ECE 4710: Lecture #17 1

Transmitters

Communication Tx generate modulated signal s(t) at the carrier frequency fc from the modulating information signal m(t) Baseband Circuits : PCM, line code generation, pulse shaping,

coding, etc. Bandpass Circuits: modulation, frequency translation, power

amplification, etc. to produce RF (Radio Frequency) signal output

Transmitted signal : Complex envelope:

Performs mapping function on m(t) : g[m(t)] modulation type Two equivalent Tx architectures defined by math representation

(polar vs. cartesian) of complex envelope g(t)

)](2cos[)()(Re)( 2 ttftRetgts ctfj c

)()()()( )( tyjtxetRtg tj

ECE 4710: Lecture #17 2

AM-PM Tx AM-PM Transmitter: baseband circuits generate R(t)

and/or (t) signals from m(t) Polar g(t) form R(t) = AM & (t) = PM

» Passed to RF circuits to modulate the carrier

Baseband Circuits» Linear or non-linear methods

Linear AM & Non-linear PM» Analog or Digital Circuits» Digital Circuits

Software algorithms for generation

Need ADC for m(t) and then two DACs for R(t) & (t)

ECE 4710: Lecture #17 3

AM-PM Tx

)](2cos[)()( ttftRts c

Product Multiplier or Mixer

ECE 4710: Lecture #17 4

AM-PM Mapping

ECE 4710: Lecture #17 5

Quadrature or IQ Tx

Quadrature Transmitter: baseband circuits generate x(t) and y(t) signals from m(t) Cartesian g(t) form x(t) = In-Phase & y(t) = Quadrature “IQ Tx”

» Passed to RF circuits to modulate two carriers

cos(2fc t) and sin(2fc t)Summed to produce quadrature output signal

Baseband circuits are usually digital» Software control

Easy algorithm updates» Multiple types of modulation done using same digital circuitry with

different software controlFlexible and cost-effective for mass production

ECE 4710: Lecture #17 6

Quadrature Tx

tfty

tftxts

c

c

2sin)(

2cos)()(

ECE 4710: Lecture #17 7

Quadrature Mapping

ECE 4710: Lecture #17 8

Transmitters RF circuits provide carrier and signal amplification Power Amplifier (PA) is usually final stage before antenna or

wired channel Class A or B Amplifiers

» Linear modulation methods (AM) Signal information contained in amplitude variation which cannot have

non-linear distortion Poor DC to RF efficiencies typically 40-65%

Class C Amplifiers» Non-linear modulation methods (FM, FSK, etc.)

Constant envelope signals Signal information contained in phase or frequency variation Excellent DC to RF efficiencies typically 80-90% Cost effective and very important for wireless systems using DC

battery supply

ECE 4710: Lecture #17 9

Transmitters AM-PM and Quadrature Tx’s have different

architecture but any type of modulated signal can be produced from either architecture Designer chooses architecture type based on

performance, cost, and state of art in circuit design In general (not always)

AM-PM for Analog Modulation Methods (AM, FM, etc.) Quadrature Tx for Digital Modulation Methods (PSK, FSK,

etc.)

ECE 4710: Lecture #17 10

Receivers Communication Rx extract estimate of source information signal, m(t),

from received signal, r(t), that may be distorted by channel and corrupted by noise Bandpass RF Circuits

» First-stage amplification of weak received signal (e.g. LNA = low noise amplifier)» First-stage filtering» Frequency translation to baseband or intermediate frequency (IF)

Bandpass IF Circuits (optional)» Most signal amplification (more cost effective than at RF)» Second-stage sharp filtering (more cost effective and higher Q than at RF)» Frequency translation to baseband

Baseband Circuits» Demodulation/detection» ADC & signal processing» Low pass and/or adaptive filtering & amplification» Error detection & correction» DAC for analog output (if needed)

˜

ECE 4710: Lecture #17 11

Receivers Three basic types:

Tuned Radio Frequency (TRF)» Low cost, low performance

SuperHeterodyne (SH)» Most widely used high-performance Rx» Radar, AM/FM/TV broadcast, satellite communications, etc.

Zero-IF (ZIF) or Homodyne» Specialized applications like wireless handsets» Enable “system on chip” Rx designs using 2 chips

1 RF MMIC and 1 baseband DSP ASIC

ECE 4710: Lecture #17 12

TRF Rx

Station tuner controls center frequency (variable) of cascaded RF amplifiers to select

desired FDMA signal :

˜

Antenna

TunableRF Amps

Detector LPFBasebandAmplifier

AnalogOutput

StationTuner

˜̃ ff1 f2 f3

ECE 4710: Lecture #17 13

TRF Rx Primary Advantages

Simple and cheap Primary Disadvantages

Used for analog modulation methods only (AM/FM radio) Low performance:

» Can’t design cascaded tunable RF amplifier chain to select appropriate channel frequency while simultaneously providing sharp rolloff so that adjacent channel signals are completely rejected

» Can’t have large gain in amplifier chain without having oscillationsMust have strong signal at Rx input for good output S/N

ECE 4710: Lecture #17 14

SuperHeterodyne Rx

IFFilter

˜

Antenna

Low NoiseRF Amp

LPF

BasebandAmplifier

Digital orAnalogOutput

LocalOscillator

Mixer

Station Tuning Circuit

IFAMP

Demod /Detector

DSP

ADC, Bit Detection, Decoding, Adaptive Filter,

Error Correction, etc.

fc

fLO

fc + fLO

fc – fLO

-fc + fLO

-fc – fLO

fIF = -fc + fLO