SIwave TrainingExercise 3

Analyzing return currents of a differential pair on a package

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Example: Differential pair on a package

Differential pairThe differential pair on a package is intended to show you how to create, simulate and analyze a power structure on a package in terms of return currents. This example assumes that the user has reviewed and understands of the sections covered in the Getting Started Section of this guide.

The following illustration details the passive device you will be creating.

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Notice

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Example: Differential Pairs

Differential Pairs – OverviewIdea: Transmit 2 signals instead of one. Send the signal you want plus a 2nd signal equal to the negative of the first. The return current of the first is positive and the 2nd is negative which cancel. At the receiver, compare the two signals to determine their logic polarity(Does not require a voltage reference)

Advantage: Ground voltage shifts between the transmitter and the receiver affect each line equally so there is no effect on the difference.

Note: If the differential signals are not exactly opposite, their currents do not cancel. This imbalance is refereed to as the common mode current.

VC VCOne Pair Supporting

Common & Differential Modes+ VD -VD

Two Separate, Single-Ended Transmission LinesV1 V2

Pair excited equally(Common Mode Zcm)

Pair excited differentially(Differential Mode Zdiff)

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Differential Pairs – ImpedanceThe characteristic impedance for a pair of lines is a 2x2 matrix

The characteristic impedance matrix relates the line voltage to the line current as follows

Replacing the nodal v & i with modal v & i, Zo becomes:

Example: Differential Pairs Theory

=

220120

1201100 ZZ

ZZ Z

=

odd

evenZ0

00 Z0

0Z

termreflectionii

Zvv

2

10

2

1 +

=

+=+=

+=+=

22112

22111

24132

22111

2

1

2

1 & with & Replace ibibiibibi

vavavvavav

ii

vv

m

m

m

m

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Example: Differential Pairs TheoryDifferential Pairs – Impedance

What is the relation ship between Zo and Zcm/Zdiff?

Set the differential-mode voltage signal to 1Volt and the common-mode voltage signal to 0Volt.(V1= 1/2, V2= -1/2)

Set the differential-mode voltage to 0Volt and the common-mode signal to 1Volt. (V1= 1, V2= 1)

00

,==

==dcm vcm

dcm

vd

ddiff i

vZivZ

2 , 21

21vvvvvv cmd

+=−=

( )

+−

+

−=

−

−

−

−=

⇒

=

− 120110

1201102120

2110110120

1201102120

21102

1

2

1

110120

120110 2/12/1

2/11 2/1

2/1ZZ

ZZZZZZ

ZZZZi

iii

ZZZZ

( ) ( ) 21

21

1201102

1201101 ZZ

iiZZ

i diff −=⇒−=

−=

( )1201102Z ZZdiff −=∴

−−

−=

−

−

−=

⇒

=

120110

1201102120

2110110120

1201102120

21102

1

2

1

110120

120110 1111

11

ZZZZ

ZZZZZZ

ZZii

ii

ZZZZ

2 1

12011012011021 ZZ

iZZ

ii cm +=⇒

+==

2120110 ZZ

Z cm+

=∴

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Example: Differential Pair TheoryDifferential Pairs – Impedance

What is the relation ship between Zodd/Zeven and Zcm/Zdiff?

Differential Pairs – Perfect Match

Example

( )1201100 2Z2 ZZZ odddiff −=⋅=

( )

( ) ( )( ) 120

2120

2110

120110120110

120110120110

0

0

120110

1201100

1201100

2ZZ2

2Z2//222

Z2

ZZZ

ZZZZZZZZ

ZZ

Z

ZZZ

ZZZZZ

ZZZZ

oddb

oddba

b

oddabdiff

evenbcm

−=

−−+−⋅+⋅

=−

⋅⋅=

+=

−=⋅==

+===

51.5813.039

13.039563.51ZZZZ

Z220120

1201100

=

=

( )

