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Using LINC2 with Sonnet® EM Software Enhances Simulation Accuracy
By Dale D. Henkes, ACS
Designing high frequency circuits requires
a varied approach to circuit simulation and verification
if good results are to be obtained from the first physical
prototype built. Different modeling and simulation techniques
will be required to address all the concerns. In fact, there
will be times when even different types of simulators will
need to be employed (such as EM simulation) to correctly
analyze the problem.
The major categories of circuit and component
attributes that require special attention in high frequency
circuits are:
1. “Parasitic” elements due to the physical
construction of the component that depart
from the ideal model
2. Stray inductance and capacitance formed by a number of
possibilities, including
component lead length, wires or
conductive paths involved in component
interconnections, capacitance between
component mounting pads and ground,
mutual inductance and capacitance
between adjacent components, and more
3. Distributed or transmission line effects of components
and traces due to their physical
size relative to a wavelength
4. Discontinuities in transmission lines, such as a step
change in microstrip width
or abrupt bend, etc.
5. EM (electromagnetic) coupling between lines or between
components and
conductive surfaces
All of the above produce more and more pronounced
effects on circuit behavior as the operating frequency increases.
Eventually, operation of the physical circuit will no longer
resemble the circuit simulation if these circuit and component
attributes are not accounted for. Fortunately, the LINC2
circuit design and simulation software from ACS (Escondido,
CA) can accurately deal with all these issues.
To address the concerns of item 1, above, LINC2 has an extensive
device library and enhanced physical component models to
accurately model parasitic elements. Circuit elements in
the LINC2 Parts menu are available to model the effects
of item 2, while EM simulation can be used to simulate the
effects of 3, 4 and 5.
This article will use the LINC2 filter synthesis software
to design a lowpass distributed filter in microstrip to
demonstrate the effects of transmission line discontinuities
and other non-ideal elements on circuit performance. It
will then be shown how EM simulation can be used to analyze
the resulting performance shift and to determine how to
correct for it.
The LINC2 Filter program has a built-in interface
to Sonnet’s EM simulation software. The latest version
of Sonnet® Lite™ is included free with LINC2 Pro
(or can be downloaded free from Sonnet Software’s
web site at www.Sonnetsoftware.com/lite). The LINC2 Sonnet
interface automatically starts the Sonnet EM program, sets
up the EM simulation environment and exports the layout
geometry into the Sonnet geometry editor (xgeom) ready for
simulation.
ACS chose Sonnet Software as its first choice for integration
into its LINC2 circuit simulation program because of Sonnet’s
outstanding overall performance, unrivaled accuracy and
speed. Sonnet also has an excellent user interface that
is easy to navigate. The Sonnet pop-up Quick Start Guide
takes you through the process of setting up an EM simulation
project from the beginning, including setting up the simulation
environment, adding the geometry, analyzing and viewing
the project results– all in wizard-like fashion! When
using Sonnet with LINC2, this process is completed automatically.
From LINC2, the entire design, geometry, and EM project
setup is automatically transferred to Sonnet with a single
menu click.
Distributed Microstrip Low-Pass Filter
Design Example
For this example, the design specifications will include
the following:
• Filter implementation: Low-pass stepped impedance
microstrip filter
• Filter type: 7th order Chebyshev
• Material: Rogers RO4003™
• 3 dB cutoff frequency: 5000 MHz
• Stop-band attenuation at 7500 MHz: 35 dB
• Operating pass-band: 2500 - 4500 MHz
• Minimum microstrip impedance: 10 ohms
• Maximum microstrip impedance: 90 ohms
• Pass-band return loss: > 10 dB
• Pass-band ripple: 0.15 dB
The LINC2 filter synthesis program is started by selecting
Filter Design from the LINC2 Tools menu as shown in Figure
1. This action opens the LINC2 Filter Synthesis
Schematic Window, shown in Figure 2. LINC2
can design single-ended filters as well as differential
filters for operation between balanced ports. For this example,
Distributed (Single-ended) is selected from the Filter menu
(Figure 2).
When the Design Specifications Form pops up, the design
parameters for a 7th order low-pass Chebyshev filter are
entered as shown in Figure 3.
The LINC2 Design Specifications Form is tailored to the
type of filter selected, so the number and content of form
tabs will vary somewhat from filter to filter. If there
are layout-related issues to deal with, they will be listed
by selecting the Layout tab. For the distributed lowpass
Chebyshev filter selected here, there are no layout issues
at this time.
The last tab, shown in Figure 4, lists
the other miscellaneous details of the filter design specifications.
On the Other form, the first element is specified as a shunt
element. The minimum and maximum T-Line impedance is specified
as 10 and 90 ohms respectively.
At this point, clicking the Synthesize Filter button (Figure
4) generates the filter schematic shown in Figure
5.
