Lecture 1 Radio Systems: an Overview - McMaster · PDF fileLecture 1 Radio Systems: an...

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Lecture 1

Radio Systems: an Overview

Acknowledgement: Most diagrams and plots are from M. Steer’s book “Microwave and RF Design”, sections 1.6, 1.7, 1.8, 3.10

Optional Reading: Steer – Sections 1.6, 1.7, 1.8, 3.10

Why Do We Need Microwave Engineers?

rapid development of new wireless communication services in the high MHz and low GHz range

• cellular/mobile voice and data (from 700 MHz to 2.7 GHz)• the advent of LTE (long term evolution) for 4G data

communications: need to redesign system architectures and antennas

• Bluetooth (2.400 GHz to 2.485 GHz)• WLAN, Wi-Fi (between 2.4 GHz and 5.8 GHz), range ~ 10 m• long-range Wi-Fi (around 2.4 GHz, 3.5 GHz, 4 GHz, and 5

GHz), range in open space ~ 1 km• GPS (L-band: uplink@1.57542 GHz, downlink@1.22760

ElecEng4FJ4, Nikolova LECTURE 01: RADIO SYSTEMS: AN OVERVIEW 2

Why Do We Need Microwave Engineers? cont.

• WiMAX (worldwide interoperability for microwave access) –wireless alternative to cable and DSL, mobile broadband connectivity (2.3 GHz, 2.5 GHz and 3.5 GHz), range ~ 100 m

• LMDS (local multipoint distribution service), 27.5 GHz to 28.35 GHz; 29.1 to 29.25 GHz; internet+TV+phone+fax

recent growth in radar and remote control• automotive: anti-collision radar (24 GHz & 79 GHz);

automatic cruise control (77 GHz)• RCV and drones

recent development in MRI – high-tesla machines need conceptually new RF system design

Basic Principles of Radio Communication

DC (or low-frequency signals) cannot be radiated – antennas are commensurate with the wavelength or larger (for high gain)

the lower the frequency, the larger the antenna size (min ~ 0.1λ)

horizontal cross-dipole antenna for AM broadcasting (λ ~100 m) in Mainflingen (Germany), by Bernd Waniewski

http://www.broadcast-transradio.com/html/antenna.html

relatively SMALL electrically

10 cmd

relatively LARGE electrically

Basic Principles of Radio Communication – 2

DC (static) signals do not propagate: 1/r2 dependence on distance

the lowest-frequency wireless links operate at VLF (3 kHz to 30 kHz) – special applications only, antennas are huge

AC signals can be radiated and they have the so called far-zonecomponent: 1/r dependence on distance

21field strength , powerD Dr r

(D – largest antenna dimension)

the typical radio link goes through the following basic steps in order to operate at reasonable frequencies• modulation• up-conversion and transmission (radiation)• reception and down-conversion• demodulation

Radio Communication System: Block Diagram

superheterodyne receiver

transmitterra

microwave engineers design equipment for the front end (the RF portion of the system)

front end

Radio Receiver Evolution: Crystal Sets

• self-powered RF receivers• AM signals• needs good antenna and strong station• cannot power loudspeakers

[Hagen, RF Electronics] rectifierBPF

amplifiers needed otherwise (RF and/or audio amplifiers)

several mW’s needed (signal strength > 0 dBm)

Using sketches of the input and output signal waveforms (no formulas needed!), explain how a diode rectifier performs AM demodulation.

Radio Receiver Evolution: TRF Sets

• TRF – tuned radio frequency• consists of cascaded tunable BPFs and RF amps• tuning all the BPFs simultaneously while preserving their

bandwidth (~ 10 kHz for AM) is difficult

vacuum tube detectortunable BPF

[Hagen, RF Electronics]

Notes: 1) AM radio bands (Americas): 535 kHz to 1605 kHz (10 kHz channel spacing)2) AM channel bandwidth is < 10 kHz with actual audio bandwidth < 5 kHz3) FM radio bands (Americas): 87.9 MHz to 107.9 MHz (200 kHz channel spacing)4) FM channel bandwidth is about 150 kHz

AM radio TRF Receiver

AM and FM Radio Bands (North America)

[http://hyperphysics.phy-astr.gsu.edu/hbase/audio/radio.html]

