RF Transmitters

1. RF Transmitters Architectures for Integration and Multi-Standard Operation Terry Yao ECE 1352

2. Outline • Motivation • Transmitter Architectures • Current Trends in Integration • State-of-the-Art Examples (3) • Direct Conversion • 2-Stage • Future Challenges • References

3. 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 multipleRF standards

4. Function Modulation Frequency Translation Power Amplification RF Transmitters Performance Specification Accuracy Spectral Emission Output Power Level

5. 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

6. 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

7. 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

8. 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

9. 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)

10. 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 concurrentlyto enable hardware and possibly power sharing

11. Direct Conversion Example • A 5-GHz CMOS transceiver frontend chipset [6] • 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

12. 2-Stage Conversion Example • 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 • A Dual Band (GSM 900-MHz/DCS1800 1.8-GHz) CMOS Transmitter [7]

13. 2-Stage Conversion Example (#2) • 1.75GHz Integrated Narrow-Band CMOS Transmitter with Harmonic-Rejection Mixers [5] • 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

14. 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

15. References [1]. B. Razavi, “RF Transmitter Architectures and Circuits”, IEEE CICC, pp. 197-204, 1999. [2]. A. Abidi, et. al., “The Future of CMOS Wireless Transceivers”, ISSCC, pp. 118-119, Feb. 1997. [3]. J. Rudell, et. al., “Recent Developments in High Integration Multi-Standard CMOS Transceivers for Personal Communication Systems”, IEEE 1998. [4]. S. Kim, et. al., “A Single-Chip 2.4GHz Low-Power CMOS Receiver and Transmitter for WPAN Applications”, IEEE 2003. [5]. 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. [6]. T. Liu, et. al., “5-GHz CMOS Radio Transceiver Front-End Chipset”, IEEE Journal of Solid-State Circuits, Vol. 35, No. 12, Dec. 2000. [7]. 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. [8]. 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. [9]. S. Aggarwal, et. al., “A Highly Integrated Dual-Band Triple-Mode Transmit IC for CDMA2000 Applications”, IEEE BCTM 3.1, pp 57-60, 2002. [10]. X. Li, et. al., “A CMOS 802.11b Wireless LAN Transceiver”, IEEE RF IC Symposium, pp. 41-44, 2003. [11]. 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.