ECE 4710: Lecture #17 1 Transmitters  Communication Tx  generate modulated signal s(t) at the carrier frequency f c 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)

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 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(2  f c t ) and sin(2  f c 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 control  Flexible and cost-effective for mass production

ECE 4710: Lecture #17 6 Quadrature Tx

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 Tunable RF Amps Detector LPF Baseband Amplifier Analog Output Station Tuner ˜˜ f f1f1 f2f2 f3f3

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 oscillations  Must have strong signal at Rx input for good output S/N

ECE 4710: Lecture #17 14 SuperHeterodyne Rx IF Filter ˜ Antenna Low Noise RF Amp LPF Baseband Amplifier Digital or Analog Output Local Oscillator Mixer Station Tuning Circuit IF AMP Demod / Detector DSP ADC, Bit Detection, Decoding, Adaptive Filter, Error Correction, etc. fcfc f LO f c + f LO f c – f LO - f c + f LO -f c – f LO f IF = -f c + f LO