Highly Efficient Design of Microwave Circuits Using Coupled EM ...

Highly Efficient Design of
Microwave Circuits
Brad Brim
Applications Engineer, Ansoft Corp.

Highly Efficient Design of
Microwave Circuits :
Application of EM/Circuit Codesign
Techniques

Objectives
1. Introduce and provide an intuitive understanding
of an Efficient Design Flow for Microwave Circuits,
“EM/Circuit Codesign”.
2. Demonstrate and explain through a set of
easy-to-understand and realistic examples.
3. Provide motivation to consider application of
EM/Circuit Codesign for microwave circuits
based on a clear understanding of the benefits.

EM/Circuit Codesign Examples
1. obvious MDL monopulse comparator
2. straight forward RS Microwave iris filter
3. more difficult couplers & filters
1
Ch.2
Input
2
Ch.1
3

� EM-only Design
Design Flows
� Benefit: accuracy, generality
� like using only one hand
� Circuit-only Design
� Benefit: speed, higher level analyses
� like using the other hand
� EM/Circuit Codesign
� Benefit: efficiency, accuracy, re-use
� like using both hands together

EM Circuit
Analyze
Parametric
Components
Draw
Composite
Geometry
EM/Circuit Codesign Flow
Synthesize
Circuit
Materials
Boundaries
Ports
Analysis Setup
user-directed loop
automated loop
Analyze, Tune
and Optimize
Circuit
automated loop
Analyze and
Optimize
Plot Circuit
Results
Plot Results
Examine Fields

Enabling Concepts
1. Definition of S-parameters
� There is a definitional difference in S-parameters between
circuit and EM analyses.
2. Parametric Component Models
� Automated schematic-level circuit design requires access to
parametric component models or libraries
3. Arbitrary Transmission Lines
� Circuit design requires TL components
� Circuit tools support only: coax, MS, SL, RWG
4. Matched Ports
� Microwave design applies matched ports,
which are not 50 ohms and are typically frequency dependent

Definition of S-parameters
� Circuit Analysis
� Y-parameter based modified nodal analysis
� S-parameter data is converted to Y using Zo
� Y-parameter results are converted to S using port reference
� Electromagnetic Analysis
� variations exist amongst tools but S-parameters are mostly
wave based as amplitudes of incident and reflected modes
� To summarize
� Circuit Analysis defines S11 = 0 for a conjugate match
� EM Analysis applies S11 = 0 for a matched load
� A difference not easily overcome

S-parameter Definition
an example
a trick you can apply to extract Zo from a built-in TL component:
Zo = sqrt(Zin_open*Zin_short)
NOTE:
very lossy line,
with complex Zo

S-parameter Definition
unexpected results for EM designers!
(expected loss, unexpected reflection)
NOTE: re_Zo & im_Zo from previous slide
an example

S-parameter Definition
a simple solution
Ansoft Designer optionally uses the EM definition for Circuit analysis,
which enables results to be viewed and optimization goals to be defined
in a more familiar manner

S-parameter Definition
more familiar results for EM designers!
“matched ports”:
no reflection, transmission loss
the same example

S-parameter Definition
the implications
If you always use real Zref you are not affected by
this issue if (and only if)
� your renormalize all EM data in the EM tool to a
real Zref for lossy lines and cutoff modes
� internal and external ports must be renormalized
implying a complication of optimization goals
This issue is “rediscovered” periodically and may well
be the basis of industry-wide under utilization of
EM/Circuit codesign techniques, especially for
circuits involving cutoff modes.

Parametric Component Models
� Parametric EM data models are required in circuit
analysis to perform “design”
� Ansoft Designer can access parametric HFSS
sub-circuit components directly from HFSS projects
tuner gap parameter
G1
H1
tap height parameter

Inserting an HFSS Component
from Ansoft Designer
1. menu item “Circuit � Add SubCircuit � Add HFSS NPort Design”
2. select HFSS project, design and analysis/sweep
3. click mouse to place component in the schematic
1 2 3

Arbitrary Transmission Lines
� Arbitrary Transmission Lines are required to enable
circuit analysis of many microwave devices
� Ansoft Designer can access parametric HFSS port
data to provide arbitrary multi-moded T-Lines
“S1” is Line length

Inserting an HFSS TL Component
from Ansoft Designer
exactly the same as shown for a 3D HFSS component
� specify “Transmission line model” and select HFSS port
� multiple modes are accommodated automatically (3 in this case)

Port Reference Impedance
� Circuit Analysis uses 50 ohm ports by default
� no concept of “matched” ports,
which yield “generalized S-parameters”
the default behavior for HFSS wave-ports
� Tedious to apply Circuit frequency dependent Zref
� equations, circuit file or subcircuit based
� potential issues with parameterization
� many circuit tools fail with imaginary Zref
� (i.e. cutoff modes)
� desire intuitive design specifications
� difficult to specify RWG coupler or filter goals in 50 ohms

Matched Ports for
Arbitrary Transmission Line
TL and Zref from HFSS:
completely automated and general
even across cutoff! (at 10 GHz)
NOTE: correspondence of component ID “23” in the
matched port reference impedance specification.

