ECE1390 RF L6 - Mixers.pdf

1 Down-Conversion

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2 Mixer Classes

2.1 Unilateral Mixer

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  • Input Impedance independent of load
  • transistors in saturation and require bias current (high flicker noise)
  • performance depends on and intrinsic gain () of transistor

2.2 Bilateral Mixer

  • Input Impedance dependent on load
  • transistors operate in triode (low flicker noise)
  • performance depends on and minimum channel length
Voltage Mode Current Mode Mix Mode
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- needs voltage LNA at output
 - needs current LNA at output
- needs TIA (RX only)		 - impedance matching with mixer load

3 Current Passive Mixer (RX)

  • best choice for low power solutions with traditional topology
    • performance scales with technology
    • can be used in mixer first architecture
    • requires current source (shared with LNA)

3.1 Noise

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3.2 Modeling

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  • parasitic capacitances appear as shunt switched capacitor resistances at output

3.3 Impedance

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3.4 Quadrature Operation

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  • needs 25% duty-cycle LO
  • 3dB lower conversion gain
  • cross-talk between I and Q for Cpar

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  • leads to asymmetry in transfer function, noise and cross-talk
  • asymmetry can be avoided with minimal Circuit Loading

4 Trans-Impedance Amplifier for Bilateral Mixer

4.1 Common-Gate Stage

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4.2 Op-Amp Configuration

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  • very low input impedance
  • large voltage swing at output

Self-Oscillating Mixers

  • merges local oscillator and mixer
  • evaluated by it's flexibility, more compact = lower flexibility

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LC Tank

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  • RF signal is injected by bias current, down-converted to DC, then filtered out by LC tank
  • oscillates if C >>

Bias Split

  • conversion gain

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