( ) ( )( ) 9435.190

2ZZ2

,602.64

533.38 Z,602.64Z

0048.772 ,301.322

120

2120

2110

120110120110

120110120110

0

0120110

12011001201100

120110120110

=−

=−−+

−⋅+⋅=

−⋅⋅

==+=

=−==+=

=−==+

=

ZZZ

ZZZZZZZZ

ZZ

ZZZZ

ZZZZ

ZZZZZ

Z

oddb

oddbab

oddeven

diffcm

22Z 1201100 ZZ

Z evencm

+==

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Example: Differential PairsDifferential Pairs – Common Terminations

Bridged Termination: A common termination scheme is to place a single resistor between the two signal nets. With the bridge termination, the odd mode is perfectly matched and the even mode is completely reflected. Since the signal is carried in the odd mode and the differential receiver will reject the even mode, the extra noise on the even mode caused by the termination is inconsequential. This is true as long as the even-mode does not couple into the odd mode through mode conversion.

Single-Ended Termination: A common termination scheme is to individually terminate each transmission line. In this configuration, the odd mode is matched while the even-mode reflection is reduced. Differential drivers often provide single-ended terminations. If the receiver uses a bridged termination then the odd mode is absorbed at both ends for good signal integrity. The driver damps the even mode to keep it from becoming a noise and EMI problem.

oddR 0bridge Z2 ⋅=

oddR 0single Z=

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Example: Defining Global Materials

Defining materials in the computer databaseStart the Maxwell Control PanelClick UtilitiesClick MaterialsScroll up and click on FR4_epoxy

Note that default FR4_epoxy is defined by Ansoft as a lossless dielectric with

Select Material/DeriveType FR4_package in a box under Material Properties Set Rel. Permittivity (Eps) to 4.1Set Elec. Loss Tan to 0.02Click EnterClick OK in the warning message window. We will not use this material in thermal simulations.You have defined a lossy package material which we will use in SIwave simulations.

Click Exit to close Materials ManagerSave changes.

4.4=rε

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Example: Starting Project

Launching SIwaveClick on the Project button

Create and open a New SIwave v1 Project named: diff_pairs

Creating the ModelSelect Edit/Layer Stackup

There are four metal layers and three dielectric layers defined by defaultSelect layer Dielectric-1

Select Edit Layer PropertiesSelect Material: FR4_package

Repeat for layers Dielectric-2 and Dielectric-3

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Example: Layer Stackup Setup

You can change names of layers as you see fit.Select Metal-1. Double-click on the layer to enter Properties dialog

Type Name: signal_topRename Metal-2 to power, Metal-3 to ground, Metal-4 to signal_bottom.Click OK when you’re finished.

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Example: Stackup Review in Main Window

Main SIwave window contains stackup information in the left panelSelect next to Dielectric-2Select next to FR4_packageReview that the material information has been updated from the materials manager into SIwave. The following values should be assigned:

Right click on Dielectric-2. Select Edit Layer PropertiesThis is another way to modify layer properties directly from main window

Define thickness: 1 mmClick OK. The stackup will be complete updatedSelect next to Dielectric-2 againVerify that the new core thickness has been assigned

02.01.4

==

δε r

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Example: Conductor Setup

Move the cursor over to the layer stackup on the left and make power the active layer by clicking on it. Its font will become bold.Choose Draw/Rectangle. You are now in Drawing Mode

The cursor changes to a cross-hair and you can select start and end points.Click at (-20,-20) to indicate the starting pointClick at (20,20) to indicate the end point

If you feel uncomfortable selecting coordinates on screen, you can enter coordinates manually in the coordinate area below the main panel

Choose Draw/PolygonChoose Draw/Subtraction Mode

Click at (20,20) to indicate the starting point. Note that the cursor changed to the bulls eye to indicate an existing geometry vertexClick on (18,20)Click on (20,18)

Click on (20,20) again to complete the triangleChoose Draw/Subtraction Mode to exit the modeChoose Draw/Geometry Selection Mode to exit the drawing modeNOTE: You can use the Drawing toolbar to speed up these functions

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Example: Conductor Setup

Click anywhere within the resulting polygon in the main panelThe polygon will be highlighted to indicate the selection

Select Edit/CopySelect layer groundSelect Edit/PasteDrag the pointer into the window. Move the polygon so that two of them overlap. Your pointer should be at coordinate (0,0)Click once to paste the polygon on the new layerVerify that the planes exist on both power and ground layers

Click on the colored boxed next to power and ground layers in the left panel to make the planes opaque.Then, hold down the CTRL key on your keyboard and drag with the left mouse button to get a nice 3D view of your work. When you hold down CTRL and Shift simultaneously and drag the left mouse button you can zoom and unzoom.

