Advances in Measurement Based Transient Simulation

Transcription

1 Time Domain Simulation in ADS, Slide - 1 Advances in Measurement Based Transient Simulation Presented by GigaTest Labs Gary Otonari and Orlando Bell March, Time Domain Simulation in ADS, Slide - 2 Transient Simulation in ADS History Microwave SPICE -- EEsof time domain simulator for microwave Convolution and Multilayer Line Models High Speed Digital design ADS - A design framework for SI Transient Simulation improvements Microwave SPICE GigaTest Founded HP EEsof Merge Usability Improvements ADS 2006 SI Convolution MLM Models ADS DCA Frontend 2 Transient Simulation 1

2 Ref Time Domain Simulation in ADS, Slide - 3 Convolution Simulation For many years, the only simulator that allowed transient simulation on models with frequency dependence How does it work? S4P SNP3 File="CABLEandCONNECTORwithoutCABLE.s4p" R R5 R=50 Ohm tdrmp xnem VtPulse SRC2 TLIN Vlow=0 V TL6 t xfem Vhigh=2 V Z=50.0 Ohm Term Delay=0 nsec E=360 Edge=erf F=5 GHz Term6 Rise=(2.2*risetime) psec Num=6 Z=50 Ohm Fall=(2.2*risetime) psec Width=50 nsec Period=50 nsec R R12 R=50 Ohm tdrmn VtPulse SRC5 TLIN Vlow=0 V TL9 t Vhigh=-2 V Z=50.0 Ohm Delay=0 nsec E=360 Edge=erf F=5 GHz Rise=(2.2*risetime) psec Fall=(2.2*risetime) psec Width=50 nsec Period=50 nsec time-domain impulse from S-parameters Term Term5 Num=8 Z=50 Ohm Convolve with input waveform Transient output WITH frequency domain effects 3 Time Domain Simulation in ADS, Slide - 4 Convolution = Key Feature Coupled with Agilent s measurement and simulation depth, this tool provided a key capability to the signal integrity designer, and leverage for things like: ADS microwave and RF transmission line models Network analyzer and TDR measurements ADS Co-simulation = CONVOLUTION m1m time, psec ML2CTL_C CLin2 Subst="Subst1" MLRADIAL2 Length=45 mil ) mil Radial1 W=ms _w mil Subst="Subst1" S=69 mil X_Offset=31 mil Layer=2 Y_Offs et=0.0 mil RLGC_File= W_Left=ms_w mil ReuseRLGC=no W_Right=ms_w mil S_Left=(ms _pitc h-ms_w) mi l All GOOD? Not exactly.. 4 Transient Simulation 2

3 Time Domain Simulation in ADS, Slide - 5 Agenda The GigaTest Wish-list for Transient Simulation Causality Improved S-parameter data processing Passivity Long Structures FUTURE Directions. 5 Time Domain Simulation in ADS, Slide - 6 What s all this Causality Stuff, Anyhow? 30 Response before 0 impulse_response Transient simulation starts from time, sec 1.0 Poor accuracy Spectrum Simulated Original Non-causal impulse response degrades simulation accuracy freq, Hz 6 Transient Simulation 3

4 Time Domain Simulation in ADS, Slide - 7 Causality Correction in ADS ADS v2006u1 introduced an improved convolution simulation algorithm with causality correction Forces all impulse responses to be causal Implements improved adaptive sampling and extrapolation of frequency domain response var("cmp1_imp(2;1).impresp" var("cmp1_imp(4;3).impresp" time, nsec Actual Impulse Response Calculated during convolution Starts at t=0 7 Time Domain Simulation in ADS, Slide - 8 Causality Check: Long Interconnects Agilent / TDR and GTL Probe Station The best way to see this is to look at measurements on a long interconnect Sample is a 1.5 meter long Infiniband Cable, measured separately for differential TDR and TDT response (below left) 4-port S-parameter response 8 Transient Simulation 4

