power amplifier design using x-parameters in the ads simulation … · 2011-10-26 · x-parameter...

© Copyright Agilent Technologies 2011

EuMW 2011 – PA Design Using X-parameters

October 2011

Power Amplifier Design Using X-Parametersin ADS Simulation Environment

*X-parameters is a trademark of Agilent Technologies, Inc.

Mihai MarcuApplications EngineerAgilent Technologies, Inc.


What Will We Talk About

• Getting over the limitation of NOT having a reliable model

• Balancing measurements versus simulations– Understanding the strengths and shortcomings of each

• Improving on known, well-established design-methodologies– Using the best method where its strengths are– Using measurements to create reliable models (X-parameters)

• Creating a model for the system level design

• Emphasis on how X-parameter models can help


October 2011


A More Detailed Agenda

• X-parameter models for RF-power devices

• Importance of LSOP for X-parameter practice

• Finding the desired LSOP (Xpar with DC and RFpwr sweep)

• Fundamental load-pull

• Source-pull

• Harmonic load-pull – synthetic load-pull capabilities

• Intermodulation Distortion – HB and Envelope simulators

• Overall amplifier behavior

• X-parameter model of the amplifier for system-level


October 2011


Models Needed for Design Work

• Reliable models for power devices are rare

• Behavioral models are very useful (…if they are accurate…)– Use measurements to observe the behavior– Build models based on these measurements

• X-parameter models:– Measurement-based behavioral models– Model-extraction is automated in both HW (NVNA) and SW (ADS)– Accurately reproduce the behavior in the region of LSOP values for

which are extracted(LSOP: frequency, power, DC-bias, load, etc.)


October 2011


X-Parameters Solve Nonlinear Problems

• Same use model as S-parameters but much more powerful


October 2011

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October 2011

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October 2011

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X-parameters allow us to simplify the general B(A) relations:Trade efficiency, practicality, for generality & accuracyPowerful, correct, and practical

1 1 2 2( )) (i i iB S DC A S DC A The simplest X-parameters are just linear S-parameters


Sensitivity to Load Variations

• X(S) and X(T) are in fact sensitivities to incident waves– Responses to small variations around the LSOP

• They can be used to estimate the behavior around the load-impedance used during measurement

• This is synthetic load-pull

• Synthetic load-pull can be used for:– Any harmonic– Load- or source-impedance


October 2011

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Simplest X-Parameters of an Amplifier


October 2011

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October 2011

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“X-parameters unify S-parameters and Load-Pull”

Synthetic load-pull:

Complete load-dependence:


X-Parameters versus Compact Models

• X-parameters are NOT a replacement of compact models

• If a compact model is not available or is not accurate enough, then use X-parameters

• X-parameters:– Valid for the range of LSOPs used at extraction time– If LSOP is swept over a large region

• A very large and/or complex X-parameter model• Might be difficult to work with

• Balance the knowledge gathered through HW experiments to create a reasonable model for the task at hand


October 2011


What Is Used in This Discussion

• Proof of concept and methodology is discussed– All X-parameter models extracted in simulation from a compact model– Compact model provided by CREE

• CGH40010: 13 W operation up to 6 GHz

• In practice – all X-parameter models are measured with NVNA

• The same transistor from CREE – used in a hardware implementation based on this methodology– Results soon to be published

• Same methodology used for higher power (100 W) in other designs


October 2011


How Do We Start the Design Process

• Measure the X-parameters of the device

• Very important to measure at the “right” LSOP– Common issue: model measured at one LSOP, but used at a different

LSOP in the simulation

• LSOP is not initially known – we need an estimate


October 2011

A1

B1

A2

B1


Finding the Desired LSOP for the Device

• Three methods available

1) Usually supplied by the device-manufacturer

2) Run HW tests to identify the desired LSOP– Vary bias, load on various harmonics, source-impedance, etc.– Search for the desired operation– usually time consuming and expensive (especially if multi-harmonic

load-pull is performed)

3) Extract a simple X-parameter model– Swept RF-power, bias but only 50 Ohm – relatively fast process– Do all the preliminary investigations in simulation


October 2011


X-parameter Model for Defining Desired LSOP

• Support both DC and RF-performance– DC-sweep for gate and drain for a sizable range– RF-power sweep – with large power range– 50 Ω load only (synthetic load-pull ensures load-dependency)

• Include small-signal characterization– Have at least 1 power point in the small-signal region– Important if you intend to observe the DC-characteristics


October 2011


Use a Good Fixture for the DUT

• Proper measurement techniques– Fixture– Calibration– Stable environment– High-power setup

• The model must characterize the device itself


October 2011


X-Parameter Measurement on 50 Ω


October 2011

• DC supplies also needed (not shown here)


Select the DCOP

• Use the model to analyze various combinations and decide the class of operation for the device

• DC without RF – this is why we need one point at low power– At high power the DC is influenced by the RF


October 2011


RF Performance – Search for Desired LSOP

• Use the model to observe the behavior

• Vary all available parameters to decide which is the optimum region for the desired LSOP

• This initial model gives a good estimate of the expected performance

• Once the desired LSOP is identified a new, more accurate model will be extracted.

