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RF Transmitters Architectures for Integration and Multi-Standard Operation Terry Yao ECE 1352 Outline Motivation Transmitter Architectures Current Trends in Integration State-of-the-Art Examples (3) DirectConversion 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  End goal: a low cost single chip radio transceiver covering multiple RF standards RF Transmitters Function Performance Specification Modulation Accuracy Frequency Spectral Translation Emission Power Output Amplification Power Level Transmitter Architectures Mixer-Based DirectConversion (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 ,,, 2-stage results in better performance (ie. reduced LO pulling) at the expense of increased complexity and hence higher cost of implementation ,,,, Transmitter and receiver designed concurrently to enable hardware and possibly power sharing Direct Conversion Example A 5-GHz CMOS transceiver frontend chipset  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  Exploits similarities of GSM and DCS1800 standards (modulation, channel spacing, antenna duplexing) to reduce hardware 2 quadrature upconverters driven by 450MHz LO to generate quadrature phases of IF signal IF signal routed to single-sideband mixers driven by a 1350MHz LO, producing either 900MHz or 1800MHz signal 2-Stage Conversion Example (#2) 1.75GHz Integrated Narrow-Band CMOS Transmitter with Harmonic-Rejection Mixers  Harmonic rejection mixer for IF up-conversion relaxes on- chip filtering requirements and even eliminates discrete IF filter 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 References . B. Razavi, “RF Transmitter Architectures and Circuits”, IEEE CICC, pp. 197-204, 1999. . A. Abidi, et. al., “The Future of CMOS Wireless Transceivers”, ISSCC, pp. 118-119, Feb. 1997. . J. Rudell, et. al., “Recent Developments in High Integration Multi-Standard CMOS Transceivers for Personal Communication Systems”, IEEE 1998. . S. Kim, et. al., “A Single-Chip 2.4GHz Low-Power CMOS Receiver and Transmitter for WPAN Applications”, IEEE 2003. . J. Weldon, et. al., “A 1.75-GHz Highly Integrated Narrow-Band CMOS Transmitter With Harmonic- Rejection Mixers”, IEEE Journal of Solid-State Circuits, Vol. 36, No. 12, Dec. 2001. . T. Liu, et. al., “5-GHz CMOS Radio Transceiver Front-End Chipset”, IEEE Journal of Solid-State Circuits, Vol. 35, No. 12, Dec. 2000. . B. Razavi, “A 900-MHz/1.8-GHz CMOS Transmitter for Dual-Band Applications”, IEEE Journal of Solid-State Circuits, Vol. 34, No. 5, May 1999. . R. Point, et. al., “An RF CMOS Transmitter Integrating a Power Amplifier and a Transmit/Receive Switch for 802.11b Wireless Local Area Network Applications”, IEEE RF IC Symposium, pp 431- 434, 2003. . S. Aggarwal, et. al., “A Highly Integrated Dual-Band Triple-Mode Transmit IC for CDMA2000 Applications”, IEEE BCTM 3.1, pp 57-60, 2002. . X. Li, et. al., “A CMOS 802.11b Wireless LAN Transceiver”, IEEE RF IC Symposium, pp. 41-44, 2003. . S. Mehta, et. al., “A CMOS Dual-Band Tri-Mode Chipset for IEEE 802.11a/b/g Wireless LAN”, IEEE RF IC Symposium, pp 427-430, 2003.
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