5 Time Domain Simulation in ADS, Slide - 9 Non-causal Simulation Results ADS 2003C Simulation Data Prior to ADS 2006, transmission line models were non-causal in ADS Convolution applied to measured S-parameter data was also not generally causal Results shown below are Convolution simulation for the 1.5 meter Infiniband cable with ML2CTL_C CLin1 Subst="cable" Length=1.42 meter W=wire mil S=(pitch-wire) mil Layer=3 RLGC_File= ReuseRLGC=no Stripline Microwave Model Multi-layer Line Model (2D EM) port S-parameter data The pulse should not arrive before the interconnect delay allows The risetime should degrade in a causal response time, nsec 9 Time Domain Simulation in ADS, Slide - 10 Non-causal Simulation Results ADS 2003C Simulation Data The complex loss description over frequency, determines whether a transmission line model is causal. For most commercially available simulators, this is determined by the frequency dependent dissipation factor, or dielectric loss tangent Measured S-parameter data is causal if measured correctly, but simulated results can become non-causal when convolution simulation is applied to it Prior to ADS 2006U1: The microwave t-line models were the most non-causal MLM models were better Measured S-parameters best tdtsp_strip-tdtsn_strip tdtmp-tdtmn tdtsp-tdtsn time, nsec 10 Transient Simulation 5

6 Time Domain Simulation in ADS, Slide - 11 ADS 2008A: Causality Enforced Exact same models and data simulated in ADS 2008 Stripline Microwave Model Multi-layer Line Model (2D EM) 4-port S-parameter data 2.0 tdtsp_strip-tdtsn_strip tdtmp-tdtmn tdtsp-tdtsn time, nsec 11 Time Domain Simulation in ADS, Slide - 12 Implications: Model Accuracy ADS 2003C vs. 2008A Eye Diagrams generated from the MLM Model for a 1.5 meter long Infiniband cable with mated connectors by ADS 2003C and ADS 2008A eye(voutp-voutn, 1.25 GHz) m6 m5 m6 time= 322.3psec m6=0.008 index= m5 time= 324.8psec m5=0.007 index= time, psec eye(voutp-voutn, 1.25 GHz) m6 time= 328.1psec eye(voutp-voutn, 1.25 GHz)=0.0 index= m5 m6 m5 time= 322.1psec eye(voutp-voutn, 1.25 GHz)=0.0 index= time, psec Datarate = 2.5 Gb/s Risetime = 100 ps 12 Transient Simulation 6

7 Ref Time Domain Simulation in ADS, Slide - 13 What About S-parameters? Properly measured / modeled S-parameter data should be causal Convolution Simulation can introduce non-causal response EXAMPLE: Simple RLC Circuit (Causal) ==> After band-limiting (Non Causal) Term Term1 Num=1 Z=50 Ohm L L1 L=1 H R= R R1 R=1 Ohm C C1 C=1.0 F 1.0 Admittance simulated from 0 to 1 Hz 30 Band-limiting induces non-causal response Spectrum impulse_response freq, Hz Non-causal response introduces simulation error time, sec Causality Bandwidth Risetime 13 Time Domain Simulation in ADS, Slide - 14 S-parameter Data Causality ADS 2008 has a new adaptive sampling algorithm with extrapolation This improvement is in addition to the causality enforcement Very helpful where source parameters slightly exceed bandwidth of SnP Example: 1.5 meter Infiniband cable at 2.5 Gb/s with 50 ps risetime S4P SNP2 File="concatenate_cable.ds" Same 1.5m cable Datarate = 2.5 Gb/s Risetime is 50 ps Jitter Measured: 13.7 ps Jitter Sim: 12.8 ps VERY good agreement! eye(voutn,1.25ghz) time= 434.2psec eye(voutn,1.25ghz)=0.296 index= time 424.2psec eye(voutn,1.25ghz)=0.024 index= m5 m time psec 14 Transient Simulation 7