• This analysis (and initial model) is often not needed– Desired LSOP provided by the device manufacturer


October 2011


• Different values, parameter sweeps, etc., as needed

• only ZL1 changed here – but harmonics may also be varied

RF Performance – Search for Desired LSOPSimulation Setup


October 2011


How the Load Variation Works

• Sensitivities to small signal perturbations– Synthetic load-pull built-into the X-parameter model (XS and XT terms)– 50 Ω model (no load-pull during measurement)

• Works accurately for small perturbations around the LSOP– Load around the center of 50 Ω Smith-Chart

• Good estimate for larger perturbations– Load further away from the center of the 50 Ω Smith-Chart


October 2011


RF Performance – Search for Desired LSOPSimulation Results - Spectra

• Displayed here for ZL = 50 Ω


October 2011


RF Performance – Search for Desired LSOPSimulation Results – Power and PAE

• Displayed here for ZL = 50 Ω


October 2011


RF Performance – Search for Desired LSOPSimulation Setup – Fundamental Load-Pull

• Synthetic load-pull (X-parameters on 50 Ω)


October 2011


RF Performance – Search for Desired LSOPSimulation Results – Fundamental Load-Pull

• Synthetic load-pull (X-parameters on 50 Ω)


October 2011

Estimated ZL on fundamentalfor improved performance


Load-Dependent X-Parameter extraction

• A new model is extracted– More accuracy in the region of interest– Reduced number of points for bias– Sweep RF-power – enough range to give good coverage for equivalent

source-pull

• The new model is targeted towards the specific application– Ensures the model is kept to the minimum required complexity and size

(but still covering the necessary features)– This could be done from the beginning if device-manufacturer provides

the desired LSOP


October 2011


Load-Dependent X-Parameter Measurement

• DC supplies also needed (not shown here)

• Swept load a single model file is created automatically


October 2011

NVNA +Load-Pull


• The same load-pull setup, but the results are more accurate

RF Performance - Load-Dependent X-ParametersSimulation Setup – Fundamental Load-Pull


October 2011


RF Performance - Load-Dependent X-ParametersSimulation Results – Fundamental Load-Pull

• Optimum fundamental load for Pdel and PAE


October 2011


Select Fundamental-Load

• A compromise between Pdel and PAE

• If more parameters are requested (current, input-match, etc.)– Can be monitored in simulation environment– No additional measurements are required– The fundamental load-selection – a compromise between all desired

performance parameters

• In this case-study – optimize Pdel– PAE can be further optimized using harmonic-loading– Selected load: ZL1 = ( 20 + j*3.7 ) Ω


October 2011


Fundamental Source-Pull

• When X-parameters are extracted:– No source-pull performed– Source-power sweep is performed– Source impedance can be 50 Ω or any other value created by a source-

tuner

• For X-parameters: source-power sweep is equivalent to source-impedance pulling


October 2011


Source-Pull in SimulationDirect Zs variation

• Varying source impedance needs careful consideration– The impedance is the same across all frequencies– This would also have an impact on the practical meaning of Pavs

• Very different than what happens in HW on the test-bench


October 2011


Source-Pull in SimulationUsing an Ideal Tuner - Recommended

• Similar to what happens in the HW on the test-bench– Pavs is on 50 Ω, independent of Zs– Zs is set by the source-tuner, independent of Pavs– Zs can be specified for each harmonic, as needed (tuner characteristic)

• Real data from the HW-tuners can be specified here


October 2011

Pavs

Zs


Source-Pull Details

• The source tuner adjusts the power of the incident wave (A1)

Pavs = |Asrc|2

|Asrc|2 - |Bsrc|2 = |A1|2 - |B1|2 (loss-free tuner)

• A1 (hence the tuner) sets the LSOP of the DUT


October 2011

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Bsrc

A1

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Practical Approach for Source-Pull

• An exact optimum value for Zs is not necessary

• In the lab:– Search for a relatively good value of Zs (not necessarily the best)– Measure X-parameters with power sweep at that Zs-value

• This will accurately cover a full source-pull process in the simulation

• Saves lots of time and data in the HW measurement process


October 2011


• Using the load-dependent model

RF Performance – Fundamental Source-PullSimulation Setup


October 2011


RF Performance – Fundamental Source-PullSimulation Results

• Power level improved, PAE will further be improved using ZL2


October 2011


Select Source-Impedance on Fundamental

• Compromise between monitored performance parameters

• In this example, the following value was selected:

Zs1 = ( 6.1 - j* 8.7 ) Ω

• Expected performance:

Pdel = ~42 dBm

PAE = ~64%


October 2011


Harmonic Load-Pull

• Sensitivities to small signal perturbations– Synthetic harmonic-pulling built into the model (XS and XT terms)– 50 Ω on harmonics during model-measurement

(no harmonic-pulling during measurement)

• Works accurately for reasonable harmonic content– < ~16 dBc is equivalent to small perturbations around the LSOP– Practically works for the entire Smith-Chart

• Good estimate for larger harmonic-content


October 2011


Verification of Synthetic Harmonic Load-PullX-Parameter Measurement with Fundamental Only Dependency


October 2011

PNA‐X

Maury Software

DUT Maury Tuner

USB

DC Supply

GPIB

Bias Tees

NVNA Firmware

Maury Tuner

9 load states at f1

Cree CGH40010 GaN HEMT

10 W packaged transistor

• 900 MHz• Measure Load-dependent X-parameters vs power at 9 impedances

• 4 harmonics measured• probe tones at 2nd and 3rd

harmonics• harmonic impedancesuncontrolled

X-parameter -> ADS for independent validation

See refs [13] and [14]


Verification of Synthetic Harmonic Load-PullMulti-Harmonic Load-Pull in the HW (for Verification)


October 2011

PNA‐X

Maury Software

DUT Maury Tuner Z1

USB

DC Supply

GPIB

Bias Tees

NVNA Firmware

Maury Tuner

Maury Tuner Z2

Maury Tuner Z2

9 states 9 states

•Waveforms measured versus power at each set of 729 harmonic loads as controlled independently by the tuners.•Fundamental, second, and third complex impedances set independently

See ref. [14]


Verification of Synthetic Harmonic Load-PullLoad Set 1


October 2011

Courtesy of J. HornSee refs [13] and [14]

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X-parameter model Harmonic time-domain load-pull measurements

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October 2011

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October 2011

6 8 10 12 14 16 184 20

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Harmonic loads



• Using the fundamental-load-dependent model

Second-Harmonic Load-PullSimulation Setup


October 2011


Second-Harmonic Load-PullSimulation Results

• If left uncontrolled could impact:– PAE with ~12%– Pdel with ~1 dBm


October 2011


Selecting Load on Second-Harmonic

• Only Pdel and PAE monitored in this example

• ZL2 little impact on Pdel

• ZL2 significant impact on PAE

• ZL2 selected to maximize PAE:

ZL2 = ( 3 + j*21 ) Ω


Pdel = ~43 dBm

PAE = ~71%


October 2011


Third-harmonic Load-Pull

• Could performance be further optimized?

• X-parameter model supports load- and source-pull up the harmonic order included in the model– User has control at time of model extraction– Only fundamental tuner is needed at time of extraction– At time of simulation – synthetic harmonic load-pull


October 2011


Third-Harmonic Load-PullSimulation Setup

• Using the fundamental-load-dependent model


October 2011


Third-Harmonic Load-PullSimulation Results

• If left uncontrolled could impact:– PAE with ~10%– Pdel with ~1 dBm


October 2011


Selecting Load on Third-Harmonic

• Only Pdel and PAE monitored in this example

• ZL3 little impact on Pdel

• ZL3 significant impact on PAE

• ZL3 selected to maximize PAE:

ZL3 = ( 5.5 - j*50 ) Ω


Pdel = ~43.5 dBm

PAE = ~75%


October 2011


Best Expectation Performance

• Desired device loading:

• Expected performance for this ideal loading:

Pdel = ~43.5 dBm

PAE = ~75%


October 2011


Practical Expectations

• In a practical design – difficult to comply to all loads– The X-parameter model – will capture performance variation due to

imperfect-compliance to the ideal loading– The imperfect compliance might be significant, but the performance

degradation might be acceptable versus the trade-offs

• A study of some practical configurations lead to the conclusion that a loading as shown here may be feasible for practical implementation:


October 2011


Expected Practical PerformanceSimulation Setup

• In practice – loading on other harmonics ≠ 50 Ω– X-parameter model takes into consideration this mismatch


October 2011


Expected Practical PerformanceSimulation Results – Spectral Content


October 2011


Expected Practical PerformanceSimulation Results – Pdel, PAE


October 2011

• Degraded performance due to no-optimal load-impedance

• Complete loading should be done iteratively– Iterations between load and source– Load was changed – source should be re-analyzed– a new source-pull is required


Fundamental Source-Pull – A Second IterationSimulation Results

• Good performance obtained for 50 Ω source– Simplifies the design


October 2011


Expected Practical Performance

• Device loading:



