Downconversion Mixers 101

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Downconversion Mixers 101

• Brian N. Limketkai
• Robert W. Brodersen
• University of California, Berkeley

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Why Do We Need Mixers?

• Besides giving me something to do
• Frequency translation of signal
• can modulate different parts of the spectrum thus
  allowing multiple channels (TV, radio, etc)
• make antenna designs practical
• higher frequencies require shorter antennas
• move signal to bands where there is less
  attenuation in the medium
• chopper stabilizer
• avoids flicker noise

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Frequency Translation

• Frequency translation requires some non-linearity
• Linear systems can only output frequencies which
  are present at the inputs
• Most mixers are basically multipliers
• Downconversion mixers keep the low-frequency terms

Linear System
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Basic Switching Mixer Topologies
Multiply RF signal by a mixing function
Topology
Mixing Function
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Gilbert Multiplier

• Most commonly used mixer topology
• RF signal multiplied by a bipolar square wave

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How Mixing Affects Noise

• Noise folds over during downconversion
• Signal also increases in a direct-conversion
  system

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Noise in the Receiver Chain

• Noise of blocks close to the input affect overall
  SNR more than blocks further down the chain
• Mixer noise significantly affects overall SNR

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Summary

• Most non-linearities can be used to make a mixer
• Switching mixers are more digital-like and
  hence more tolerant to errors in device models
  than other types of mixers
• Mixers can be a significant contributor to the
  total noise output
• Larger gain in the LNA helps