RF Transmitters

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

				
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