, e.g. CB radio
  • Full Duplex (FDX) – transmit and receive simultaneously between two stations, e.g. standard telephone system
  • Full/Full Duplex (F/FDX) - transmit and receive simultaneously but not necessarily just between two stations, e.g. data communications circuits
time and frequency domains
Time and Frequency Domains
  • Time domain: an oscilloscope displays the amplitude versus time
  • Frequency domain: a spectrum analyzer displays the amplitude or power versus frequency
  • Frequency-domain display provides information on bandwidth and harmonic components of a signal
non sinusoidal waveform
Non-sinusoidal Waveform
  • Any well-behaved periodic waveform can be represented as a series of sine and/or cosine waves plus (sometimes) a dc offset:

e(t)=Co+SAn cosnw t + SBn sin nw t (Fourier series)

effect of filtering
Effect of Filtering
  • Theoretically, a non-sinusoidal signal would require an infinite bandwidth; but practical considerations would band-limit the signal.
  • Channels with too narrow a bandwidth would remove a significant number of frequency components, thus causing distortions in the time-domain.
  • A square-wave has only odd harmonics
mixers
Mixers
  • A mixer is a nonlinear circuit that combines two signals in such a way as to produce the sum and difference of the two input frequencies at the output.
  • A square-law mixer is the simplest type of mixer and is easily approximated by using a diode, or a transistor (bipolar, JFET, or MOSFET).
dual gate mosfet mixer
Dual-Gate MOSFET Mixer

Good dynamic range and fewer unwanted o/p frequencies.

balanced mixers
Balanced Mixers
  • A balanced mixer is one in which the input frequencies do not appear at the output. Ideally, the only frequencies that are produced are the sum and difference of the input frequencies.

Circuit symbol:

f1

f1+ f2

f2

equations for balanced mixer
Equations for Balanced Mixer

Let the inputs be v1 = sin w1t and v2 = sin w2t.

A balanced mixer acts like a multiplier. Thus

its output, vo = Av1v2 = A sin w1t sin w2t.

Since sin X sin Y = 1/2[cos(X-Y) - cos(X+Y)]

Therefore, vo = A/2[cos(w1-w2)t-cos(w1+w2)t].

  • The last equation shows that the output of the balanced mixer consists of the sum and difference of the input frequencies.
balanced ring diode mixer
Balanced Ring Diode Mixer

Balanced mixers are also called balanced modulators.

external noise
External Noise
  • Equipment / Man-made Noise is generated by any equipment that operates with electricity
  • Atmospheric Noise is often caused by lightning
  • Space or Extraterrestrial Noise is strongest from the sun and, at a much lesser degree, from other stars
internal noise
Internal Noise
  • Thermal Noise is produced by the random motion of electrons in a conductor due to heat. Noise power, PN = kTB

where T = absolute temperature in oK

k = Boltzmann’s constant, 1.38x10-23 J/oK

B = noise power bandwidth in Hz

Noise voltage,

internal noise cont d
Internal Noise (cont’d)
  • Shot Noise is due to random variations in current flow in active devices.
  • Partition Noise occurs only in devices where a single current separates into two or more paths, e.g. bipolar transistor.
  • Excess Noise is believed to be caused by variations in carrier density in components.
  • Transit-Time Noise occurs only at high f.
noise spectrum of electronic devices
Noise Spectrum of Electronic Devices

Device

Noise

Transit-Time or

High-Frequency

Effect Noise

Excess or

Flicker Noise

Shot and Thermal Noises

f

1 kHz

fhc

signal to noise ratio
Signal-to-Noise Ratio
  • An important measure in communications is the signal-to-noise ratio (SNR or S/N). It is often expressed in dB:

In FM receivers, SINAD = (S+N+D)/(N+D)

is usually used instead of SNR.

noise figure
Noise Figure
  • Noise Factor is a figure of merit that indicates how much a component, or a stage degrades the SNR of a system:

F = (S/N)i / (S/N)o

where (S/N)i = input SNR (not in dB)

and (S/N)o = output SNR (not in dB)

  • Noise Figure is the Noise Factor in dB:

NF(dB)=10 log F = (S/N)i (dB) - (S/N)o (dB)

equivalent noise temperature and cascaded stages
Equivalent Noise Temperature and Cascaded Stages
  • The equivalent noise temperature is very useful in microwave and satellite receivers.

Teq = (F - 1)To

where To is a ref. temperature (often 290 oK)

  • When two or more stages are cascaded, the total noise factor is:
high frequency effects
High-Frequency Effects
  • Stray reactances of components(including the traces on a circuit board) can result in parasitic oscillations / self resonance and other unexpected effects in RF circuits.
  • Care must be given to the layout of components, wiring, ground plane, shielding and the use of bypassing or decoupling circuits.
narrow band rf amplifiers
Narrow-band RF Amplifiers
  • Many RF amplifiers use resonant circuits to limit their bandwidth. This is to filter off noise and interference and to increase the amplifier’s gain.
  • The resonant frequency (fo), bandwidth (B), and quality factor (Q), of a parallel resonant circuit are:
narrowband amplifier cont d
Narrowband Amplifier (cont’d)
  • In the CE amplifier, both the input and output sections are transformer-coupled to reduce the Miller effect. They are tapped for impedance matching purpose. RC and C2 decouple the RF from the dc supply.
  • The CB amplifier is quite commonly used at RF because it provides high voltage gain and also avoids the Miller effect by turning the collector-to-base junction capacitance into a part of the output tuning capacitance.
wideband rf amplifiers
Wideband RF Amplifiers
  • Wideband / broadband amplifiers are frequently used for amplifying baseband or intermediate frequency (IF) signals.
  • The circuits are similar to those for narrowband amplifiers except no tuning circuits are employed.
  • Another method of designing wideband amplifiers is by stagger-tuning.
amplifier classes
Amplifier Classes

An amplifier is classified as:

  • Class A if it conducts current throughout the full input cycle (i.e. 360o). It operates linearly but is very inefficient - about 25%.
  • Class B if it conducts for half the input cycle. It is quite efficient (about 60%) but would create high distortions unless operated in a push-pull configuration.
class c amplifier
Class C Amplifier
  • Class C amplifier operates for less than half of the input cycle. It’s efficiency is about 75% because the active device is biased beyond cutoff.
  • It is commonly used in RF circuits where a resonant circuit must be placed at the output in order to keep the sine wave going during the non-conducting portion of the input cycle.
frequency multipliers

Input

fi

Frequency Multipliers
  • One of the applications of class C amplifiers is in “frequency multiplication”. The basic block diagram of a frequency multiplier:

High Distortion

Device +

Amplifier

Tuning

Filter

Circuit

Output

N x fi

principle of frequency multipliers
Principle of Frequency Multipliers
  • A class C amplifier is used as the high distortion device. Its output is very rich in harmonics.
  • A filter circuit at the output of the class C amplifier is tuned to the second or higher harmonic of the fundamental component.
  • Tuning to the 2nd harmonic doubles fi ; tuning to the 3rd harmonic triples fi ; etc.
neutralization
Neutralization
  • At very high frequencies, the junction capacitance of a transistor could introduce sufficient feedback from output to input to cause unwanted oscillations to take place in an amplifier.
  • Neutralization is used to cancel the oscillations by feeding back a portion of the output that has the opposite phase but same amplitude as the unwanted feedback.
review of filter types responses
Review of Filter Types & Responses
  • 4 major types of filters: low-pass, high-pass, band pass, and band-reject or band-stop
  • 0 dB attenuation in the passband (usually)
  • 3 dB attenuation at the critical or cutoff frequency, fc (for Butterworth filter)
  • Roll-off at 20 dB/dec (or 6 dB/oct) per pole outside the passband (# of poles = # of reactive elements). Attenuation at any frequency, f, is:
review of filters cont d
Review of Filters (cont’d)
  • Bandwidth of a filter: BW = fcu - fcl
  • Phase shift: 45o/pole at fc; 90o/pole at >> fc
  • 4 types of filter responses are commonly used:
    • Butterworth - maximally flat in passband; highly non-linear phase response with frequecny
    • Bessel - gentle roll-off; linear phase shift with freq.
    • Chebyshev - steep initial roll-off with ripples in passband
    • Cauer (or elliptic) - steepest roll-off of the four types but has ripples in the passband and in the stopband
low pass filter response
Low-Pass Filter Response

Gain (dB)

BW = fc

0

Vo

Ideal

-20

1

-20 dB/dec

-40

-60 dB/dec

0.707

-40 dB/dec

Passband

-60

BW

0

f

f

fc

fc

10fc

100fc

1000fc

Basic LPF response

LPF with different roll-off rates

high pass filter response
High-Pass Filter Response

Gain (dB)

0

Vo

-20

1

-20 dB/dec

-40

0.707

-40 dB/dec

-60 dB/dec

Passband

-60

0

fc

f

0.01fc

0.1fc

fc

f

Basic HPF response

HPF with different roll-off rates

band pass filter response
Band-Pass Filter Response

Centre frequency:

Vout

1

Quality factor:

0.707

Q is an indication of the

selectivity of a BPF.

Narrow BPF: Q > 10.

Wide-band BPF: Q < 10.

BW

f

fc1

fo

fc2

Damping Factor:

BW = fc2 - fc1

band stop filter response
Band-Stop Filter Response
  • Also known as band-reject, or notch filter.
  • Frequencies within a certain BW are rejected.
  • Useful for filtering interfering signals.

Gain (dB)

0

-3

Pass

band

Passband

f

fc1

fo

fc2

BW

filter response characteristics
Filter Response Characteristics

Av

Chebyshev

Bessel

Butterworth

f

damping factor
Damping Factor

The damping factor (DF)

of an active filter sets

the response characteristic

of the filter.

Frequency

selective

RC circuit

Vin

Vout

+

_

R1

R2

Its value depends on the

order (# of poles) of the

filter. (See Table on next

slide for DF values.)

General diagram of active filter

active filters
Active Filters
  • Advantages over passive LC filters:
    • Op-amp provides gain
    • high Zin and low Zout mean good isolation from source or load effects
    • less bulky and less expensive than inductors when dealing with low frequency
    • easy to adjust over a wide frequency range without altering desired response
  • Disadvantage: requires dc power supply, and could be limited by frequency response of op-amp.
single pole active lpf
Single-pole Active LPF

R

Vin

Vout

+

_

C

R1

R2

Roll-off rate for a single-pole

filter is -20 dB/decade.

Acl is selectable since DF is

optional for single-pole LPF

sallen key low pass filter
Sallen-Key Low-Pass Filter

CA

Selecting RA = RB = R,

and CA = CB = C :

RA

RB

Vin

Vout

+

_

CB

The roll-off rate for a

two-pole filter is

-40 dB/decade.

R1

Sallen-Key or VCVS

(voltage-controlled

voltage-source) second-

order low-pass filter

R2

For a Butterworth 2nd-

order response, DF = 1.414;

therefore, R1/R2 = 0.586.

cascaded low pass filter
Cascaded Low-Pass Filter

CA1

Roll-off rate: -60 dB/dec

RA1

RB1

RA2

Vin

+

_

+

Vout

CB1

CA2

_

R1

R3

R2

R4

2 poles

1 pole

Third-order (3-pole) configuration

single pole high pass filter
Single-Pole High-Pass Filter
  • Roll-off rate, and formulas for fc , and Acl are similar to those for LPF.
  • Ideally, a HPF passes all frequencies above fc. However, the op-amp has an upper-frequency limit.

C

Vin

Vout

+

_

R

R1

R2

sallen key high pass filter
Sallen-Key High-Pass Filter

RA

Again, formulas and

roll-off rate are similar

to those for 2nd-order

LPF.

CA

CB

Vin

Vout

+

_

RB

To obtain higher roll-

off rates, HPF filters

can be cascaded.

