ECE 6361:Mixer Design Review

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ECE 6361Mixer Design Review

• 2.4 GHz Single Balanced Mixer
• Jan Brosi
• Vasileios Iliopoulos

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Project Objectives I

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Project Objectives II

• Board4-layer PPE (1.7x2.6 inches)
• Schottky diodes dual series connected

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Physical Construction I

• Reduced-size rat-race balun with coupled lines on
  different layers
• Radial stubs for matching at RF/LO frequency
• Lumped elements for matching at IF and for
  DC-return

27mm
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Physical Construction II

• Board parameters
• er2.85 0.5
• tand0.002-0.003
• core thickness1mm
• resin thickness50µm
• copper thickness12µm
• Diodes
• Zetex ZC2812ECT
• IS9.5nA, RS16.3Ohm
• N1.27, Cj01.1pF
• Lumped Elements
• Toko 2021 series chip inductors
• Panasonic chip capacitors

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Model Description Simulation

• Simulation with Agilent ADS using a combination
  of Multilayer Microstrip models
• Diode Model ADS P-N diode model with parameters
  from manufacturers data sheet
• Inclusion of parasitic elements (bends, steps, T-
  junctions, vias, pads, parasitics of lumped
  elements).
• Balun design with S-parameter simulation and
  gradient method optimization for best performance
• Mixer harmonic balance simulation with 8 orders
  and gradient method optimization

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Simulation Results

• Simulated circuit meets all specs

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Conversion Loss Measurement

• Measurement of conversion loss in down-conversion
  using the frequency offset function of HP89441
  Network analyzer
• The LO port is fed by E4432 Signal Generator with
  8.5dBm input power (assuming 0.5 dB cable loss)
• Use of a low-pass filter at the input port of the
  Network analyzer to ensure better phase-locking
• The filter response was calibrated out
• Accuracy of the measurement
• Signal generator 0.5dB
• Network analyzer 0.2dB
• Total accuracy 0.7dB

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Conversion Loss Measurement Results

• Conversion loss increases with increasing
  LO-frequency

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Spurious Isolation Measurement

• Measurement of spurious signals and LO-RF
  isolation for up-conversion using HP8994E
  Spectrum Analyzer.
• Measurement of LO-IF isolation for
  down-conversion
• Sweep over the LO frequency range
• Accuracy of the spurious signals measurement
• Relative accuracy of spectrum analyzer 0.5dB
• Accuracy of the LO-RF isolation measurement
• Absolute accuracy of spectrum analyzer 1dB
• Signal generator 0.5dB
• Total accuracy 1.5dB

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Spurious Isolation Measurement Graph
LO-IF
LO
RF
LO-2IF
LO2IF
LO-3IF
LO3IF
LO4IF
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Spurious Measurement Results
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Isolation Measurement Results
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Changes made to the Circuit

• Circuit assembled as designed has bad performance
  and does not meet any of the specs
• Slight change of inductor capacitor values and
  interchange of their position increased mixer
  performance
• The mixer seems to be shifted to lower
  frequencies, it works much better at fLO1.9 GHz
• Thus it was tried to reduce the size of the mixer
• Coupled lines were shortened by use of new vias
• Line edges were smoothened with copper material
  to reduce length
• Resulting circuit had best performance and was
  used for measurements

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Comparison to Simulations

• The measurements dont match with the simulations
• Trying to fit ADS-model to measurements by change
  of dielectric constant, distance between layers,
  coupled line offset, length and width of lines,
  stub length and width, vias and pads, losses and
  diode model.
• This should account for production tolerances and
  previously not considered effects
• Changes didnt have a significant effect on the
  simulated mixer performance
• Indication that ADS multilayer-model is not
  accuarate

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Summary Conclusions I

• A single-balanced mixer using a reduced-size
  rat-race balun with mulilayer coupled-lines was
  designed, simulated, fabricated and measured
• Great differences between model and measurements
  of the produced mixer occurred
• Performance could be increased by change of
  lumped elements and reducing the size of lines
• Specifications for Conversion loss are almost
  met, for Isolation and some spurious signals not
• Re-simulation was not successful

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Summary Conclusions II

• The ADS multilayer-model does not seem to work
  well in our case, another software or model
  should be tried
• Board parameters like layer thickness and
  dielectric constant should be reevaluated
• The mixer works much better at lower frequencies,
  the balun seems to be downshifted
• For the next production cycle, the line lengths
  should be decreased to account for that