Synopsys, Inc. · NASDAQ:SNPS
Synopsys — Technical Presentation (2012)
30 slides from Synopsys's technical presentation, May 9, 2012
Slide 1: S-parameter Modeling and Simulation for Signal Integrity Analysis
S-parameter Modeling and Simulation for Signal Integrity Analysis S-parameter Modeling Brad Brim Sigrity, Product Marketing Manager bradb@sigrity.com
Slide 2: Agenda
Agenda Ò Definition and Properties of S-parameters Ò File Formats and Connectivity Ò Reduced Order Models of S-parameters Ò Application of S-parameters
Slide 3: This material is introductory
Ò This material is introductory Ò … but it is critical to remember because improper generation and application of S-parameters is often directly attributable to violating these basic concepts Ò The verbiage will have an AMS chip-centric bias Ò due to track sponsorship Ò all concepts apply equally we
Slide 4: Much of this material is already available elsewhere
Ò Much of this material is already available elsewhere Ò technical journal archives Ò college texts Ò EM, circuits and distributed circuits courses Ò trade publications Ò conference archives Ò SI/RF educational companies Ò wiki Ò http://en.wikipedia.org/wiki/Scattering_parameters Ò The focus of this
Slide 5: Where will you encounter S-parameters?
Where will you encounter S-parameters? Ò package and system models Ò commonly applied for chip/package/system simulation Ò device/circuit models Ò whether individual devices or larger on-chip circuits Ò reduced-order models Ò whether macromodel S-parameters or I.P. or measurements
Slide 6: Circuit Simulation
Ò Circuit Simulation Ò Modified nodal analysis Ò SPICE and most (if not all) circuit simulators in both the time and frequency domain are based on modified nodal analysis Ò S-parameters are not applied directly in any of these simulators, but are first converted to Y-parameters for integration with
Slide 7: The "wave" definition
Ò The “wave” definition Ò very natural for EM theory and computations Ò voltage wave amplitude ratios Ò reference impedance is that of the transmission line at each port
Slide 8: The "circuit" definition
Ò The “circuit” definition Ò Most common definition applied is from Kurokawa Ò Kurokawa, K., "Power Waves and the Scattering Matrix", IEEE Trans. Micr. Theory & Tech., Mar. 1965, pp194-202 Ò Notice the complex conjugate in the definition for reference impedance due to “power” definition instead of w
Slide 9: The "measurement" definition
Ò The “measurement” definition Ò power is measured internally to a VNA (vector network analyzer) Ò reference impedance can be selected based on the “calibration” scheme Ò measured reference, line impedance, renormalized to Rref Ò Calibration is a critical step Ò Deembedding removes “feed structure”
Slide 10: How the definitions differ
How the definitions differ Ò conj(Zref) Ò voltage waves have no reflection when Zload=Zref Ò circuits transfer maximum power when Zload=conj(Zsource) Ò The difference is in the transformation between S- and Y- parameters Ò a simple conjugate of reference impedance Ò If Zref is complex (e.g. highly l
Slide 11: S-parameters are "differential" in nature
S-parameters are “differential” in nature Ò only the local voltage difference is relevant Ò there is no such thing as global ”ground” Ò you can never know the voltage at any of the S-parameter reference nodes … NEVER! Ò all “ports” are 2-terminal for generation of S- parameters Ò the current into on
Slide 12: "2N" and "N+1" circuit components
“2N” and “N+1” circuit components Ò 2N: nodelist: port1 ref1 port2 ref2 … Ò N+1: nodelist: port1 port2 … ref Ò Either can be applied in a circuit simulator Ò the 2N is identical to the N+1 with ideal n=1:1 transformers wrapped around it Ò The information contained in the S-parameter file is exactly
Slide 13: Mixed-mode S-parameters
Mixed-mode S-parameters Ò “single-ended” Ò single uncoupled traces - Sij Ò “mixed-mode” - {SDD, SCC, SDC, SCD}ij Ò diff-pairs Ò if you ignore the common mode you may be in serious trouble Ò the two modes can mix together internal to the circuit Ò emission is much greater from the common mode Ò “mult
Slide 14: Common language
Common language (see the wiki web page cited earlier for much more) Ò Return Loss: reflection, Sii Ò Insertion Loss: thru transmission, S21 Ò Coupling, Crosstalk, etc: Sij Ò Near-end and Far-end Crosstalk (NEXT, FEXT) Ò distinguishes the Sij for coupled thru paths for the source and receiver ends Ò
Slide 15: Common displays
