BasebandEquivalent Model for System Simulation of RF Circuits

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Baseband-Equivalent Model for System Simulation
of RF Circuits

• Dennis Yee
• Robert Brodersen
• University of California, Berkeley

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Receiver Performance Issues
wireless environment weak desired signal strong
adjacent interferers
circuit nonidealities noise distortion
system requirement signal-to-noise ratio

• Examples of other receiver nonidealities

close-in phase noise
ideal QPSK constellation
I/Q quadrature phase mismatch
I/Q gain mismatch
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Framework for System/Hardware Codesign

• Simulation to examine system/hardware interactions

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Simulation Issues

• RF signals occupy a narrow bandwidth relative to
  carrier
• Example 25MHz W-CDMA signal centered at 2GHz
• Simulation sampling rate determined by Nyquist
  Sampling Theorem
• fs gt 2fRF
• Simulation of RF signals can be really, really
  slow

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Baseband-Equivalent Model Introduction

• Represent any real signal as

• Example an ideal RF signal centered at wc with
  baseband I and Q components, sI1(t) and sQ1(t)

• Eliminate dependence on wc by keeping track of
  time-varying coefficients sDC(t), sIn(t), and
  sQn(t)

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Baseband-Equivalent Model LNA

• LNA transfer function

• Includes gain, DC offsets, nonlinearities
• Let xi(t) and xo(t) have the form

• Solve for xoDC(t), xoIn(t), and xoQn(t) in terms
  of xiDC(t), xiIn(t), and xiQn(t)
• Sample time based on symbol period
• Complexity depends on number of harmonics

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Baseband-Equivalent Model Mixer

• Mixer transfer function

• Let yi(t), yLO(t), and yo(t) have the form

• Solve for yoDC(t), yoIn(t), and yoQn(t) in terms
  of yiDC(t), yiIn(t), yiQn(t), yLODC(t), yLOIn(t),
  and yLOQn(t)

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Baseband-Equivalent Model LO

• Model accounts for phase offsets, frequency
  offsets, phase noise and frequency spurs of the
  LO
• Example in-phase LO with frequency offset Dw and
  phase offset Df

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Simulink Implementation

• Baseband-equivalent model described by
  S-functions
• S-functions written in C for compatibility with
  RTW
• N 3

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Simulink Example