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Example: Conductor Setup

When you are satisfied, click on Top-down View button on the toolbarClick on the colored boxed next to power and ground layers again in the left panel to make the planes transparent

Select File/SaveKeep default diff_pairs.siw file name. Click Save

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Example: Trace Setup

Select layer signal_topChoose Draw/Set Trace Width (use toolbar )

Type 0.17 mm. That will achieve 50Ω on this microstrip layerSelect Draw/Trace (use toolbar)In the coordinate entry window,

type x = 0 y = -1. Click EnterEnter dx = -16 dy = 16. Click Enter

If you make a mistake, press Esc to ignore previous point

Click twice at (-16,20) to complete the trace

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Example: Trace Setup

Draw the second traceChoose (0,0)Choose (-14,14)Click twice at (-14,20) to complete the trace

Select Draw/Trace to exit the drawing mode

Select Nets tabs in the left panel

Select boxes next to NET-3 and NET-4Verify that these nets correspond to the signal nets

Select File/Save

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Example: Solution Setup

Select Simulation/Global OptionsYou can use defaults for the solution settings

Verify that Lossy Model Type is usedLossless simulations typically run faster than lossy ones

Verify that Mesh Refinement is automatic This settings allows you to manually control the mesh density

Click OK

Select Simulation/Compute Resonant ModesLeave Minimum Frequency at defaultSet # of Modes to Compute: 6Select to plot voltage between layers power and ground

Note: SIwave solves for resonant modes in power/ground structure. If you select layers which have traces only, SIwave will find no resonances. In order to include effects of traces, other simulations methods are used

Click OK to start the simulation

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Example: Viewing Resonant Modes

Using CTRL key and left mouse button, switch to 3D modeToggle the visibility of the mesh by selecting the View/Mesh menu item or corresponding icon on the toolbarSelect frequency of 3.7229 GHz. Select Phase Animation…Select Generate Frames. Once complete, click the Play buttonWhen done, Close both windows

50.007690.0567390.0528325.283716110.73858

50.00750.0717720.0417664.17698587.543137

50.00750.0721440.0415514.15549387.092696

50.00750.0805260.0372263.72292378.02675

50.00750.0807170.0371383.71413477.84254

50.00750.1140810.0262762.62788655.076413

50.00750.1614660.0185651.85669138.913372

50.00750.1618650.0185191.85211938.817541

Qlambda(m)Im(f) (GHz)f (GHz)k#

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Example: Resonant Modes Analysis

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Example: Source definition

Switch to Top-down viewEditing is disabled in 3D view

Select Circuit Element/Voltage SourceThe corresponding toolbar icon is V

Place a voltage source on one of the tracesClick twice at (0,-1)Select positive terminal as signal_top to connect to the traceSelect negative terminal as ground to reference the groundClick OKLeave magnitude at 1 VoltClick OK

Place a voltage source on another traceClick twice at (0,0)Select the same layersChange magnitude to -1 VoltClick OK

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Example: Termination definition

Select Circuit Element/Voltage SourceThis will exit the Voltage Source definition mode

Select Circuit Element/ResistorThe corresponding toolbar icon is R

Place a termination on one of the tracesClick twice at (-16,20)Select positive terminal as signal_top to connect to the traceSelect negative terminal as power to reference the nearby planeClick OKLeave magnitude at 50 OhmClick OK

Place a voltage source on another traceClick twice at (-14,20)Select the same layersLeave magnitude to 50 OhmClick OK

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Example: Editing Circuit Element Properties

Select Edit/Circuit Elements PropertiesYou can change magnitudes and layers of the positive and negative terminals in this menu. You cannot change geometrical locations of the terminals here. However, you can delete a circuit element here and redraw in the main window without closing this dialogClick OK

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Example: Setup Frequency Sweep

Make sure that nets NET-3 and NET-4 are checked in the left panel

Unselected traces are not included in frequency sweep simulation

Select Simulation/Compute Frequency Sweep…

Start frequency: 1E+009 HzStop frequency: 1E+010 HzNumber of Solution Points n: 91Voltage Surface Plot Options:

Layer power and Layer ground

Click OK to start simulation

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Example: Solution

While holding CTRL key, double-click at the top left corner of the main panel.