8 Time Domain Simulation in ADS, Slide - 15 Transient Wish List #2 S-parameter Data Noisy, resonant and lossy structures have historically been difficult to TDR using convolution Improvements to the S-parameter data sampling and extrapolation have other implications beyond causality db(s(3,3)) db(s(1,1)) db(s(4,3)) db(s(2,1)) freq, GHz freq, GHz 15 Time Domain Simulation in ADS, Slide - 16 Example: Noisy and Lossy S-parameters Sample is an Infiniband PCB + Connector Measured with 2-sided probing using N5230A 4-port PNA calibrated from 50 MHz to 10 GHz 86100A / 54754A Differential TDR 16 Transient Simulation 8

9 Time Domain Simulation in ADS, Slide - 17 Demo: Two-sided Probing GTL 5050 Rotatating Stage Probing System 17 Time Domain Simulation in ADS, Slide - 18 Example: Noisy and Lossy S-parameters Single ended S-parameters are shown including return loss at each port (left) and attenuation for each complementary net in the differential pair. NOTE the small resonances in band, as well as the rapid increase in both return loss and attenuation above 9 GHz 18 Transient Simulation 9

10 Time Domain Simulation in ADS, Slide - 19 ADS 2003C TDR on Bandlimited Data ADS 20003C / Measured TDR Risetime = 50 ps 9.1 GHz 19 Time Domain Simulation in ADS, Slide - 20 ADS 2003C TDR on Bandlimited Data ADS 20003C / Measured TDR Risetime = 45 ps 10.1 GHz 20 Transient Simulation 10

11 Time Domain Simulation in ADS, Slide - 21 ADS 2003C TDR on Bandlimited Data ADS 20003C / Measured TDR Risetime = 40 ps 11.4 GHz! Note the improved accuracy at the higher bandwidth (which exceeds the 10 GHz data) 21 Time Domain Simulation in ADS, Slide - 22 ADS 2008A TDR on Bandlimited Data ADS 2008A / Measured TDR Risetime = 50 ps 9.1 GHz 22 Transient Simulation 11

12 Time Domain Simulation in ADS, Slide - 23 ADS 2008A TDR on Bandlimited Data ADS 2008A / Measured TDR Risetime = 45 ps 10 GHz 23 Time Domain Simulation in ADS, Slide - 24 ADS 2008A TDR on Bandlimited Data ADS 2008A / Measured TDR Risetime = 40 ps 10 GHz 24 Transient Simulation 12

13 Time Domain Simulation in ADS, Slide - 25 How Does ADS Do This? ADS 2008 has an improved adaptive sampling algorithm when converting the frequency domain data to time domain impulse response to better handle complex time domain response typical of noisy, lossy and resonant structures New impulse response calculation has smart extrapolation to DC The plot shows measured S11 vs. Extrapolated/ Re-sampled S11 Convolution settings should be left at default values freq, GHz 25 Time Domain Simulation in ADS, Slide - 26 Before and After vs. 50 ps risetime. Summarizing 26 Transient Simulation 13

14 Time Domain Simulation in ADS, Slide - 27 Transient Wish List #3: Passivity Non passive data can come from EM field solvers, measurements or user processing Package w/ S21 > 0 db de-embedded PCB w/ S11 > 0dB ADS 2006 implemented a new passivity enforcement for S- parameter data files Two common sources of non-passive data in measurement Dissimilar probing surface between calibration and measurement De-embedding 27 Time Domain Simulation in ADS, Slide - 28 Non-passive Measured Data Usually, measured data will be non-passive only on short, low-loss structures Often due to probe pads and DUT interfaces that differ from calibration structures Example: package measurements on solder balls or solder bumps The probe has a tendency to dig into the soft solder, resulting in an contact point that is behind the calibration reference plane Flip-chip BGA bumps Calibration Substrate 28 Transient Simulation 14