R1

R2

Basic Sallen-Key

second-order HPF

bpf using hpf and lpf
BPF Using HPF and LPF

CA1

RA2

Vin

+

Vout

_

+

RA1

_

CA2

R1

R3

Av (dB)

R2

R4

0

-3

-20 dB/dec

HP response

-20 dB/dec

LP response

f

fc1

fo

fc2

more on bandpass filter
More On Bandpass Filter

If BW and fo are given, then:

A 2nd order BPF obtained by combining a LPF and a HPF:

BiFET op-amp

has FETs at

input stage and

BJTs at output

stage.

notes on cascading hpf lpf
Notes On Cascading HPF & LPF
  • Cascading a HPF and a LPF to yield a band-pass filter can be done as long as fc1 and fc2 are sufficiently separated. Hence the resulting bandwidth is relatively wide.
  • Note that fc1 is the critical frequency for the HPF and fc2 is for the LPF.
  • Another BPF configuration is the multiple-feedback BPF which has a narrower bandwidth and needing fewer components
multiple feedback bpf
Multiple-Feedback BPF

C1

Making C1 = C2 = C,

R2

C2

R1

_

Vin

Vout

Q = fo/BW

+

R3

Max. gain:

R1, C1 - LP section

R2, C2 - HP section

Ao < 2Q2

broadband band reject filter
Broadband Band-Reject Filter

A LPF and a HPF can also be combined to give a broadband

BRF:

2-pole band-reject filter

narrow band band reject filter
Narrow-band Band-Reject Filter

Easily obtained by combining the inverting output of a

narrow-band BPF and the original signal:

The equations for R1, R2, R3, C1, and C2 are the same as for BPF.

RI = RF for unity gain and is often chosen to be >> R1.

multiple feedback band stop filter
Multiple-Feedback Band-Stop Filter

C1

The multiple-feedback

BSF is very similar to

its BP counterpart. For

frequencies between fc1

and fc2 the op-amp will

treat Vin as a pair of

common-mode signals

thus rejecting them

accordingly.

R2

C2

R1

_

Vin

Vout

+

R3

R4

When

C1 = C2 =C

filter response measurements

Spectrum

analyzer

Sweep

Generator

Filter

Filter Response Measurements
  • Discrete Point Measurement: Feed a sine wave to the filter input with a varying frequency but a constant voltage and measure the output voltage at each frequency point.
  • A faster way is to use the swept frequency method:

The sweep generator outputs a sine wave whose frequency

increases linearly between two preset limits.

signal generation oscillators
Barkhausen criteria for sustained oscillations:

The closed-loop gain, |BAV| = 1.

The loop phase shift = 0o or some integer multiple of 360o at the operating frequency.

AV = open-loop gain

B = feedback factor/fraction

Signal Generation - Oscillators

AV

Output

B

basic wien bridge oscillator
Basic Wien-Bridge Oscillator

R4

R1

R1

Voltage

Divider

_

C1

_

R2

+

Vout

C1

R4

Vout

+

R2

R3

Lead-lag

circuit

C2

R3

C2

Two forms of the same circuit

notes on wien bridge oscillator
Notes on Wien-Bridge Oscillator
  • At the resonant frequency the lead-lag circuit provides a positive feedback (purely resistive) with an attenuation of 1/3 when R3=R4=XC1=XC2.
  • In order to oscillate, the non-inverting amplifier must have a closed-loop gain of 3, which can be achieved by making R1 = 2R2
  • When R3 = R4 = R, and C1 = C2 = C, the resonant frequency is:
phase shift oscillator
Phase-Shift Oscillator

Rf

_

C1

C2

C3

Vout

+

Choosing

R1 = R2 = R3 = R,

C1 = C2 = C3 = C,

the resonant

frequency is:

R1

R2

R3

Each RC section provides 60o of

phase shift. Total attenuation of

the three-section RC feedback,

B = 1/29.

clapp oscillator
Clapp Oscillator

The Clapp oscillator is a variation of the Colpitts circuit. C4 is added in series with L in the tank circuit. C2 and C3 are chosen large enough to “swamp” out the transistor’s junction capacitances for greater stability. C4 is often chosen to be << either C2 or C3, thus making C4 the frequency determining element, since CT = C4.

voltage controlled oscillator
Voltage-Controlled Oscillator
  • VCOs are widely used in electronic circuits for AFC, PLL, frequency tuning, etc.
  • The basic principle is to vary the capacitance of a varactor diode in a resonant circuit by applying a reverse-biased voltage across the diode whose capacitance is approximately:
crystals
Crystals
  • For high frequency stability in oscillators, a crystal (such as quartz) has to be used.
  • Quartz is a piezoelectric material: deforming it mechanically causes the crystal to generate a voltage, and applying a voltage to the crystal causes it to deform.
  • Externally, the crystal behaves like an electrical resonant circuit.
ic waveform generation
IC Waveform Generation
  • There are a number of LIC waveform generators from EXAR:
    • XR2206 monolithic function generator IC
    • XR2207 monolithic VCO IC
    • XR2209 monolithic VCO IC
    • XR8038A precision waveform generator IC
  • Most of these ICs have sine, square, or triangle wave output. They can also provide AM, FM, or FSK waveforms.
phase locked loop
Phase-Locked Loop
  • The PLL is the basis of practically all modern frequency synthesizer design.
  • The block diagram of a simple PLL:

Vp

fr

fo

Phase

Detector

Loop

Amplifier

LPF

VCO

  • Examples of a PLL I.C.: XR215, LM565, and CD4046
operation of pll
Operation of PLL
  • Initially, the PLL is unlocked, i.e.,the VCO is at the free-running frequency, fo.
  • Since fo is probably not the same as the reference frequency, fr , the phase detector will generate an error/control voltage, Vp.
  • Vp is filtered, amplified, and applied to the VCO to change its frequency so that fo = fr. The PLL will then remain in phase lock.
pll frequency specifications
PLL Frequency Specifications

There is a limit on how far apart the free-running

VCO frequency and the reference frequency can be

for lock to be acquired or maintained.

