10-04-2017, 08:37 PM
RF Transmitters
Architectures for Integration and
Multi-Standard Operation
Outline
Motivation
Transmitter Architectures
Current Trends in Integration
State-of-the-Art Examples (3)
Direct Conversion
2-Stage
Future Challenges
References
Motivation
Increase in demand for low-cost, small-form-factor, low-power transceivers
Proliferation of various wireless standards pushes for multi-standard operation
CMOS is well suited for high levels of mixed signal radio integration [2]
End goal: a low cost single chip radio transceiver covering multiple RF standards
Transmitter Architectures
Mixer-Based
Direct Conversion (Homodyne)
2-Stage Conversion (Heterodyne)
Both architectures can operate with constant and non-constant envelope modulation
Well-suited for multi-standard operation
PLL-Based
Show promise with respect to elimination of discrete components
Fundamentally limited to constant-envelope modulation schemes not suitable for multi-standard operation
Transmitter Architectures
Direct Conversion
Attractive due to simplicity of the signal path suitable for high levels of integration
Output carrier frequency = local oscillator (LO) frequency
Important drawback: LO disturbance by PA output
Transmitter Architectures
Direct Conversion LO Pulling
Noisy output of PA corrupts VCO spectrum - injection pulling or injection locking
VCO frequency shifts toward frequency of external stimulus
If injected noise frequency close to oscillator natural frequency, then LO output eventually locks onto noise frequency as noise level increases
Transmitter Architectures
Direct Conversion LO Frequency Offset Technique
LO pulling can be alleviated by moving the PA output spectrum sufficiently far from the LO frequency
LO offset can be achieved by mixing 2 VCO outputs 1 and 2 and filtering the result; leading to a carrier frequency of 1+ 2, far from either 1 or 2
BPF1 must have high selectivity to suppress spurs of the form m 1+m 2 to avoid degradation in quadrature generation and spurs in the up-converted signal
Transmitter Architectures
2-Stage Up-Conversion
Another approach to solving the LO pulling problem
Up-convert in 2 stages so PA output spectrum is far from VCO frequency
Quadrature modulation at IF ( 1), up-convert to 1+ 2 by mixing and filtering
BPF1 suppresses the IF harmonics, while BPF2 removes the unwanted sideband 1- 2
Advantages: no LO pulling; better I/Q matching (less crosstalk between the 2 bit streams)
Current Trends in Integrated Transceivers
Both direct and 2-stage architectures are used (with modifications for better integration and multi-standard operation)
Direct architecture achieves a low-cost solution with a high level of integration [3],[4],[6],[8]
2-stage results in better performance (ie. reduced LO pulling) at the expense of increased complexity and hence higher cost of implementation [5],[7],[9],[10],[11]
Transmitter and receiver designed concurrently to enable hardware and possibly power sharing
For more information abut this article,please follow the link:
http://eecg.toronto.edu/ kphang/papers/2003/TYao_RF_TX_circuits.ppt