15 Time Domain Simulation in ADS, Slide - 29 Non-passive Measured Data Flip-chip BGA differential pair w/ +S21 Reflection is only slightly affected Turning on the passivity enforcement causes the data to change from Blue (non-passive) to Red (passive) below m2 m2 freq= 2.350GHz db(s(2,1))= Time Domain Simulation in ADS, Slide - 30 Non-Passive Data: De-embedding De-embedding often results in non-passive data This can be made worse if: measured data is used to perform the de-embedding the structure to be de-embedded is electrically shorter than the DUT it s being de-embedded from This non-physical data can severely impact the transient simulation accuracy Example: Test Board & Package Measured from test port to BGA device solder bump pads Measured from test port to BGA pads Desired Output: Package deembedded from PCB 30 Transient Simulation 15

16 1 1 Ref Ref Ref Time Domain Simulation in ADS, Slide - 31 De-embedding Results PCB + Package / PCB Only / De-embedded Package De-embedding is accomplished in ADS with the schematic shown 10 2-port DEEMBED component used 0 4-port component available Term Term14 Num=1 Z=50 Ohm Term Term12 Num=4 Z=50 Ohm freq GHz Term Term13 Num=2 Z=50 Ohm Term Term11 Num=3 Z=50 Ohm freq, GHz 31 Time Domain Simulation in ADS, Slide - 32 Passivity Enforced ADS 2008 can enforce passivity on the S-parameter data The Transient simulation control (left) shows how this is activated Before and After plots are shown for S(1,1) and TDR Impedance S(1,1) DiffZ time, nsec 32 Transient Simulation 16

17 Time Domain Simulation in ADS, Slide - 33 Transient Wish List #4 Long Interconnects Generating TDR on long structures requires low frequency s-parameter data to avoid aliasing in the convolution FFT Limitations in network analysis hardware can make this low frequency data difficult to obtain The adaptive sampling capability implemented in ADS 2008 includes better extrapolation to DC, which greatly improves the accuracy of the TDR response on long structures 33 Time Domain Simulation in ADS, Slide Backplane / Flex Circuit This DUT consisted of two differential flex transmission lines connected to each other through a backplane channel. Each side of the flex is attached directly to a backplane connector wafer, then mated to backplane connector mounted on the multilayer PCB. Flex probed at these points Transient Simulation 17

18 Time Domain Simulation in ADS, Slide - 35 VNA Measurements The raw data from 25 MHz to 20 GHz is shown on the left with differential S(2,1) and S(1,1) The data on the right is S(1,1) at low frequency with the ORIGINAL data in Green, the ADS processed (extrapolated, re-sampled) data in Orange freq, GHz freq, MHz The improved extrapolation at low frequencies greatly improves the TDR 35 Time Domain Simulation in ADS, Slide - 36 TDR Measurements Diff Z 1.20E E E+02 impedance (ohms) 1.05E E E+01 diff Z 9.00E E E E E E E E E E-08 time (ns) TDR measured using an Agilent 86100B Infiniium Oscilloscope with 54745A TDR Plug-in The data was captured from the scope and re-plotted using EXCEL (left) Risetime ~ ps Transient Simulation 18

19 Time Domain Simulation in ADS, Slide - 37 TDR Simulation TDR Data (from S-parameter measurements) C vs. 2008A 1.20E E E impedance (ohms) 1.05E E E time, nsec ADS 2003C is much lower than the actual TDR profile, due to aliasing while ADS 2008A agrees well. Risetime = 100 ps 37 Time Domain Simulation in ADS, Slide - 38 Conclusions ADS 2006 and beyond, significantly improves what was already the most advanced transient simulation tool for high frequency design Improves accuracy by addressing: Causality S-parameter data processing Passivity Aliasing Future possible improvements: Impedance peeling PNA measurements to DC 38 Transient Simulation 19

20 Time Domain Simulation in ADS, Slide - 39 Who is GigaTest? PCB Probing Systems and other measurement accessories Signal Integrity Engineering Measurement Simulation Design Training 39 Transient Simulation 20