Lock Range

Capture Range

Free-Running

Frequency

f

fLL

fLC

fo

fHC

fHL

basic pll frequency synthesizer
Basic PLL Frequency Synthesizer

fr

Phase

comparator

LPF

VCO

fout = Nfr

N

fc = fout/N

For output frequencies in the VHF range and higher,

a prescaler is required. The prescaler is a fixed divider

placed ahead of the programmable divide by N counter.

frequency synthesizer using prescaling
Frequency Synthesizer Using Prescaling

fr

Phase

comparator

fout

=(NP+M)fr

LPF

VCO

Prescaler

P or (P+1)

N

M

2-modulus prescaler divides by P+1 when M counter is non zero;

it divides by P when M counter reaches zero. N counter counts

down (N-M) times. E.g. of I.C. prescaler: LMX5080 for UHF

operation.

am waveform
AM Waveform

AM signal:

es = (Ec + em) sin wct

ec = Ec sin wct

em = Em sin wmt

modulation index
Modulation Index
  • The amount of amplitude modulation in a signal is given by its modulation index:

where, Emax = Ec + Em; Emin = Ec - Em (all pk values)

When Em = Ec , m =1 or 100% modulation.

Over-modulation, i.e. Em>Ec , should be avoided

because it will create distortions and splatter.

effects of modulation index
Effects of Modulation Index

m = 1

m > 1

In a practical AM system, it usually contains many

frequency components. When this is the case,

am in frequency domain
AM in Frequency Domain
  • The expression for the AM signal:

es = (Ec + em) sin wct

can be expanded to:

es = Ec sin wct + ½ mEc[cos (wc-wm)t-cos (wc+wm)t]

  • The expanded expression shows that the AM signal consists of the original carrier, a lower side frequency, flsf = fc - fm, and an upper side frequency, fusf = fc + fm.
am spectrum
AM Spectrum

Ec

mEc/2

mEc/2

fm

fm

f

fusf

flsf

fc

fusf = fc + fm ; flsf = fc - fm ; Esf = mEc/2

Bandwidth, B = 2fm

am power
AM Power
  • Total average (i.e. rms) power of the AM signal is: PT = Pc + 2Psf , where

Pc = carrier power; and Psf = side-frequency power

  • If the signal is across a load resistor, R, then: Pc = Ec2/(2R); and Psf = m2Pc/4. So,
am current
AM Current
  • The modulation index for an AM station can be measured by using an RF ammeter and the following equation:

where I is the current with modulation and

Io is the current without modulation.

complex am waveforms
Complex AM Waveforms
  • For complex AM signals with many frequency components, all the formulas encountered before remain the same, except that m is replaced by mT. For example:
transmitter stages
Transmitter Stages
  • Crystal oscillator generates a very stable sinewave carrier. Where variable frequency operation is required, a frequency synthesizer is used.
  • Buffer isolates the crystal oscillator from any load changes in the modulator stage.
  • Frequency multiplier is required only if HF or higher frequencies is required.
transmitter stages cont d
Transmitter Stages (cont’d)
  • RF voltage amplifier boosts the voltage level of the carrier. It could double as a modulator if low-level modulation is used.
  • RF driver supplies input power to later RF stages.
  • RF Power amplifier is where modulation is applied for most high power AM TX. This is known as high-level modulation.
transmitter stages cont d93
Transmitter Stages (cont’d)
  • High-level modulation is efficient since all previous RF stages can be operated class C.
  • Microphone is where the modulating signal is being applied.
  • AF amplifier boosts the weak input modulating signal.
  • AF driver and power amplifier would not be required for low-level modulation.
impedance matching networks
Impedance Matching Networks
  • Impedance matching networks at the output of RF circuits are necessary for efficient transfer of power. At the same time, they serve as low-pass filters.

Pi network

T network

trapezoidal pattern
Trapezoidal Pattern
  • Instead of using the envelope display to look at AM signals, an alternative is to use the trapezoidal pattern display. This is obtained by connecting the modulating signal to the x input of the ‘scope and the modulated AM signal to the y input.
  • Any distortion, overmodulation, or non-linearity is easier to observe with this method.
trapezoidal pattern cont d
Trapezoidal Pattern (cont’d)

m<1

m=1

m>1

Improper

phase

-Vp>+Vp

am receivers
AM Receivers
  • Basic requirements for receivers:
  • ability to tune to a specific signal
  • amplify the signal that is picked up
  • extract the information by demodulation
  • amplify the demodulated signal
  • Two important receiver specifications:

sensitivity and selectivity

tuned radio frequency trf receiver
Tuned-Radio-Frequency (TRF) Receiver
  • The TRF receiver is the simplest receiver that meets all the basic requirements.
drawbacks of trf receivers
Drawbacks of TRF Receivers
  • Difficulty in tuning all the stages to exactly the same frequency simultaneously.
  • Very high Q for the tuning coils are required for good selectivity  BW=fo/Q.
  • Selectivity is not constant for a wide range of frequencies due to skin effect which causes the BW to vary with fo.
superheterodyne receiver
Superheterodyne Receiver

Block diagram of basic superhet receiver:

antenna and front end
Antenna and Front End
  • The antenna consists of an inductor in the form of a large number of turns of wire around a ferrite rod. The inductance forms part of the input tuning circuit.
  • Low-cost receivers sometimes omit the RF amplifier.
  • Main advantages of having RF amplifier: improves sensitivity and image frequency rejection.
mixer and local oscillator
Mixer and Local Oscillator
  • The mixer and LO frequency convert the input frequency, fc, to a fixed fIF:

High-side injection: fLO = fc + fIF

autodyne converter
Autodyne Converter
  • Sometimes called a self-excited mixer, the autodyne converter combines the mixer and LO into a single circuit:
if amplifier and agc
IF Amplifier and AGC
  • Most receivers have two or more IF stages to provide the bulk of their gain (i.e. sensitivity) and their selectivity.
  • Automatic gain control (AGC) is obtained from the detector stage to adjusts the gain of the IF (and sometimes the RF) stages inversely to the input signal level. This enables the receiver to cope with large variations in input signal.
diagonal clipping distortion
Diagonal Clipping Distortion

Diagonal clipping distortion is more pronounced at

high modulation index or high modulation frequency.

sensitivity and selectivity
Sensitivity and Selectivity
  • Sensitivity is expressed as the minimum input signal required to produce a specified output level for a given (S+N)/N ratio.
  • Selectivity is the ability of the receiver to reject unwanted or interfering signals. It may be defined by the shape factor of the IF filter or by the amount of adjacent channel rejection.
image frequency
Image Frequency
  • One of the problems with the superhet receiver is that an image frequency signal could interfere with the reception of the desired signal. The image frequency is given by: fimage = fsig + 2fIF

where fsig = desired signal.

