Single Balanced Mixer Design ECE 6361

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Single Balanced Mixer Design ECE 6361
July 30, 2002 Prof. J. S. Kenney

• L01 Group1
• Dennis Mahoney
• Adam Toner
• Minsik Ahn

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Objectives

• Single-balanced mixer
• 2.400-2.485 GHz RF, 2.260-2.345 GHz LO, 140 MHz
  IF (5 MHz bandwidth)
• 4-layer PPE printed circuit boards
• 2.6 by 1.7
• Upconversion and downconversion

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Description of Design

• Balun was realized as a 380 mil long broadside
  coupled line on layers 1 and 2.
• RF was fed into diode through a Wilkinson power
  divider for balance.
• Microstrip stubs were used for reflective
  terminations.

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Alternate Design

• Similar design, but balun outputs on single layer
• Simulated conversion loss 8 dB
• Measured conversion loss 10.5 dB
• Tapered LO input and moved vias in CAD
• LO-RF transmission null centered at 2.8 GHz
  instead of 2.4 GHz indicating poor balun
  performance
• Poor RF port return loss

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Simulation

• Design, simulation, optimization, and layout in
  Agilent ADS 2001
• Harmonic balance simulation and optimization with
  seven harmonics
• Curve fit diode for n, Rs Cj from datasheet

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ADS Simulation Model
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Wilkinson Power Divider in ADS
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ADS Simulation Results Downconverter
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ADS Simulation Results Upconverter
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Mixer Layout
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Specifications and Results Downconverter
Actual Results (Simulated Results) RF2400 MHz RF2442 MHz RF2485 MHz Specification
Conversion Loss 8.9 dB (9.325 dB) 9.5 dB (9.253 dB) 11.2 dB (9.456 dB) lt 9.5 dB
LO-RF Rejection 29.4 dB (33.545 dB) 33.3 dB (49.606 dB) 38.0 dB (30.543 dB) lt -20 dB
LO-IF Rejection 36.7 dB (58.052 dB ) 40.1 dB (42.908 dB) 43.3 dB (54.393 dB) lt -30 dB
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Specifications and ResultsUpconverter Spurious
Response
Actual Results (Simulated Results) RF2400 MHz RF2442 MHz RF2485 MHz Specification
2400-2485 No Spurs (No Spurs) No Spurs (No Spurs) No Spurs (No Spurs) -50 dBc
2485-2900 MHz -24.3 dBc at 2.520 GHz (-43.461 dBc at 2.520 GHz) -25.4 dBc at 2.582 GHz (-49.912 dBc at 2.582 GHz) -28.4 dBc at 2.765 GHz (-48.305 dBc at 2.765 GHz) -40 dBc
2325-2400 MHz No spurs (No spurs) No spurs (No spurs) Only LO (Only LO) -40 dBc
1000-2120 MHz -21.9 dBc at 1.980 GHz (-24.28 dBc at 1.840 GHz) -22.0 dBc at 2.022 GHz (-38.619 dBc at 1.882 GHz) 22.2 dBc at 2.065 GHz (-32.607 dBc at 1.925 GHz) -30 dBc
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Measurement Notes

• Downconverter conversion loss measured with VNA,
  accuracy 0.2 dB
• Increased signal generator amplitude to
  compensate for measured cable loss
• Frequency offset mode, calibrated IF output with
  filter and cables, connected RF directly to test
  port
• Upconverter conversion loss measured with
  spectrum analyzer, results were better than
  downconverter but accuracy was 1 dB
• Spurious response measurements with spectrum
  analyzer set to appropriate RBW and span
• Verified IF bandwidth (gtgt 5 MHz) with swept
  input, measured output on spectrum analyzer

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Problems

• Simulated without vias
• Vias greatly increased simulation time
• Performance margin built into design
• One via to ground not drilled
• Repaired by drilling, inserting wire, soldering
• Top balun trace not centered
• Decreases coupling efficiency, increases
  conversion loss and spurious products
• Trace moved in CAD

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Problems (continued)

• Simulated with straight reactive terminations,
  fabricated curved stubs
• Mixer operating range shifted down in frequency
• Conversion loss improved from simulation in low
  end but increased in high end
• Low end spurs increased to out-of-spec from
  simulation (stub tuning offset down)
• Simulated with nominal dielectric constant

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Improvements

• Increase spacing around stubs to reduce parasitic
  coupling
• Reduce loss in RF input bends
• Optimize further in ADS
• Limited time for first prototype
• Agilent diode improved conversion loss in
  simulation by 3 dB
• Investigate alternate design problems

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Conclusions

• Two designs fabricated
• One was consistent with simulation
• Conversion loss for alternate design was 3 dB
  worse than simulation
• Primary design met most specifications
• Some conversion loss and spurious specs missed
• Room for improvement in both designs
• Can reduce size and improve performance