Clicking Analyze now would yield simulation results for
the schematic shown in Figure 5 with ideal
electrical transmission line components. The simulation
results thus rendered on graphs and Smith chart displays
of S21, S11 and S22 give a quick indication whether the
final design is likely to meet specifications.
The next step is to use the Convert T-Lines to | Microstrip
feature from the Auto menu. This action automatically converts
all the ideal transmission line elements on the schematic
page to physical microstrip based on a pop-up substrate
definition form, as shown in Figure 6.
The resulting converted schematic is shown in Figure
7. All of the physical dimensions required to build
the filter are now contained on the schematic, including
a description of the circuit board material. Anyone who
has used a transmission line calculator to go through a
schematic and manually convert all of the electrical (modeled)
transmission lines to physical lines will appreciate the
way LINC2 automates this process.
Clicking Layout produces the layout view shown in Figure
8. The length of de-embedding from the sides of
the EM box is set to 50 mils. The EM box is the space over
which the EM simulation will be performed. Simply clicking
EM Simulation (Sonnet) pops up the Sonnet geometry editor
(xgeom) with the LINC2 filter geometry already loaded.
View | View 3D now displays the filter geometry in three
dimensions, as shown in Figure 9. The only
action that is needed to get EM simulation results for the
entire filter is to select Analyze from the Sonnet project
editor; the software is really that easy to use!
When the EM simulation is completed, selecting Projects
| View Response | New Graph displays the filter response
plots on the graph shown in Figure 10.
The heavy curves show the initial filter response before
adjustment for the discontinuity effects of the large impedance
steps between the shunt and series elements. The result
is that the -3 dB cutoff frequency has shifted down by nearly
10% to 4540 MHz. This should not be surprising since the
lumped circuit theory model for the step discontinuity is
itself a lowpass structure with series inductance and shunt
capacitance that acts to lower the cutoff frequency.
Both LINC2 and LINC2 Filter Pro have built-it Parts menus
that include microstrip and stripline stepped width/impedance
models for modeling these discontinuities. When the stepped-impedance
lowpass filter in this example is simulated in LINC2 with
these circuit theory discontinuity models added, the result
is also a downward shift in the frequency response. With
discontinuity modeling applied, the LINC2 simulation predicts
a downward shift in the -3 dB cutoff frequency of about
8% to 4612.5 MHz.
The difference between Sonnet EM and LINC2 simulations is
only 2% at 5GHz with this particular circuit and geometric
structure. Circuit simulations of other structures may depend
much more on EM simulation for accurate results. Also, as
the operating frequency increases, EM simulation will almost
always be needed to close the gap between simulated and
measured performance.
Improved Synthesis Using EM Analysis
The EM simulation indicated that the effect of the stepped-impedance
microstrip discontinuities was a downward shift of 460 MHz
in the cutoff frequency. In less than a minute, another
filter synthesis run can be completed, this time compensating
for the frequency shift by specifying a cutoff frequency
of exactly 460 MHz higher (or Fc = 5460 MHz). This was done
and the results from the new EM simulation are shown as
the light colored plots overlaid the original (heavy plots)
in Figure 10. The new -3 dB cutoff frequency
is now 5.02 GHz. This reduced the error in cutoff frequency
from 10% to 0.4% in one iteration. All of the other original
filter goals have been met, including return loss and stop-band
attenuation.
Summary and Conclusions
This stepped-impedance lowpass filter design was used to
show how EM simulation can be used to uncover the effects
of transmission line discontinuities in distributed filters,
thus improving the accuracy of the simulation. As was shown
in this example, lumped (circuit theory) discontinuity models
can be included in the schematic based simulation to improve
the accuracy. However, these transition models are usually
based on lumped LC approximations. This means that there
is a limit on the size of the discontinuity, the substrate
dielectric, and in many cases, there are limitations on
the frequency range as well.
In this example, the ratio of the line widths at the impedance
steps was nearly 28. The schematic based line width/impedance
step models typically are only accurate to within a few
percent for steps less than 10:1. Since these kinds of filters
have such large impedance steps, it is a good idea to use
EM simulation for the best results.
LINC2 is a high performance RF and microwave design and
simulation program from ACS. In addition to schematic based
circuit simulation, optimization and statistical yield analysis,
LINC2 Pro includes many value-added features for automating
design tasks, including circuit synthesis. LINC2 directly
interfaces to leading RF and microwave design suites, allowing
it to be used stand-alone or by leveraging its capabilities
with those of other major packages. The subject of this
article, the LINC2 Sonnet interface, uses a single button
click to automatically transfer the LINC2 layout geometry
into Sonnet for world-class EM analysis. More information
about LINC2 can be found on the ACS web site at www.appliedmicrowave.com.
Sonnet is a trademark of Sonnet Software, Inc.
All trademarks and registered trademarks are the property
of their respective owners.
Applied
Computational Sciences
www.appliedmicrowave.com
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