1. Sub – 1 Ohm Broadband Impedance Matching Network
Design Methodology for High Power Amplifiers
W. McCalpin
dBm Engineering, Inc., Boulder, CO
dBm Engineering
5446 Conestoga Ct.
Boulder, CO 80301
dBmEngineering.com
September 18, 2018

2. High Power Amplifier Design Challenges
A fixed-tuned broadband 50-Ohm test fixture is required with the following design constraints:
Prototype RF Power Transistor: Single-Ended 250W Pulsed LDMOS part
The design bandwidth required is 15% (1200 to 1400 MHz) as compared to ~ 3% for Wireless bands
High Power Pulsed Load Pull measurements have produced sub-1 Ohm target Load / Source impedances (ZLoad=0.8 – j1.0) to be held nearly constant over the bandwidth of interest
September 18, 2018

3. Prototype 250W Pulsed Power LDMOS Transistor
September 18, 2018

4. Outline Motivation Load Pull Impedance Accuracy
Proposed Simulation / Design Method
Matching Network Topology Selection
Simulation of “Ideal” Topology / Trajectory
Conversion of “Ideal” Circuit Simulation to a Physically-based Circuit Simulation
Fabrication of TRL Verification and DUT fixtures
Comparison of Measured vs. Simulated Trajectory and ZLoad
Summary
September 18, 2018

5. Is the simulation / design process wrong?
Motivation
Q: Why is it difficult to go from High Power Load Pull measurements to working hardware?
Q: Why do RF Power engineers still commonly rely heavily upon intuition, tuning and iteration?
Is Load Pull wrong?
The target impedances generated from doing Load Pull may not be accurate enough to truly represent what the DUT actually sees.
and / or
Is the simulation / design process wrong?
Common simulation techniques may not produce hardware that faithfully recreates the target impedances (when the designer fabricates the physical circuit from the simulated circuit).
September 18, 2018

6. Load Pull Tuner Impedance Accuracy
(calibrated impedance vs. actual impedance during measurement)
e.g. for a ZCal point, what is the Output Tuner’s Load reflection coefficient when ZMeas is normalized to the ZCal point
ZMeas
ZCal
50 Ω
ZMeas
September 18, 2018

7. Load Pull Tuner Impedance Accuracy (cont.)
Focus Microwaves - PMT Load Pull System
The Tuner by itself: *
50 Ω
ZMeas
Tuner
Tuning to the indicated points:
Worst Case R.L.(dB) meas. = ~ - 51 dB
Tuning repeatability:
Worst Case R.L.(dB) meas. = ~ - 47 dB
(Based on a TRL 7/16” Connectorized VNA calibration with Sii ≤ -60 dB)
September 18, 2018

8. ZLoad Impedance Accuracy at the DUT plane
The impedance presented to the DUT:
Worst Case R.L.(dB) meas. = ~ - 31 dB
Tuning to the indicated points:
Tuning repeatability:
50 Ω
ZMeas
Load Pull
Test Fixture
Tuner *
(Based on a TRL Impedance Transforming PCB VNA calibration with Sii ≤ -41 dB)
September 18, 2018

9. Load Pull Tuner Impedance Accuracy (cont.)
Given the proper discipline in calibration and setup, the accuracy of the Load Pull targets is very good
Load Pull targets should be useful for design as well as device characterization
September 18, 2018

10. Proposed Simulation / Design Method
Ideal impedance matching network topology / trajectory simulation (using circuit models and lumped elements)
Identification of the circuit response to tuning variations
Conversion of distributed elements to EM based multi-port S-parameter blocks
Conversion of lumped element models to measured one-port and two-port S-parameter blocks
Simulation refinement and design centering
September 18, 2018

11. Proposed Simulation / Design Method (cont.)
Design and fabrication of a TRL verification fixture and the break-apart DUT impedance matching fixture
Assembly of each half of the DUT fixture with component by component verification of the impedance matching trajectory using the TRL verification fixture
Full DUT fixture assembly
Top-level tuning and design centering
Break-apart measurement of the final Load and Source impedances.
September 18, 2018

12. Ideal Topology / Trajectory Simulation
Output Matching Network
Z0 = 4.48 Ohms
50 Ohms
September 18, 2018

13. Conversion of Distributed Elements to EM based Simulation (using Momentum)
Generates a 13 port .s13p file to import into ADS to combine with the measured .s1p files of the components
September 18, 2018

14. Conversion of Lumped element circuit models to measured s1p and s2p files
Measured using 50-Ohm TRL standards either wafer-probed (preferred) or connectorized (as shown)
Using the application substrate and component mounting configuration to include all substrate parasitic effects
Commercially available Model Libraries are ideal – parameterized to substrate material, full range of values by product type, accurate and wideband to include harmonic frequencies, etc.
September 18, 2018

15. Design and Fabrication of a Break-Apart DUT Impedance Matching Fixture
ZLoad
Fabricated to be Break-apart using the substrate selected for the application
September 18, 2018

16. Design and Fabrication of an Impedance Transforming TRL Verification Test Fixture
Fabricated on the substrate to be used in the application with a line width equal to the lead width of the DUT (500 mils)
September 18, 2018

17. Measurement of the DUT Break-apart fixture using the TRL Verification fixture
ZLoad
After TRL calibration using the Verification fixture, multiple measurements can be made as each lumped component is added
September 18, 2018

18. Simulated vs. Measured ZLoad Impedance Trajectory
September 18, 2018

19. September 18, 2018

20. Invest in your Methodology!
Summary
Invest in your Methodology!
High Power Load Pull is useful for design as well as characterization
Choose a topology that will hit the required impedance targets and achieve the desired matching network trajectory
Replace the ideal circuit elements with EM and measurement based elements as early as possible
Verify each simulated step by measuring at every step
Know the impacts of tuning locations and keep them small
Center and finalize the tuning and
measure the final impedances
Improve the process
September 18, 2018

21. References
[1] D. Williams and D. Walker, “On-Wafer Measurement Accuracy”,
ARFTG Short Course on Measurements and Metrology for RF
Telecommunications, November 2000
[2]
September 18, 2018