• super(sonic) hetero-dyne (from Greek – “different power”)• heterodyne refers to a beat or “difference” frequency (IF) produced

when two or more RF signals are fed to a nonlinear device (mixer)• a single-tone signal (from LO) is mixed with the received RF signal

to produce a lower-frequency version of the signal (the IF)• inventors: Reginald Fessenden (1901), Edwin Armstrong (1917)• key advantage: use fixed filters while tuning the LO• main problem: image interference

MIXERRF IF

LOFREQUENCYIF RF

(a) (b)

Radio Receiver Evolution: Superheterodyne Receivers, Principles

Standard AM Superheterodyne Receiver

• consists of a frequency converting front end plus an IF back-end receiver which is in effect a fixed-frequency TRF

• AM radios use an IF frequency of 455 kHz

[Hagen, RF Electronics]

TRF back-end receiver

front-end

Note: FM radio receivers use an IF frequency of 10.7 MHz.

Double Conversion Superhet

Mixing and Trigonometry

• the mixer can be viewed as a multiplier

• the mixer contains: (a) nonlinear device (e.g., diode, FET), (b) biasing circuits, and may be (c) filter, (d) amplifier

• the signal multiplication is described by trigonometric identities

• the mixer output in general contains both sum and difference terms

• to retain only the down-converted signal and maintain low conversion loss, a double-mixer circuit is used

Down-conversion Double-mixer Circuit

RFsin( )t

RF LO RF LO

RF LO RF LOsin ( ) sin ( ) / 2cos ( ) cos ( ) / 2t t

O RF L

sin (cos ( ) cos

) sin ( ) /( ) 2

Rsin( )cos( )t

RF LO IFsin ( ) sin( )t t

• up-converted terms cancel upon summation

Heterodyne Frequency Conversion: Image Channel

• the image of the desired RF signal (above LO frequency) is an RF signal below the LO frequency and equidistant from it

FREQUENCYDC

PUMPMAINCHANNEL

RF PRESELECTFILTER

• the image signal is a problem because its IF is the same as that of the desired signal

RFLOIF

imageRF IF LO RF

Image-frequency Definition:2 , if 2 , if

f f f ff

f f f f

Heterodyne Frequency Conversion: Image Interference

FREQUENC

DOWN-CONVERTEDMAINCHANNEL

IFFILTERIFFILTER

–IF +IF

FREQUENCYDC

MAINCHANNELWITH IMAGE

FREQUENCYDC

PUMP MAINCHANNEL

RF PRESELECTFILTERRF PRESELECTFILTER

RF image LOLet , , A t I t B t

cos( )

cos sin( ) / 2

sin co

cos cos( ) / 2

cos cos cos(

s sin( ) / 2

sin( )

A B A B

I B I B

down-converted termsElecEng4FJ4, Nikolova LECTURE 01: RADIO SYSTEMS: AN OVERVIEW 16

Image Rejection Using Filtering

• RF preselect filter can suppress the image before mixing• higher IF means that the image frequencies are more distant from

the RF and hence less troublesome to remove by preselect filtering

FREQUENC

PUMPMAINCHANNEL

RF PRESELECTFILTER

FREQUENC YDC

PUMP MAINCHANNEL

RF PRESELECTFILTERRF PRESELECTFILTER

ElecEng4FJ4, Nikolova 17

Image Rejection Using Filtering – 2

• preselect filtering with BPFs is usually applied before each down-conversion stage

Image Rejection with Quadrature Mixing

• quadrature-mixing down-conversion with Hartley’s circuit

channelI

channelQ

opposite signs!

ElecEng4FJ4, Nikolova

I: in-phase channelQ: quadrature channel

Confused About I and Q Channels?

ElecEng4FJ4, Nikolova 20

see http://www.ni.com/tutorial/4805/en/

• I and Q are nothing but the coefficients multiplying the cosine and sine components of a harmonic signal

( ) ( ) ( ) ( ) ( ) ( )

( ) ( )

cos( ) cos cos( ) sin sin( )t t t t t t

I t Q t

m t m t m t

( ) ( ) ( ) ( )cos( ) cos( ) sin( )t t t tm t I t Q t

• in modulation, information is imprinted onto I and Q instead of on m and φ

Hartley’s circuit: math detail for the RF down-conversion

cos( )sin( )

a tb t

LOcos( )t

LOsin( )t

RF LL OO

cos ( ) / 2sin (

cos ( )) n ( ) / 2si

0.5 cos( )0.5 sin( )

a tb t

F OO R L

sin ( ) / 2cos (sin ( )

cos ( ) ) / 2a tb

0.5 sin( )0.5 cos( )

a tb t

0.5 cos( )0.5 sin( )

a tb t

cos( )sin( )

a tb t

RF LOhere:

