INTRODUCTION TO TRANSCEIVER DESIGN ECE3103 ADVANCED TELECOMMUNICATION SYSTEMS

Transcription

1 INTRODUCTION TO TRANSCEIVER DESIGN ECE3103 ADVANCED TELECOMMUNICATION SYSTEMS

2 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

3 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

4 MODULATION STANDARDS

5 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 Re(X(t)) Output signal Re(x(t) A c cosω c t + Im(xt)) A c cosω c t Im(X(t)) Note: For frequency and phasemodulated signals, conversion must provide quadrature outputs to avoid loss of information.

6 DRAWBACK OF DIRECT CONVERSION 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 of external stimulus due to the feedback. 3. If injected noise frequency is close to oscillator frequency and the noise increases, then the output locks onto noise frequency.

7 SOLUTION TO LOCAL OSCILLATOR (LO) PULLING LO pulling is usually minimized by moving the PA output spectrum sufficiently far from the LO frequency through a process called LO offset. LO offset is achieved by mixing 2 VCO outputs ω 1 and ω 2 and filtering the result; leading to a carrier frequency of ω 1 + ω 2, which is far from either ω1 or ω2. BPF1 must have high selectivity to suppress harmonics from Output signal Re(x(t) A c cos(ω 1 +ω 2 )t + Im(xt)) A c cos(ω 1 + ω 2 )t

8 MHz MHz HOMODYNE RECEIVER

9 TWO-STAGE SUPERHETRODYNE Hetrodyne conversion uses Quadrature modulation at IF (ω 1 )followed by a second up-conversion stage to yield ω 1 + ω 2 by mixing and filtering. Advantages: Has no LO pulling; Has better I/Q matching, i.e less crosstalk between the 2 bit streams. BPF1 suppresses the IF harmonics BPF2 removes the unwanted sideband ω 1 - ω 2

10 2-STAGE SUPERHETRODYNE RECEIVER

11 HETERODYNE RADIO TRANSMITTER WITH SINGLE IF STAGE

12 TRENDS IN TRANSCEIVER INTEGRATION 1. Both Direct conversion and hetrodyne architectures are used with minor modifications for better integration and multi-standard operation. 2. Direct architecture achieves a low-cost solution with a high level of integration 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

13 EXAMPLE - 5-GHZ CMOS TRANSCEIVER FRONTEND CHIPSET Voltage-Controlled Oscillator on-chip quadrature VCO and buffers to improve frequency purity Duplexer Allows simultaneo us Tx and Rx, Buffers isolate sensitive VCO circuits from high-power, large voltage or current swing circuit blocks

14 A DUAL BAND GSM 900/1800-MHZ CMOS TRANSMITTER Transmitter exploits similarities of GSM 900 and 1800 standards (modulation, channel spacing, antenna duplexing) to reduce hardware f 1 = 450 MHz f 2 = 1350 MHz Two quadrature upconverters driven by 450MHz LO to generate quadrature phases of IF signal. f 1800 = IF signal routed to singlesideband mixers driven by a 1350MHz Local Oscillator, producing either 900MHz or 1800MHz signal f 1 = 450 MHz f 2 = 1350 MHz f 900 =

15 EXAMPLES OF CMOS OSCILLATORS Crystal Crystal (a) Piece Oscillator (b) Miller Oscillator

16 CONVENTIONAL CMOS MIXER LO Signal from Local Oscillator IF Intermediate frequency signal LO Signal from RF Image filter

17 PRINCIPLE OF OPERATION OF CMOS MIXER (a) CMOS Mixer Circuit (b) Current flow in the resistors

18 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.

19 PLL TRANSMITTER