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

BPF1 must have high selectivity to suppress spurs of the form mω1+mω2 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 (ω1), up-convert to ω1+ ω2 by mixing and filtering
BPF1 suppresses the IF harmonics, while BPF2 removes the unwanted sideband ω1- ω2
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


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RF Transmitters
Architectures for Integration and Multi-Standard Operation
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
 RF Transmitters
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 à not suitable for multi-standard operation
 Direct Conversion
• Attractive due to simplicity of the signal path à suitable for high levels of integration
• Output carrier frequency = local oscillator (LO) frequency
• Important drawback: LO disturbance by PA output
 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
 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 ω1 and ω2 and filtering the result; leading to a carrier frequency of ω1+ ω2, far from either ω1 or ω2
• BPF1 must have high selectivity to suppress spurs of the form mω1+mω2 to avoid degradation in quadrature generation and spurs in the up-converted signal
 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 (ω1), up-convert to ω1+ ω2 by mixing and filtering
• BPF1 suppresses the IF harmonics, while BPF2 removes the unwanted sideband ω1- ω2
• 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 à achieves a low-cost solution with a high level of integration [3],[4],[6],[8]
 2-stage à 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
Direct Conversion Example
• Homodyne architecture for better integration, lower cost and lower power consumption
• Uses on-chip quadrature VCO and buffers to improve frequency purity
• On-chip VCO minimizes radiation leakage from strong PA output back to core oscillator
• Buffers isolate sensitive VCO circuit from high-power, large voltage or current swing circuit blocks
 2-Stage Conversion Example
 A Dual Band (GSM 900-MHz/DCS1800 1.8-GHz) CMOS Transmitter
 2-Stage Conversion Example (#2)
• Harmonic rejection mixer for IF up-conversion relaxes on-chip filtering requirements and even eliminates discrete IF filter à better integration!
• HRM not only does frequency translation, but also attenuates the 3rd and 5th IF harmonics by multiplying the baseband signal by a 3-bit, amplitude-quantized sinusoid
Future Challenges
 Implementation of highly integrated radio transceivers will remain as one of the greatest challenges in IC technology
 New architectures and circuit techniques should be investigated for higher flexibility in CMOS transmitters
 Further improvement needed in the design of on-chip inductors, filters and oscillators in a standard CMOS process
 Continued improvement in high frequency CMOS device modeling and simulation