Image Rejection with Quadrature Mixing – 2

Hartley’s circuit: math detail for the image down-conversion

cos( )sin( )

a tb t

LOcos( )t

LOsin( )t

cos ( )sin ( )

cos ( ) / 2sin ( ) / 2

0.5 cos( )0.5 sin( )

a tb t

O OI L

sin ( ) / 2cos ( ) /sin ( )

c 2os ( )a tb

0.5 sin( )0.5 cos( )

a tb t

0.5 cos( )0.5 sin( )

a tb t

I LOhere:

Image Rejection with Quadrature Mixing – 3

Pros and Cons of Hartley’s Circuit

• together with a pre-select filter, it offers near-ideal suppression of image interference

• main drawback: the receiver is sensitive to phase errors of the LO signal (such errors lead to incomplete image rejection)

Homodyne or Direct-Conversion (zero-IF) Receivers

• LO frequency is the same as the RF carrier and is synchronized in phase with it (note: in AM, the carrier is directly available)

• mixing leads to an IF signal centered around zero frequency• for AM signals, simple rectifier can be used to demodulate

rectifierin AM, all information is contained in the upper band

( )cos[ ( )]M t t t

LOpump cos( )M t

Homodyne Mixing with I/Q• for frequency and phase modulated signals, I and Q signals (at

baseband) are extracted and processed digitally

RFcos( )m t cosm

sinm IQ

arctan( / )m I Q

basebandRFcos( )t

RFsin( )t

Pros and Cons of the Homodyne Architecture

• main advantage: avoids the image interference problem altogether (the desired channel is the image of itself)

• simplicity: no need for a BPF, which is usually implemented off-chip in heterodyne receivers – less off-chip components

• main disadvantage: leakage from the LO (or VCO) mixes with the LO signal itself leading to DC offset, which affects the signal recovery

• the flicker noise of the mixer may corrupt significantly the output baseband signal (important for the detection of very weak RF signals)

Transmitter Architectures

main goals:• spectral efficiency• power efficiency• detectability (good SNR)

direct conversion architecture: general block diagram

Transmitter Architectures: Single Side-band Up-conversion

• suppressed carrier single side-band (SCSS) up-conversion provides both spectral and power efficiency

• done with a quadrature up-conversion circuit• the quadrature up-converter is suitable for both constant-envelope

and variable-envelope modulation schemes

m( ) cos( )i t t

m( ) sin( )q t t

VCO ccos( )t

c m c mcos ( ) cos ( ) / 2t t

csin( )t

c m c mcos ( ) cos ( ) / 2t t

c mcos ( )t 90

frequencycf

c mf fc mf f

frequencycf

c mf f

c mf ffrequencycf

c mf f

Direct Conversion TX Architecture• single-stage mixing (ωVCO is the same as ωc)

ccos( )t

csin( )t

baseband I

baseband Q

PA matchingnetwork

major drawback: leakage from the wideband “noisy” PA output disturbs the VCO, whereby the VCO frequency tends to drift toward that of an external stimulus through a feedback mechanism, esp. if the two frequencies are close (aka “injection pulling”)[Razavi, “RF Transmitter Architectures and Circuits”, IEEE Custom ICs Conf., 1999]

VCO pulling by PA

Direct Conversion TX Architecture with Two VCOs

• main principle: move the VCO frequency far from the carrier –achieved with two VCOs and a mixer

[Razavi, 1999]

• neither ω1 nor ω2 are close to the carrier c 1 2

Two-stage Conversion TX Architecture

[Razavi, 1999]

• the “injection pulling” problem is again avoided because

• the 2nd mixing step may employ quadrature up-converter• advantage over single-stage conversion: I/Q modulation at

lower frequency – easier filtering for spectral purity

c 1 2 1 2,

Summary

wireless links operate at high frequencies to achieve efficient power transmission with antennas of reasonable size

the front end of a receiver includes: antenna, transmission lines, low-noise amplifiers, bandpass filters, mixers, oscillators, switches, etc.

the two most common receiver architectures are the heterodyne architecture with image rejection and the homodyne architecture

image rejection is achieved with pre-select filtering and with Hartley’s circuit

the front end of a transmitter includes : antenna, transmission lines, power amplifiers, bandpass filters, mixers oscillators, switches, etc.

the most common up-conversion scheme is the suppressed carrier single-side band scheme

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