  • An image signal must be rejected by tuning circuits prior to mixing.
image frequency rejection ratio
Image-Frequency Rejection Ratio
  • For a tuned circuit with a quality factor of Q, its image-frequency rejection ratio is:

In dB, IFRR(dB) = 20 log IFRR

if transformers
IF Transformers
  • The transformers used in the IF stages can be either single-tuned or double-tuned.

Double-tuned

Single-tuned

loose and tight couplings
Loose and Tight Couplings
  • For single-tuned transformers, tighter coupling means more gain but broader bandwidth:
under over critical coupling
Under, Over, & Critical Coupling
  • Double-tuned transformers can be over, under, critically, or optimally coupled:
coupling factors
Coupling Factors
  • Critical coupling factor kc is given by:

where Qp, Qs = prim. & sec. Q, respectively.

  • IF transformers often use the optimum coupling
  • factor, kopt = 1.5kc, to obtain a steep skirt and
  • flat passband. The bandwidth for a double-tuned
  • IF amplifier with k = kopt is given by B = kfo.
  • Overcoupling means k>kc; undercoupling, k< kc
piezoelectric filters
Piezoelectric Filters
  • For narrow bandwidth (e.g. several kHz), excellent shape factor and stability, a crystal lattice is used as bandpass filter.
  • Ceramic filters, because of their lower Q, are useful for wideband signals (e.g. FM broadcast).
  • Surface-acoustic-wave (SAW) filters are ideal for high frequency usage requiring a carefully shaped response.
suppressed carrier am systems
Suppressed-Carrier AM Systems
  • Full-carrier AM is simple but not efficient in terms of transmitted power, bandwidth, and SNR.
  • Using single-sideband suppressed-carrier (SSBSC or SSB) signals, since Psf = m2Pc/4, and Pt=Pc(1+m2/2), then at m=1, Pt= 6 Psf .
  • SSB also has a bandwidth reduction of half, which in turn reduces noise by half.
generating ssb filtering method
Generating SSB - Filtering Method
  • The simplest method of generating an SSB signal is to generate a double-sideband suppressed-carrier (DSB-SC) signal first and then removing one of the sidebands.

Balanced

Modulator

USB

DSB-SC

BPF

or

AF

Input

LSB

Carrier

Oscillator

waveforms for balanced modulator
Waveforms for Balanced Modulator

V2, fm

Vo

V1, fc

f

fc-fm

fc+fm

mathematical analysis of balanced modulator
Mathematical Analysis of Balanced Modulator
  • V1 = A1sin ct; V2 = A2sin mt
  • Vo = V1V2 = A1A2sin ct sin mt

= ½A1A2{cos(c- m)t – cos(c+ m)t}

  • The equation above shows that the output of the balanced modulator consists of a lower side-frequency (c - m) and an upper side-frequency (c+ m)
filter for ssb
Filter for SSB
  • Filters with high Q are needed for suppressing the unwanted sideband.

fa = fc - f2

fb = fc - f1

fd = fc + f1

fe = fc + f2

where X = attenuation of

sideband, and f = fd - fb

generating ssb phasing method
Generating SSB - Phasing Method
  • This method is based on the fact that the lsf and the usf are given by the equations:

cos {(wc - wm)t} = ½(cos wct cos wmt + sin wct sin wmt)

cos {(wc + wm)t} = ½(cos wct cos wmt - sin wct sin wmt)

  • The RHS of the 1st equation is just the sum of two products: the product of the carrier and the modulating signal, and the product of the same two signals that have been phase shifted by 90o.
  • The 2nd equation is similar except for the (-) sign.
diagram for phasing method
Diagram for Phasing Method

Balanced Modulator 1

Modulating

signal

Em cos wmt

Carrier

oscillator

Ec cos wct

SSB

output

+

90o phase

shifter

90o phase

shifter

Balanced Modulator 2

phasing vs filtering method
Phasing vs Filtering Method

Advantages of phasing method :

  • No high Q filters are required.
  • Therefore, lower fm can be used.
  • SSB at any carrier frequency can be generated in a single step.

Disadvantage:

Difficult to achieve accurate 90o phase shift across the whole audio range.

peak envelope power
Peak Envelope Power
  • SSB transmitters are usually rated by the peak envelope power (PEP) rather than the carrier power. With voice modulation, the PEP is about 3 to 4 times the average or rms power.

where Vp = peak signal voltage

and RL = load resistance

coherent ssb bfo receiver
Coherent SSB BFO Receiver

RF SSBRC

IF SSBRC

RF amplifier

and

preselector

IF amp. &

bandpass

filter

IF

mixer

Demod.

info

RF mixer

RF

input

signal

RF LO

Carrier recovery

and frequency

synthesizer

BFO

notes on ssb receivers
Notes On SSB Receivers
  • The input SSB signal is first mixed with the LO signal (low-side injection is used here).
  • The filter removes the sum frequency components and the IF signal is amplified.
  • Mixing the IF signal with a reinserted carrier from a beat frequency oscillator (BFO) and low-pass filtering recovers the audio information.
ssb receivers cont d
SSB Receivers (cont’d)
  • The product detector is often just a balanced modulator operated in reverse.
  • Frequency accuracy and stability of the BFO is critical. An error of a little more than 100 Hz could render the received signal unintelligible.
  • In coherent or synchronous detection, a pilot carrier is transmitted with the SSB signal to synchronize the RF local oscillator and BFO.
angle modulation
Angle Modulation
  • Angle modulation includes both frequencyand phase modulation.
  • FM is used for: radio broadcasting, sound signal in TV, two-way fixed and mobile radio systems, cellular telephone systems, and satellite communications.
  • PM is used extensively in data communications and for indirect FM.
comparison of fm or pm with am
Comparison of FM or PM with AM

Advantages over AM:

  • better SNR, and more resistant to noise
  • efficient - class C amplifier can be used, and less power is required to angle modulate
  • capture effect reduces mutual interference