This is a shortcut to a pre-defined angle in 3D view

Select 1GHz row in Frequency Sweep Results window

If you accidentally closed this window, select Results/Frequency Sweep/Surface Voltages

Using arrows on the keyboard or animation, scroll down the frequencies. Observe when the wave on the power plane is visible

The result matches modes #6 and #7 in the resonant mode solution. This resonance is caused by return currents in odd mode.

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Example: Common Mode Setup

Select Edit/Circuit Elements PropertiesSelect Voltage Sources tabClick on V2 and select Modify (or simply double-click on V2)Set Magnitude = 1 VoltClick OKLeave positive and negative layers unchanged. Click OK

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Example: Frequency Sweep Setup

Select Simulation/Compute Frequency Sweep…Leave all fields unchanged from the previous simulationClick OK to start simulationRepeat the same steps for viewing the solutionA very strong resonance can be seen at 3.7GHz. This corresponds numerically to the resonant modes #4 and #5. However, a field structure observation (see page 20) demonstrates that the even modereturn current excites mode #5 at 3.7229GHz

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Example: Port Setup

Select Edit/Circuit Elements PropertiesSelect Voltage Sources tabSelect both V1 and V2 by clicking and dragging the mouseClick DeleteSelect Resistors tabSelect both R1 and R2Click DeleteClick OK

Select Top-down View to allow editing

Select Edit/Grid Spacing…Select User-Defined, 1.00 mm

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Example: Port Setup

Assign ports to the tracesSelect □ □ p □ icon (or select Circuit Elements/Port)Click twice at (0, -1)

Make sure that the cursor changes to a bulls eyeSelect signal_top as a positive terminal and ground as a negative terminal. Click OKClick OK to define PORT1.Click twice at (0, 0)Click OK to accept “sticky” layer definitions. Click OK to define PORT2Repeat at (-16, 20) for PORT3Repeat at (-14, 20) for PORT4

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Example: Port Setup

Assign ports to the power planeZoom in on the central area of the package, near ports 1 and 2

Use toolbar icons

Click twice at (1, 0)Select power as a positive terminal. Leave ground as a negative terminal. Click OKClick OK to accept port name PORT5

Zoom out using Zoom in near ports 3 and 4Click twice at (-15, 20)Click OK to accept “sticky” layer definitions. Click OK to define PORT6

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Example: Setup SPICE Solution

Important! Select File/SaveSIwave starts a solver as soon as Setup is complete

Select Simulation/Compute SPICE Solution…

Mesh Refinement: Automatic (default value)SPICE Subcircuit Format: Maxwell SPICEChoose Frequency Dependent (Full Wave SPICE)Number Points: 1000Max Frequency: <do not modify>Min. Rise/Fall Time: 1e-10 s

Once you enter rise time, the max frequency will be automatically updated to 50 GHz

Click OK to start simulation

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Example: SPICE Differential Pairs

Differential PairsThe differential pairs example is intended to show you how to create, simulate, and analyze the effects of return current discontinuities on the noise induced on power planes using Maxwell Full Wave Spice and the SPICE circuit created by Ansoft SIwave

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Example: Circuit Construction

Launching Schematic CaptureStart the Maxwell Control PanelClick the Projects button

Create and open a New Schematic Capture Project named: diff_pairs_spiceIf several versions of the software are listed select the newest version (Highest Release Number)

