Probes and Setup for Measuring Power-Plane Impedances with Vector Network Analyzer

Probes and Setup for Measuring Power-Plane Impedances with Vector Network Analyzer Plane impedance measurement with VNA 1

Outline Introduction, Y, and S parameters Self and transfer impedances VNA One-port impedance measurement Two-port impedance measurement DUTs Measured self and transfer impedances Correlation to simulations Resources References Plane impedance measurement with VNA

Requirements in Power Distribution In digital and mixed analog/digital systems: Core and signaling voltages drop Noise margin goes down Core and total I/O current go up Bandwidth goes up Requirement: Few milliohms over hundreds of MHz bandwidth. Solution: In multilayer boards, power and ground are distributed over (solid) planes. Plane impedance measurement with VNA 3

, Y, and S Parameters v v i v 1 1 = = = = v i Y Y = i 11 1 i i 1 1 + + = Y v 11 1 v 1 v 1 + Y 1 1 + Y i i v v b b 1 = = b S S 11 1 = Sa a a 1 1 + + S S 1 a a ero wave is easier to set Calibration plane is not critical ero volt/current is hard to set Calibration plane is critical Plane impedance measurement with VNA 4

Which Parameters Do We Need At high frequencies, S parameters are easier to measure, but Digital designers deal with voltages and currents Good power-distribution network is a voltage source >>> parameters needed I i V i V j ji i = 0 i j ii = = v i i i v i i j i j = 0 j Plane impedance measurement with VNA 5

Measuring Power-Distribution Network TDR LCR Bridge VNA Γ or S11, and S1 Plane impedance measurement with VNA 6

What is a VNA Tuned sinewave generator Directional couplers Tracking receiver(s) Source Directional coupler Receiver b 1 a 1 S S Port1 11 1 = = b a 1 1 b a 1 a a = 0 = 0 b DUT Port Plane impedance measurement with VNA 7

One-Port Self-Impedance Measurement S 11 = in in + 50 50 11 = The impedance is measured between the ground and power planes at the selected point in = 1+ 50 1 S S 11 11 Plane impedance measurement with VNA 8

Errors of One-Port Self-Impedance Measurement VNA accuracy is lower at high reflections Connecting discontinuity is in series of low- DUT Plane impedance measurement with VNA 9

VNA Error in S11 Measurement S11 uncertainty of HP870D is 1.5% at Γ ~1 in the 50-000MHz range Impedance uncertainty is 0.375 ohms For low measurement errors, DUT must be in the ohms range But we want to measure fractions of an ohm Plane impedance measurement with VNA 10

Errors Due to Discontinuities 50 mils of pigtail connector/cable discontinuity is L p ~0.4nH 0.4 nh is p ~.4 ohms at 1GHz Problem: p is in series to DUT Port1 of VNA and cable: 50 ohm L p V S DUT Plane impedance measurement with VNA 11

Two-Port Self-Impedance Measurement S 1 instead of S 11 is measured S 1 uncertainty is less p is in series to 50 ohms instead of DUT Cable and Port1 of VNA: 50 ohm V S DUT Cable and Port of VNA: 50 ohm Plane impedance measurement with VNA 1

Two-Port Self-Impedance Reading First-order calculation: Assume that L p ~ 0 DUT << 0 Port1 and Port of VNA: 5 ohm DUT = 11 = S 1 * 5 [ohm] V S DUT Plane impedance measurement with VNA 13

Transfer Impedance Measurement Cable and Port1 of VNA: 50 ohm Cable and Port of VNA: 50 ohm V S V 1 DUT V Plane impedance measurement with VNA 14

Transfer Impedance Reading First-order calculation: Assume that L p ~ 0 11 << 0 << 0 1 << 0 1 = 1 = S 1 * 5 [ohm] Plane impedance measurement with VNA 15

S1 Uncertainty S 1 uncertainty of HP870D: <1dB in the S 1 > -60dB range <3dB in the S 1 > -70dB range Impedance uncertainty: 1dB (10%) for DUT > 5milliohms 3dB (40%) for DUT > 8milliohms Plane impedance measurement with VNA 16

Measurement Setup Vector Network Analyzer: HP 4396 or HP 870C VNA Probe: pieces of 1-inch long semirigid coax DUT Plane impedance measurement with VNA 17