October 2011


IMD PerformanceEnvelope Simulation

• Current implementation of X-parameters – no dynamic memory effects

• Envelope simulation can be used with good accuracy for multi-tone stimulus if device has no significant memory effects

• Using envelope significant advantages:– Any modulated signal can be used as stimulus

• Possible to monitor performance for modulated signals: ACLR, EVM– Co-simulation with system-level signal-processing - supported in ADS


October 2011


IMD PerformanceEnvelope Simulation Setup• All stimuli and responses – within simulation bandwidth


October 2011


IMD PerformanceEnvelope Simulation Setup - Loading• Identical with the one for 1-tone HB


October 2011


IMD PerformanceEnvelope Simulation Results• IMD results


October 2011


IMD PerformanceHB Simulation

• Works accurately for devices with dynamic memory effects

• Requires multi-tone X-parameter models:– 2-tones supported in the latest HW-release IMD measurements– The model is more complex

• X-parameters predict accurately the phase of IMD

• Advantages:– Very accurate– Efficient simulation

• Limitation – CW signals only


October 2011


IMD PerformanceHB Simulation Setup

• Multi-tone setup


October 2011


IMD PerformanceHB Simulation Setup - Loading• Proper loading for all large signals


October 2011

…if ( freq==Freq1 || freq==Freq2 ) then ZL1 else …


IMD PerformanceHB Simulation Results – Ideal Loads• IMD results – asymmetry for some loads

– Accurately captured by HB


October 2011


IMD PerformanceHB Simulation Results – Practical Loads• IMD results for expected implementation


October 2011


The Design

• Use known methods to implement the bias and matching networks

• Use the X-parameter model for all simulations– A very flexible design environment

• Design the layout– Use EM-simulations where needed– Co-simulate layout components (EM-results) with the X-parameter

model easy to observe complex interdependencies

• Complete all phases for implementation:– Simulation, LVS, DRC– Various design iterations with minimal cost-impact


October 2011


The DesignSchematic

• An example of such implementation

• Practically implemented and tested– Simulations versus measurements– Results to be published soon


October 2011

Measured X-parameters


The DesignLayout

• Usual ADS-layout methods


October 2011


The DesignThe Layout - A 3D-View

• EM-simulation (Momentum or FEM)– Co-simulate with X-parameters high accuracy


October 2011


The DesignSimulation Results – Spectral Content

• Very close to the results with desired loading

• Take into consideration all known effects– Filtering effects of the low-pass matching network


October 2011


The DesignSimulation Results – Pdel and PAE• Very close to the results predicted by the simplified loading


October 2011


Physical Implementation

• A physical implementation was done based on such a process

• A physical device measured with the NVNA (X-parameters)

• Design performed with this model

• Results to be published soon

• A brief review shown here


October 2011


Physical ImplementationThe Amplifier


October 2011


Physical ImplementationSimulation versus Measurement Results


October 2011


X-Parameters for the Amplifier

• As the amplifier is manufactured – the design can be used for higher-level system design

• X-parameters for the overall amplifier extracted in the simulation environment– Preserve IP– System-level performance can be shared outside the group/company

• When the amplifier-HW is available, model can be directly replaced with the measured (HW-extracted) X-parameters


October 2011


X-Parameters for the AmplifierSimulation Setup

• In the PA-schematic – measured X-parameters of the device


October 2011


X-Parameters for the AmplifierSimulation Setup Guidelines

• Small variations around the desired LSOP at the system-level– System-designers can study the impact on system-performance

• Examples of small variations:– Power supply– RF-power– Source and load impedances

• In general, load-dependent X-parameters are not necessary:– Fully supported, if needed, but…– System-load varies around a known, desired– Covered by synthetic load-pull (XS and XT parameters)

• Resulting model is, in general, relatively simple and easy to use


October 2011


• The entire design process is supported by X-parameter models

• X-parameter model – not a replacement of the compact model:– Multiple X-parameter models may be used along the design process– Each step of the design may use a model tailored for it simpler model

• Find the right balance between measurement time and model complexity

• ADS can handle very large and complex X-parameter model-files

ConclusionsX-Parameter Model Setup


October 2011


• Set up the device-measurement on the NVNA and use it interactively during the design process:– Each step is less time-consuming– Get results faster than with other methods– Balance device-measurements and simulations for the optimum

development time

• Very good integration with system-level design (IP protected)– The process continues at the system-level– Models for sub-systems can be refined as the design progresses– Simulation and/or measurement based X-parameter models– X-parameters co-exist with all other models

ConclusionsX-Parameters – an Integral Part of the Design Process


October 2011

power amplifier design using x-parameters in the ads simulation … · 2011-10-26 · x-parameter...

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