Common displays Ò most common display Ò X-axis: linear frequency Ò Y-axis: dB for magnitude, [-180,180] degrees for phase Ò Smith Charts Ò simply a bilinear mapping of magnitude and phase to a 2D plot Ò relevant to reflection S-parameters Ò many GUIs provide readout of equivalent impedance Ò bottom
Slide 16: Properties of S-parameters
Properties of S-parameters Ò Reciprocity Ò Sij = Sji … except for loss and active circuits Ò Passivity Ò (I-SS*) >= 0 Ò Causality Ò no response before T=0, or appropriate physical delay Ò Smoothness Ò not really a property so don’t insist on it
Slide 17: Practical issues for modeling with S-parameters
Practical issues for modeling with S-parameters Ò Noise Ò measurements Ò calibration errors, repeatability, noise Ò EM computations Ò numerical error, convergence, design specification (geometrical representation, material properties) Ò Phase Ò phase is critical to get correct, often more so than am
Slide 18: File formats
File formats Ò “Touchstone” Ò been around for many years Ò extension of SNP, where N=integer number of ports Ò version 2.0 specified by IBIS Ò mixed mode parameters, per-port Zref, etc. Ò version 2.1 available soon Ò sparse data, binary Ò future versions Ò rational functions instead of discrete freq
Slide 19: Model Connectivity
Model Connectivity Ò Two proprietary formats are available Ò Sigrity MCP: Model Connection Protocol Ò public domain format, multiple EDA companies read/write Ò Apache CPP: Chip Package Protocol Ò covered under NDA, multiple EDA companies read/write Ò Need to connect two high pin count models Ò netli
Slide 20: Model Connectivity Example
Model Connectivity Example Ò Connection Modes Ò by name or by physical location Ò Electrical resolution may vary Ò May merge two models Ò Pwr/Gnd nodes highlighted Ò Supports both MCP and CPP
Slide 21: Reduced Order Models
Reduced Order Models Ò Motivation Ò It is difficult to deal with frequency domain data in a transient circuit simulator Ò pin counts are increasing Ò data bandwidth is increasing Ò data is often not “well conditioned” Ò Benefits relative to direct usage of S-parameters Ò faster, more accurate transi
Slide 22: Reduced Order Models
Reduced Order Models Ò Vector Fitting Ò the underlying technical algorithms of most (if not all) macromodel generators Ò essentially a pole-zero fit of the data Ò Difficulties Ò high pin count – memory, time and numerical convergence Ò previously 100 was considered good, now 500 Ò preserving passivi
Slide 23: Macromodel generation flow
acromodel generation flow 7 Input: SPICE model 5 ij “ S-parameters | & | extration Time-domain model TD model checking checking i 1. Step response 2. Pulse response eres Faster and more accurate >| Tuning time-domain simulation |» Frequency-domain resu Its model checking eee” SPICE simulator id chec
Slide 24: Macromodel 4-port serial channel example
Macromodel 4-port serial channel example
Slide 25: Applications of S-parameters to chip-package PDN
die Vdpg Vdsg Applications of S-parameters to chip-package PDN Vdie-brd Ò Do not try to measure voltage at reference nodes when working with S-parameters Vbpg Vbsg board
Slide 26: Applications of S-parameters to parallel buses and serial channels
Applications of S-parameters to parallel buses and serial channels Ò Return paths Ò be consistent amongst all your models Ò use an IO buffer model to properly push/pull current into the PDN Ò Power Ports Ò model signals and power together Ò use a common return path Ò IBIS 5.0 buffer models Ò power-a
Slide 27: Applications of S-parameters to parallel buses
Applications of S-parameters to parallel buses Ò Single versus dual referencing PDN noise Ò required simultaneous signal/power modeling Ò 15mv single vs. 45mv dual, though two extra layers in the package Ò Memory interface Ò silicon vendor and system house worked together to model system Ò eye openi
Slide 28: Applications of S-parameters to parallel buses (memory)
Applications of S-parameters to parallel buses (memory) Lessons: (1) Use a driver model to push/pull current from the real return path(s), since it varies with rising/falling edges. (2) Model signals, power and ground together. Eye Diagram #1 Eye Diagram #2 (a) VSS and VDD return path (a) VSS return
Slide 29: Lessons learned … and re-learned regularly
Lessons learned … and re-learned regularly Ò There is no such thing as global ground Ò You never know Vref when working with S-parameters Ò Use a consistent return path amongst all models Ò Model signals and power together Ò use a common return path Ò use a power-aware IO buffer model Ò S-parameters
Slide 30: Thank You!
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