Disadvantages:

  • much wider bandwidth is required
  • slightly more complex circuitry is needed
frequency shift keying fsk
Frequency Shift Keying (FSK)

Carrier

Modulating

signal

FSK

signal

fsk cont d
FSK (cont’d)
  • The frequency of the FSK signal changes abruptly from one that is higher than that of the carrier to one that is lower.
  • Note that the amplitude of the FSK signal remains constant.
  • FSK can be used for transmission of digital data (1’s and 0’s) with slow speed modems.
frequency modulation
Frequency Modulation

Carrier

Modulating

Signal

FM

signal

frequency modulation cont d
Frequency Modulation (cont’d)
  • Note the continuous change in frequency of the FM wave when the modulating signal is a sine wave.
  • In particular, the frequency of the FM wave is maximum when the modulating signal is at its positive peak and is minimum when the modulating signal is at its negative peak.
frequency deviation
Frequency Deviation
  • The amount by which the frequency of the FM signal varies with respect to its resting value (fc) is known as frequency deviation: Df = kf em, where kf is a system constant, and em is the instantaneous value of the modulating signal amplitude.
  • Thus the frequency of the FM signal is:

fs (t) = fc + Df = fc + kf em(t)

maximum or peak frequency deviation
Maximum or Peak Frequency Deviation
  • If the modulating signal is a sine wave, i.e., em(t) = Emsin wmt, then fs = fc + kfEmsin wmt.
  • The peak or maximum frequency deviation:

d = kf Em

  • The modulation index of an FM signal is:

mf = d / fm

  • Note that mf can be greater than 1.
relationship between fm and pm
Relationship between FM and PM
  • For PM, phase deviation, Df = kpem, and the peak phase deviation, fmax = mp = mf.
  • Since frequency (in rad/s) is given by:

the above equations suggest that FM can be

obtained by first integrating the modulating

signal, then applying it to a phase modulator.

equation for fm signal
Equation for FM Signal
  • If ec = Ec sin wct, and em = Em sin wmt, then the equation for the FM signal is:

es = Ec sin (wct + mf sin wmt)

  • This signal can be expressed as a series of sinusoids: es = Ec{Jo(mf) sin wct

- J1(mf)[sin (wc - wm)t - sin (wc + wm)t]

+ J2(mf)[sin (wc - 2wm)t + sin (wc + 2wm)t]

- J3(mf)[sin (wc - 3wm)t + sin (wc + 3wm)t]

+ … .}

bessel functions
Bessel Functions
  • The J’s in the equation are known as Bessel functions of the first kind:

mf Jo J1 J2 J3 J4 J5 J6 . . .

0 1

0.5 .94 .24 .03

1 .77 .44 .11 .02

2.4 0.0 .52 .43 .20 .06 .02

5.5 0.0 -.34 -.12 .26 .40 .32 .19 . . .

notes on bessel functions
Notes on Bessel Functions
  • Theoretically, there is an infinite number of side frequencies for any mf other than 0.
  • However, only significant amplitudes, i.e. those |0.01| are included in the table.
  • Bessel-zero or carrier-null points occur when mf = 2.4, 5.5, 8.65, etc. These points are useful for determining the deviation and the value of kf of an FM modulator system.
fm side bands
FM Side-Bands
  • Each (J) value in the table gives rise to a pair of side-frequencies.
  • The higher the value of mf, the more pairs of significant side- frequencies will be generated.
power and bandwidth of fm signal
Power and Bandwidth of FM Signal
  • Regardless of mf , the total power of an FM signal remains constant because its amplitude is constant.
  • The required BW of an FM signal is:

BW = 2 x n x fm ,where n is the number of pairs of side-frequencies.

  • If mf > 6, a good estimate of the BW is given by Carson’s rule: BW = 2(d + fm (max) )
narrowband wideband fm
Narrowband & Wideband FM
  • FM systems with a bandwidth < 15 kHz, are considered to be NBFM. A more restricted definition is that their mf < 0.5. These systems are used for voice communication.
  • Other FM systems, such as FM broadcasting and satellite TV, with wider BW and/or higher mf are called WBFM.
pre emphasis
Pre-emphasis
  • Most common analog signals have high frequency components that are relatively low in amplitude than low frequency ones. Ambient electrical noise is uniformly distributed. Therefore, the SNR for high frequency components is lower.
  • To correct the problem, em is pre-emphasized before frequency modulating ec.
pre emphasis circuit
Pre-emphasis circuit
  • In FM broadcasting, the high frequency components are boosted by passing the modulating signal through a HPF with a 75 ms time constant before modulation.
  • t = R1C = 75 ms.
de emphasis circuit
De-emphasis Circuit
  • At the FM receiver, the signal after demodulation must be de-emphasized by a filter with similar characteristics as the pre-emphasis filter to restore the relative amplitudes of the modulating signal.
fm stereo broadcasting baseband spectra
FM Stereo Broadcasting: Baseband Spectra
  • To maintain compatibility with monaural system, FM stereo uses a form of FDM or frequency-division multiplexing to combine the left and right channel information:

19 kHz Pilot

Carrier

SCA

(optional)

L+R

(mono)

L-R

L-R

kHz

.05

15

23

38

53

60

67

74

fm stereo broadcasting
FM Stereo Broadcasting
  • To enable the L and R channels to be reproduced at the receiver, the L-R and L+R signals are required. These are sent as a DSBSC AM signal with a suppressed subcarrier at 38 kHz.
  • The purpose of the 19 kHz pilot is for proper detection of the DSBSC AM signal.
  • The optional Subsidiary Carrier Authorization (SCA) signal is normally used for services such as background music for stores and offices.
block diagram of fm transmitter
Block Diagram of FM Transmitter

FM

Modulator

Frequency

Multiplier(s)