Enabling Concepts Wrap-Up
1. Definition of S-parameters
2. Parametric Component Models
3. Arbitrary Transmission Lines
4. Matched Ports
� Ansoft Designer to HFSS Dynamic Link
� HFSS and Ansoft Designer have both been augmented to
address all these issues
� A unique contribution unmatched by any other
combination of tools from any vendor(s)
� EM/Circuit Codesign is uniquely enabled and
completely automated

Efficient Coupler Design
� Couplers to be examined
� 2-hole (narrowband), 16-hole (broadband)
� Discussion
� EM-based design
� Circuit based design
� EM/Circuit Codesign

An Isolated Coupling Hole
the ‘one’ HFSS component
Coupling Hole HFSS simulation time ≈ 60 sec
(includes refinement and frequency sweep time for each unique hole radius)

2-hole RWG Coupler
Design Objectives
� -30dB forward coupling at 15GHz (S41)
� no reflection & reverse coupling (S11, S31)
Initial Design Parameters (quick approximations)
� R1 half power from each hole (3mm)
� S1 quarter wavelength separation (7mm)

2-hole RWG Coupler
Circuit optimization in only seconds in Ansoft Designer
by applying Dynamic Link HFSS components and TLs.
Optimization Goals

Circuit-level design parameters
R1 = 2.89254 mm
S1 = 6.57886 mm
HFSS verification performed
simulation time ≈ 2.5 min
2-hole RWG Coupler
GOOD: Coupling within 1.3 dB while nearlymeeting
bothreflection and reverse coupling isolation goals!

2-hole RWG Coupler
HFSS optimization performed with new SNLP optimizer
same optimization goals, desired response after 6 steps
HFSS verification and optimization in less than 15 minutes
R1 = 3.0059 mm
S1 = 6.6275 mm
BETTER: -30dB forward coupling while meeting both
reflection and reverse coupling isolation specs!!!

16-hole Coupler Design
� broad bandwidth objective
� symmetry implies: 8 hole radii, 8 separations

16-hole Coupler Design
� circuit analysis time of seconds despite much larger circuit
� first perform circuit-level random optimization
� then gradient optimization from best case result

3D view
16-hole Coupler Design
HFSS design with
circuit-optimized parameter values
side view
end view
top
view

Circuit
Prediction
AWESOME!!!
Design is Completed!
16-hole Coupler Design
EM
Verification
HFSS analysis results

top
side
Cutoff Mode S-parameters
an EM/Circuit Codesign Example
an evanescent mode attenuator
and its step-in-height/width “component”
attenuator
step
top
side

Cutoff Mode Attenuator
HFSS results for step component
deembedded to common face where waveguides meet
∑
1
= i n
i=
NOTE: Sij
≠ 1
2
Port 2 cutoff (im_Zo) transmission phase 45 deg

Cutoff Mode Attenuator
EM and Circuit analysis yield the same results for the attenuator
using HFSS Dynamic Link step component and TLs.

Cutoff Mode S-parameters
You can perform circuit-level design with cutoff modes!!!
(despite what you might read in literature or hear at conferences)
There is confusion concerning S-parameters for cutoff modes
� stems from EM vs. Circuit S-parameter definitional difference
� For cutoff mode loss free devices:
if ‘n’ includes only loss free
propagating modes/devices,
then the sum is 1.0
∑
1
= i n
i=
Sij
2
≠
1

Resonator Filter Design
� Comb-Line Resonator Filters
� capacitively loaded resonators
� in a cutoff rectangular waveguide

HFSS Parametric Components
� 3D components
� round-to-square coax, tap-fed resonator, isolated resonator
� Transmission Lines (not shown) from 3D component ports
� round coax, rectangular coax, multi-moded cutoff waveguide

� Ansoft Designer
2-resonator Filter
Circuit-level Verification
� circuit analysis of two back-to-back HFSS components,
each half-filter with multi-moded cutoff rectangular waveguide

Only 3 Modes Required
Accumulated dB(S11) plot for {5,4,3} modes
� HFSS effectively shorts modes
not specified, as emulated here
using Ansoft Designer
5 modes, 4 modes
3 modes

3-resonator Filter Design
High Level Filter Goal
� -20dB bandwidth of 10%
� center frequency of 10.0 GHz
Detailed Filter Behavior (as tuned during manufacturing)
� bandwidth controlled mostly by S2, partially by S1
� center frequency controlled by G2
� passband match controlled by G1

3-resonator Filter Design
� Ansoft Designer Tune capability applied to quickly find
� S1 = 200 mil
� S2 = 234 mil
� G1 = 12.5 mil
� G2 = 19.5 mil

3-resonator Filter Design
AMAZING CORRESPONDENCE !!!
HFSS verification of circuit-level filter design
� Bandwidth 10%
� Center frequency 10.03 GHz

Wrap-up
� Ansoft Designer and HFSS have been augmented
to uniquely support an EM/Circuit Codesign flow.
� flow is applicable to a broad class of microwave designs
� each of four “Enabling Concepts” must be addressed to
implement such an efficient design flow
� Impressive correspondence between Circuit level
design and full EM analysis.
� even for cutoff modes
� Benefits
� huge savings for total design time
� tuning, optimization and synthesis techniques available

Stated Objectives Were
1. Introduce and provide an intuitive understanding
of an Efficient Design Flow for Microwave Circuits,
“EM/Circuit Codesign”.
2. Demonstrate and explain through a set of
easy-to-understand and realistic examples.
3. Provide motivation to consider application of
EM/Circuit Codesign for microwave circuits
based on a clear understanding of the benefits.

EM Circuit
Analyze
Parametric
Components
EM/Circuit Codesign Flow
(more familiar than first perceived)
Draw
Composite
Geometry
Synthesize
Circuit
Materials
Boundaries
Ports
Analysis Setup
user-directed loop
automated loop
Analyze, Tune
and Optimize
Circuit
automated loop
Analyze and
Optimize
Plot Circuit
Results
Plot Results
Examine Fields