Creating the CircuitSelect the menu item Add/Full Wave N-port Subcircuit

Click the Edit buttonClick the Import buttonFile Open: diff_pairs.spcNote: If you cannot find diff_pairs.spc, switch back to SIwave. Select Results/SPICE solution/SPICE subcircuit. Click Save As… . Verify that the .spc file is visible and find the location of this file. Click Cancel when done

Select diff_pair in Definitions Loaded: windowReview the circuit in Definition:

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Example: Circuit Construction

Ports are defined by two SPICE nodes in Full Wave Circuit. Place chip-side ports 1, 2 and 5 at the left side of the circuit and place ball-side ports 3, 4 and 6 at the right side of the circuit

Place cursor inside Definition: window. Edit circuit such thatleft: 1 2 3 4 9 10right: 5 6 7 8 11 12Click UpdateClick DoneClick OK

Component PlacementTo place the new component, single click with the left mouse button to place the component. Move the mouse until the size and orientation of the component are acceptable. Click the left mouse button to finish placing the componentSince we only need to place one of these components, click the right mouse button and select Done

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Example: Circuit Construction

Select View/Terminal NamesSelect the menu item Add/Resistor

Place 3 components on the schematic as shown

Name R1, Value: 50Name R2, Value: 50Name R3, Value: 50Name R4, Value: 50Name R5, Value: 1

Select the menu item Add/Source/Voltage

Place 3 voltage sources on the schematic as shown

Name: V1Transient Waveshape: PWL

Click OK

VoltsTime

11e-101

01e-110

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Example: Circuit Construction

Continue with voltage source definitions

Name: V2Transient Waveshape: PWL

Click OK

Name: V3Transient Waveshape: DC Value: 1.5Click OK

Select the menu item Add/GroundPlace the components on the schematic

VoltsTime

-11e-101

01e-110

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Example: Circuit Construction

Select the menu item Add/Voltage Difference Probe

Place 5 components on the schematic on ports 1, 2, 3, 4 and 5

Name: dv_1nearName: dv_2nearName: dv_powerName: dv_1farName: dv_2far

Select the menu item Add/WireClick the left mouse button to connect the first wire terminalMove the cursor to the place where you want to connect the next terminalClick the left mouse to connect the terminalRepeat these steps to continue adding to the wire segmentsClick the right mouse button, and choose End Wire from the menu that appears. Or double click the left mouse buttonRepeat these steps for each wire in the schematic. Click the right mouse button and select Done when you are finished

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Example: Solve Circuit

Solve the circuitSelect the menu item Solve/Number Nodes

Verify that the Node visibility is turned on. Select the menu item View/Nodes (Nodes should be checked)Use the node numbers to verify that the schematic is wired correctly

Select the menu item Solve/Run SPICEClick the Transient Analysis buttonTime Step: 1pOutput Stop: 1nClick the OK button

Click the OK button to start the simulation

After the simulation finishes,Select Load All Signals (default)Click the Close button

Plot the resultsSelect the menu item Results/New Plot

Loaded Signals: dv1_near(t), dv1_far(t),dv2_near(t), dv2_far(t)Type: CartesianX Scale: LinearY Scale: LinearClick the OK button

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Example: Post Processing - Circuit

Select the menu item Results/New PlotLoaded Signals: dv_power(t)

Type: CartesianX Scale: LinearY Scale: LinearClick the OK button

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Example: Common Mode

Redefine the PWL sourceGo back to the circuit windowDouble-click on V2Modify the voltage source so that both lines are driven positively

Rerun the simulationSelect from the menu item Solve/Run SPICEClick the OK button to start the simulation

After the simulation finishes,Select Load All SignalsClick the Close button

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Example: Differential and Common Modes

Plot the results on the differential pairSelect the menu item Results/New Plot

Loaded Signals: dv1_near(t) (2), dv1_far(t) (2),dv2_near(t) (2), dv2_far(t) (2)Type: CartesianX Scale: LinearY Scale: LinearClick the OK button

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Example: Differential and Common Modes

Plot the resultsLoaded Signals: dv_power(t), dv_power(t) (2)

Type: CartesianX Scale: LinearY Scale: LinearClick the OK button

Exit Schematic CaptureSelect the menu item File/Exit

Click the Yes button if prompted save