Antenna

Buffer

Power

Amp

Driver

Pre-emphasis

Audio

direct fm modulator
Direct-FM Modulator
  • A simple method of generating FM is to use a reactance modulator where a varactor is put in the frequency determining circuit.
crosby afc system
Crosby AFC System
  • An LC oscillator operated as a VCO with automatic frequency control is known as the Crosby system.
phase locked loop fm generators
Phase-Locked Loop FM Generators
  • The PLL system is more stable than the Crosby system and can produce wide-band FM without using frequency multipliers.
indirect fm modulators
Indirect-FM Modulators
  • Recall earlier that FM and PM were shown to be closely related. In fact, FM can be produced using a phase modulator if the modulating signal is passed through a suitable LPF (i.e. an integrator) before it reaches the modulator.
  • One reason for using indirect FM is that it’s easier to change the phase than the frequency of a crystal oscillator. However, the phase shift achievable is small, and frequency multipliers will be needed.
fm receivers
FM Receivers
  • FM receivers, like AM receivers, utilize the superheterodyne principle, but they operate at much higher frequencies (88 - 108 MHz).
  • A limiter is often used to ensure the received signal is constant in amplitude before it enters the discriminator or detector. The limiter operates like a class C amplifier when the input exceeds a threshold point. In modern receivers, the limiting function is built into the FM IF integrated circuit.
fm demodulators
FM Demodulators
  • The FM demodulators must convert frequency variations of the input signal into amplitude variations at the output.
  • The Foster-Seeley discriminatorand itsvariant, the ratio detector are commonly found in older receivers. They are based on the principle of slope detection using resonant circuits.
pll fm detector
PLL FM Detector
  • PLL and quadrature detectors are commonly found in modern FM receivers.

Phase

Detector

Demodulated

output

FM IF

Signal

f

LPF

VCO

quadrature detector
Quadrature Detector
  • Both the quadrature and the PLL detector are conveniently found as IC packages.
types of transmission lines
Types of Transmission Lines
  • Differential or balanced lines(where neither conductor is grounded): e.g. twin lead, twisted-cable pair, and shielded-cable pair.
  • Single-ended or unbalanced lines(where one conductor is grounded): e.g. concentric or coaxial cable.
  • Transmission lines for microwave use: e.g. striplines, microstrips, and waveguides.
transmission line equivalent circuit
Transmission Line Equivalent Circuit

L

L

L

L

R

R

Zo

Zo

C

C

C

C

G

G

Lossless Line

“Lossy” Line

notes on transmission line
Notes on Transmission Line
  • Characteristics of a line is determined by its primary electrical constants or distributed parameters: R (/m), L (H/m), C (F/m), and G (S/m).
  • Characteristic impedance, Zo, is defined as the input impedance of an infinite line or that of a finite line terminated with a load impedance, ZL = Zo.
formulas for some lines
Formulas for Some Lines

For parallel two-wire line:

D

d

m = momr; e = eoer; mo = 4px10-7 H/m; eo = 8.854pF/m

For co-axial cable:

D

d

transmission line wave propagation
Transmission-Line Wave Propagation

Electromagnetic waves travel at < c in a transmission

line because of the dielectric separating the conductors.

The velocity of propagation is given by:

m/s

Velocity factor, VF, is defined as:

propagation constant
Propagation Constant
  • Propagation constant, , determines the variation of V or I with distance along the line: V = Vse-x; I = Ise-x, where VS, and IS are the voltage and current at the source end, and x = distance from source.
  •  =  + j, where  = attenuation coefficient (= 0 for lossless line), and  = phase shift coefficient = 2/ (rad./m)
incident reflected waves
Incident & Reflected Waves
  • For an infinitely long line or a line terminated with a matched load, no incident power is reflected. The line is called a flat or nonresonant line.
  • For a finite line with no matching termination, part or all of the incident voltage and current will be reflected.
reflection coefficient
Reflection Coefficient

The reflection coefficient is defined as:

It can also be shown that:

Note that when ZL = Zo,  = 0; when ZL = 0,  = -1;

and when ZL = open circuit,  = 1.

standing waves
Standing Waves

Vmax = Ei + Er

Voltage

Vmin = Ei - Er

l

2

With a mismatched line, the incident and reflected

waves set up an interference pattern on the line

known as a standing wave.

The standing wave ratio is :

other formulas
Other Formulas

When the load is purely resistive:

(whichever gives an SWR > 1)

Return Loss, RL = Fraction of power reflected

= ||2, or -20 log || dB

So, Pr = ||2Pi

Mismatched Loss, ML = Fraction of power

transmitted/absorbed = 1 - ||2 or -10 log(1-||2) dB

So, Pt = Pi (1 - ||2) = Pi - Pr

simple antennas
Simple Antennas
  • An isotropic radiator would radiate all electrical power supplied to it equally in all directions. It is merely a theoretical concept but is useful as a reference for other antennas.
  • A more practical antenna is the half-wave dipole:

/2

Symbol

Balanced Feedline

half wave dipole
Half-Wave Dipole
  • Typically, the physical length of a half-wave dipole is 0.95 of l/2 in free space.
  • Since power fed to the antenna is radiated into space, there is an equivalent radiation resistance, Rr. For a real antenna, losses in the antenna can be represented by a loss resistance, Rd. Its efficiency is then:
gain and directivity
Gain and Directivity
  • Antennas are designed to focus their radiation into lobes or beams thus providing gain in selected directions at the expense of energy reductions in others.
  • The ideal l/2 dipole has a gain of 2.14 dBi (i.e. dB with respect to an isotropic radiator)
  • Directivity is the gain calculated assuming a lossless antenna
eirp and effective area
EIRP and Effective Area
  • When power, PT, is applied to an antenna with a gain GT (with respect to an isotropic radiator), then the antenna is said to have an effective isotropic radiated power, EIRP= PTGT.
  • The signal power delivered to a receiving antenna with a gain GR is PR = PDAeff where PD is the power density, and Aeff is the effective area.
impedance and polarization
Impedance and Polarization
  • A half-wave dipole in free space and centre-fed has a radiation resistance of about 70 W.
  • At resonance, the antenna’s impedance will be completely resistive and its efficiency maximum. If its length is < l/2, it becomes capacitive, and if > l/2, it is inductive.
  • The polarization of a half-wave dipole is the same as the axis of the conductor.
ground effects
Ground Effects
  • Ground effects on antenna pattern and resistance are complex and significant for heights less than one wavelength. This is particularly true for antennas operating at HF range and below.
  • Generally, a horizontally polarized antenna is affected more by near ground reflections than a vertically polarized antenna.
folded dipole
Folded Dipole
  • Often used - alone or with other elements - for TV and FM broadcast receiving antennas because it has a wider bandwidth and four times the feedpoint resistance of a single dipole.
monopole or marconi antenna
Monopole or Marconi Antenna

