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: [email protected] GHz, [email protected]
GHz)
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
ElecEng4FJ4, Nikolova LECTURE 01: RADIO SYSTEMS: AN OVERVIEW 3
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
d
relatively LARGE electrically
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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
ElecEng4FJ4, Nikolova LECTURE 01: RADIO SYSTEMS: AN OVERVIEW 5
Radio Communication System: Block Diagram
superheterodyne receiver
transmitterra
dio
chan
nel
microwave engineers design equipment for the front end (the RF portion of the system)
front end
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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.
ElecEng4FJ4, Nikolova LECTURE 01: RADIO SYSTEMS: AN OVERVIEW 7
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
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AM radio TRF Receiver
AM and FM Radio Bands (North America)
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[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
f fLO
RF
LO
IFDC
(a) (b)
f
Radio Receiver Evolution: Superheterodyne Receivers, Principles
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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.
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Double Conversion Superhet
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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
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Down-conversion Double-mixer Circuit
LO
RFsin( )t
RF LO RF LO
RF LO RF LOsin ( ) sin ( ) / 2cos ( ) cos ( ) / 2t t
t t
RF L
RF L
O RF L
O R
O
F LO
sin (cos ( ) cos
) sin ( ) /( ) 2
2/t
tt
t
R
F
F
Rsin( )cos( )t
t
RF LO IFsin ( ) sin( )t t
RF LO
IF LO
• up-converted terms cancel upon summation
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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
IMAGE
• the image signal is a problem because its IF is the same as that of the desired signal
RFLOIF
imageRF IF LO RF
imageRF IF LO RF
Image-frequency Definition:2 , if 2 , if
f f f ff
f f f f
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IFf
Heterodyne Frequency Conversion: Image Interference
FREQUENC
(b)
(a)
(c)
YDC
DOWN-CONVERTEDMAINCHANNEL
IFFILTERIFFILTER
IMAGE
–IF +IF
FREQUENCYDC
MAINCHANNELWITH IMAGE
+IF
FREQUENCYDC
PUMP MAINCHANNEL
RF PRESELECTFILTERRF PRESELECTFILTER
IMAGE
RF image LOLet , , A t I t B t
s
c
i
os
n
cos( )
cos(
cos sin( ) / 2
sin co
cos cos( ) / 2
cos cos cos(
s sin( ) / 2
)
sin( )
/
)
) 2
sin(
A
A B A B
I B I
B A B
I
A B
A B
I
B
B
I B I B
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
(a)
(b)
YDC
PUMPMAINCHANNEL
RF PRESELECTFILTER
IMAGE
FREQUENC YDC
PUMP MAINCHANNEL
RF PRESELECTFILTERRF PRESELECTFILTER
IMAGE
IFf
ElecEng4FJ4, Nikolova 17
Image Rejection Using Filtering – 2
• preselect filtering with BPFs is usually applied before each down-conversion stage
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I
Q
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?
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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
I
Q m
Hartley’s circuit: math detail for the RF down-conversion
I
Q
RF
RF
cos( )sin( )
a tb t
LOcos( )t
LOsin( )t
RF LO
RF
RF LO
RF LL OO
cos ( ) / 2sin (
cos ( )) n ( ) / 2si
a tb
ttt
IF
IF
0.5 cos( )0.5 sin( )
a tb t
LO RF
RF L
RF LO
F OO R L
sin ( ) / 2cos (sin ( )
cos ( ) ) / 2a tb
ttt
IF
IF
0.5 sin( )0.5 cos( )
a tb t
IF
IF
0.5 cos( )0.5 sin( )
a tb t
IF
IF
cos( )sin( )
a tb t
RF LOhere:
Image Rejection with Quadrature Mixing – 2
21
Hartley’s circuit: math detail for the image down-conversion
I
Q
I
I
cos( )sin( )
a tb t
LOcos( )t
LOsin( )t
II LO
I LO
LO
I LO
cos ( )sin ( )
cos ( ) / 2sin ( ) / 2
a tb
ttt
IF
IF
0.5 cos( )0.5 sin( )
a tb t
I LO
I
LO
L
I
O OI L
sin ( ) / 2cos ( ) /sin ( )
c 2os ( )a tb
tt t
IF
IF
0.5 sin( )0.5 cos( )
a tb t
IF
IF
0.5 cos( )0.5 sin( )
a tb t
00
I LOhere:
Image Rejection with Quadrature Mixing – 3
22
Pros and Cons of Hartley’s Circuit
23
• 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
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( )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
25
RFcos( )m t cosm
sinm IQ
2 2
arctan( / )m I Q
Q I
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)
26
Transmitter Architectures
main goals:• spectral efficiency• power efficiency• detectability (good SNR)
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c
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
ElecEng4FJ4, Nikolova LECTURE 01: RADIO SYSTEMS: AN OVERVIEW 28
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)
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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
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[Razavi, 1999]
• neither ω1 nor ω2 are close to the carrier c 1 2
Two-stage Conversion TX Architecture
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[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
VCO 1
c 1 2 1 2,
VCO 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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