TRANSCEIVER DESIGN ECE 2526 MOBILE COMMUNICATION Monday, 07 - - PowerPoint PPT Presentation

IN INTRODUCTION TO MOBILE TRANSCEIVER DESIGN

ECE 2526– MOBILE COMMUNICATION Monday, 07 September 2020

FUNCTIONS OF A TRANSMITTER

The basic functions of a transmitter are: a) up-conversion: move signal to desired RF carrier frequency. b) power amplification: amplify signal to deliver wanted power to antenna for emission. The main functions of a radio receiver are: a) To intercept the RF signal by using the receiver antenna b) Select the desired RF signal and reject everything else c) Amplify the RF signal d) Detect the signal and demodulate to yield the original baseband signal e) Amplify the baseband signal

TYPES OF RADIO TRANSMITTERS

Radio transmitters are basically of two types, i.e

• 1. Mixer-Based
• Which can be Direct Conversion (Homodyne) or 2-Stage Conversion

(Heterodyne)

• Both architectures can operate with constant and non-constant envelope

modulation

• Well-suited for multi-standard operation
• 2. PLL-Based
• Fundamentally limited to constant-envelope modulation schemes
• Show promise with respect to elimination of discrete components
• not suitable for multi-standard operation

MODULATION STANDARDS

DIRECT CONVERSION

Direct Conversion has the following features

• 1. It is attractive due to simplicity of the signal path

2. It is suitable for high levels of integration

• 3. Output carrier frequency is same as local oscillator (LO) frequency
• 4. Major drawback: LO disturbance by PA output

Output signal Re(x(t) 𝑩𝒅𝒅𝒑𝒕𝝏𝒅𝒖 + Im(xt)) 𝑩𝒅𝒅𝒑𝒕𝝏𝒅t Re(X(t)) Im(X(t)) Note: For frequency and phase- modulated signals, conversion must provide quadrature outputs to avoid loss of information.

• 1. “Injection pulling” or “injection locking”

results from the noisy output of power amplifier corrupting the output of the VCO.

• 2. VCO frequency shifts toward frequency
• f external stimulus due to the feedback.
• 3. If injected noise frequency is close to
• scillator frequency and the noise

increases, then the output “locks” onto noise frequency.

DRAWBACK OF DIRECT CONVERSION

• LO pulling is usually minimized by moving the PA output spectrum

sufficiently far from the LO frequency through a process called LO

• ffset.
• LO offset is achieved by mixing 2 VCO outputs ω1 and ω2 and

filtering the result; leading to a carrier frequency of ω1+ ω2, which is far from either ω1 or ω2.

SOLUTION TO ‘LOCAL OSCILLATOR (LO) PULLING’

BPF1 must have high selectivity to suppress harmonics from mω1+mω2

Output signal Re(x(t) 𝑩𝒅𝒅𝒑𝒕(𝝏𝟐+𝝏𝟑)𝒖 + Im(xt)) 𝑩𝒅𝒅𝒑𝒕(𝝏𝟐 + 𝝏𝟑)t

HOMODYNE RECEIVER

880-960 MHz 1710 – 1880 MHz

• Hetrodyne conversion uses Quadrature modulation at IF (ω1)followed

by a second up-conversion stage to yield ω1+ ω2 by mixing and filtering.

• Advantages: Has no LO pulling; Has better I/Q matching, i.e less

crosstalk between the 2 bit streams.

TWO-STAGE SUPERHETRODYNE

BPF1 suppresses the IF harmonics BPF2 removes the unwanted sideband ω1- ω2

2-STAGE SUPERHETRODYNE RECEIVER

HETERODYNE RADIO TRANSMITTER WITH SINGLE IF STAGE

TRENDS IN TRANSCEIVER INTEGRATION

• 1. Both Direct conversion and hetrodyne architectures are used with

minor modifications for better integration and multi-standard

• peration.
• 2. Direct architecture achieves a low-cost solution with a high level of

integration.

• 3. 2-stage superhetrodyne architecture results in better performance

(i.e. reduced LO pulling) at the expense of increased complexity and hence higher cost of implementation.

• 4. Transmitter and receiver parts of a transceiver are usually

designed concurrently to enable hardware and possibly power sharing

EXAMPLE - 5-GHZ CMOS TRANSCEIVER FRONTEND CHIPSET

Voltage-Controlled Oscillator

• n-chip quadrature VCO and buffers

to improve frequency purity Buffers isolate sensitive VCO circuits from high-power, large voltage

• r current swing circuit blocks

Duplexer Allows simultaneo us Tx and Rx,

A DUAL BAND GSM 900/1800-MHZ CMOS TRANSMITTER

• Transmitter exploits similarities
• f GSM 900 and 1800 standards

(modulation, channel spacing, antenna duplexing) to reduce hardware

• Two quadrature upconverters

driven by 450MHz LO to generate quadrature phases of IF signal.

• IF signal routed to single-

sideband mixers driven by a 1350MHz Local Oscillator, producing either 900MHz or 1800MHz signal

𝒈𝟐 = 𝟓𝟔𝟏 𝑵𝑰𝒜 𝒈𝟐 = 𝟓𝟔𝟏 𝑵𝑰𝒜 𝒈𝟑 = 𝟐𝟒𝟔𝟏 𝑵𝑰𝒜 𝒈𝟑 = 𝟐𝟒𝟔𝟏 𝑵𝑰𝒜 𝒈𝟐𝟗𝟏𝟏 = 𝟐𝟒𝟔𝟏 + 𝟓𝟔𝟏 𝒈𝟘𝟏𝟏 = 𝟐𝟒𝟔𝟏 − 𝟓𝟔𝟏

EXAMPLES OF CMOS OSCILLATORS

(a) Piece Oscillator

Crystal

(b) Miller Oscillator

Crystal

CONVENTIONAL CMOS MIXER

LO Signal from RF Image filter LO Signal from Local Oscillator IF Intermediate frequency signal

PRINCIPLE OF OPERATION OF CMOS MIXER

(a) CMOS Mixer Circuit (b) Current flow in the resistors

CHALLENGES IN THE DESIGN OF TRANCEIVER INTEGRATED CIRCUITS

• 1. Implementation of highly integrated radio transceivers will remain

as one of the greatest challenges in IC technology.

• 2. New architectures and circuit techniques are currently under

investigation for higher flexibility in CMOS transmitters.

• 3. Further improvement are required in the design of on-chip

inductors, filters and oscillators in a standard CMOS process.

• 4. There is need for continued improvement in high frequency CMOS

device modelling and simulation.

PLL TRANSMITTER