Main characteristics:

  • vertical and l/4
  • good ground plane is required
  • omnidirectional in the horizontal plane
  • 3 dBd power gain
  • impedance: about 36W
loop antennas
Loop Antennas

Main characteristics:

  • very small dimensions
  • bidirectional
  • greatest sensitivity in the plane of the loop
  • very wide bandwidth
  • efficient as RX antenna with single or multi-turn loop
antenna matching
Antenna Matching
  • Antennas should be matched to their feedline for maximum power transfer efficiency by using an LC matching network.
  • A simple but effective technique for matching a short vertical antenna to a feedline is to increase its electrical length by adding an inductance at its base. This inductance, called a loading coil, cancels the capacitive effect of the antenna.
  • Another method is to use capacitive loading.
inductive and capacitive loading
Inductive and Capacitive Loading

Inductive Loading

Capacitive Loading

collinear array
Collinear Array
  • all elements lie along a straight line, fed in phase, and often mounted with main axis vertical
  • result in narrow radiation beam omnidirectional in the horizontal plane
2 way mobile communications
2-Way Mobile Communications
  • 1) Mobile radio, half-duplex, one-to-many, no dial tone:
    • e.g. CB, amateur (ham) radio, aeronautical, maritime, public safety, emergency, and industrial radios
  • 2) Mobile Telephone, Full-duplex, one-to-one:
    • Analogue cellular (AMPS) using FDMA or TDMA
    • Digital cellular (PCS) using TDMA, FDMA, and CDMA
    • Personal communications satellite service (PCSS) using both FDMA and TDMA
mobile telephone systems
Mobile Telephone Systems
  • Mobile telephone began in the early 1980s first as the MTS (Mobile Telephone Service) at 40 MHz and later as the IMTS (Improved MTS) at 150 and 450 MHz.
  • Narrowband FM and relatively high transmit power were used.
  • Limited channels (total of only 33) and interference were problems.
advanced mobile phone system
Advanced Mobile Phone System
  • AMPS divide area into cells with low power transmitters in each cell.
  • Max. 4 W ERP for mobile radios; max. 600 mW for portable phones; to reduce interference min. power needed for communications is used at all times.
  • Base station: 869.040 – 893.970 MHz; mobile unit’s frequency is 45 MHz below.
  • Total of 790 duplex voice channels and 42 control channels available at 30 kHz each.
  • Channels are divided in 7- or 12-cell repeated pattern and frequencies are reused
block diagram of analogue cell phone
Block Diagram Of Analogue Cell Phone

Antenna

Speaker

IF

amp

IF

detector

Audio

amp

mixer

RF amp

De-emphasis

Display

Frequency

synthesizer

Duplexer

Microprocessor

Keypad

Data

RF power

amp

FM

modulator

Audio preamp

& Pre-emphasis

Mic

6 mW – 3W

7 cell pattern
7-Cell Pattern
  • Each cell has a base station.
  • All cell sites in a region are tied to a mobile switching centre (MSC) or mobile telephone switching office (MTSO) which in turn is connected to other MSCs.

6

3

6

5

7

1

5

4

2

1

3

4

In a real situation, the cells are

more likely to be approximately

circular, with some overlap.

cellular radio network
Cellular Radio Network

BSC: Base Station Controller

MSC: Mobile Switching Centre

BSC

BSC

BSC

BSC

BSC

BSC

BSC

To other

MSCs

Gateway

MSC

MSC

MSC

To other BSCs

BSC

To Public Switched

Telephone Network

cell site control
Cell-Site Control
  • BSC assigns channels and power levels, transmitting signaling tones, etc.
  • MSC routes calls, authorizing calls, billing, initiating handoffs between cells, holds location and authentication registers, connects mobile units to the PSTN, etc.
  • Sometimes BSC and MSC are combined.
  • Cells can be subdivided into mini and micro cells to increase subscriber capacity in a region.
digital cellular telephone
Digital Cellular Telephone
  • The United States Digital Cellular (USDC) system is backward compatible with the AMPS frequency allocation scheme but using digitized signals and PSK modulation.
  • It uses TDMA (Time-Division Multiple Access) to increase the number of subscribers threefold with the same 50-MHz frequency spectrum.
  • It provides higher security and better signal quality.
  • TDMA Service in the 1900 MHz band is also in use since there is no room in the 800 MHz band for expansion.
code division multiple access system
Code-Division Multiple-Access System
  • CDMA is a totally digital cellular telephone system.
  • It is more commonly found in the 1900 MHz PCS band with up to 11 CDMA RF channels.
  • Each CDMA RF channel has a bandwidth of 1.25 MHz, using a single carrier modulated by a 1.2288 Mb/s bitstream using QPSK.
  • Each RF channel can provide up to 64 traffic channels.
  • It uses a spread-spectrum technique so all frequencies can be used in all cells – soft handoff possible.
  • Each mobile is assigned a unique spreading sequence to reduce RF interference.
global system for mobile communications
Global System For Mobile Communications
  • GSM uses frequency-division duplexing and a combination of TDMA and FDMA techniques.
  • Base station frequency: 935 MHz to 960 MHz; mobile frequency: 45 MHz below
  • 1800 MHz is allocated for PCS in Europe while North America utilizes the 1900 MHz band.
  • RF channel bandwidth is 200 kHz but each can hold 8 voice/data channels.
personal communications satellite system
Personal Communications Satellite System
  • PCSS uses either low earth-orbit (LEO) or medium earth-orbit (MEO) satellites.
  • Advantages: can provide telephone services in remote and inaccessible areas quickly and economically.
  • Disadvantages: high risk due to high costs of designing, building and launching satellites; also high cost for terrestrial-based network and infrastructure. Mobile unit is more bulky and expensive than conventional cellular telephones.