Dual Semirigid Probe Cross-sectional view of the Dual Semirigid Probe NOT TO SCALE Semirigid coax 1 31 mils Coax pigtail: L p Semirigid coax Coax sleeve soldered all around to upper plane Parallel Cu planes 80 mils: 1.5% of λ at 1GHz Plane impedance measurement with VNA 18

Device Under Test (1) Top view: 31 mil plane separation 10-inch by 10-inch FR4 two-sided PCB Bare PCB or 5.1 ohm/inch DET Signal input and output: 1 Dual Semirigid Probe Test points: on a 1-inch by 1-inch grid Side view without and with DET: 805 SMD 5.1 ohm per inch Plane impedance measurement with VNA 19

Device Under Test () Top view: mil plane separation 10-inch by 10-inch FR4 PCB Bare PCB or 1 ohm/half-inch DET Probes: 1 Dual Semirigid Probe Test points: on a 1-inch by 1-inch grid Side view with DET: 805 SMD 1 ohm / half inches Plane impedance measurement with VNA 0

Residual Probe Response (1).5 0.5 Log impedance magnitude [ohm] DUT is not in place Dual Semirigid Probe, 50-mil pigtails, disconnected 0.05 0.005 Log frequency [Hz] 0.1G 0.5G 1G 5G Plane impedance measurement with VNA 1

Residual Probe Response ().5 0.5 0.05 Log impedance magnitude [ohm] L of -mil planes L of 0.-mil planes Probe sleeves are soldered to 31-mil DUT 50-mil probe tails in place, but not connected 0.005 Log frequency [Hz] 0.1G 0.5G 1G 5G Plane impedance measurement with VNA

Measured Self Impedance of Bare Board at Center.5 0.5 Log impedance magnitude [ohm] 31-mil DUT, edge open Dual Semirigid Probe at center of planes 0.05 0.005 Log frequency [Hz] 0.1G 0.5G 1G 5G Plane impedance measurement with VNA 3

.5 0.5 0.05 Measured Self Impedance of Board with DET at Center Log impedance magnitude [ohm] 31-mil DUT, edge terminated with 40x5.1 ohms Dual Semirigid Probe, at center of planes 0.005 Log frequency [Hz] 0.1G 0.5G 1G 5G Plane impedance measurement with VNA 4

Measured vs. Simulated Self-Impedance of DUT 10 x 10 x 31mil FR4 with DET Measured with HP870C VNA Simulated with 1-inch grid at center node: Freq [Hz] magn[ohm] 1.00E+08 0.3143 1.58E+08 0.4413.51E+08 0.6574 3.98E+08 1.33 6.31E+08 1.744 1.00E+09.645.5 0.5 0.05 Measured at center node Log Log magnitude of of impedance [ohm] [ohm] x x x x x x Measured Simulated Log Log frequency [Hz] [Hz] 50M 100M 0.5G 1G 5G Plane impedance measurement with VNA 5

Measured Transfer Impedance Log Log impedance magnitude [ohm] [ohm].5 0.5 0.05 S1 S1 [db] [db] -0-40 -60 31-mil DUT, edge terminated with 40x5.1 ohms Two separate semirigid probes, at middle of side and at corner 0.005 Log frequency [Hz] 0.1G 0.5G 1G 5G Plane impedance measurement with VNA 6

Measured vs. Simulated Transfer Impedance of DUT oomed responses: -40-50 S1 S1 [db] [db] Measured Simulated -60 0.1G 0.5G 1G Log frequency [Hz] Plane impedance measurement with VNA 7

Self Impedance, -mil DUT 1 0.1 Self impedance magnitude [ohm] -mil DUT, edge terminated with 80x(1ohm + 100nF) Dual Semirigid Probe, at center of planes 0.01 0.001 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 Frequency [Hz] Plane impedance measurement with VNA 8

Equivalent Circuit of Probes Connection Coax and Port1 of VNA: 50 ohm Self impedance: L p1 L p V S 11 = DUT Coax and Port of VNA: 50 ohm Transfer impedance: Cable and Port1 of VNA: 50 ohm L p1 L p V S V 1 DUT V Cable and Port of VNA: 50 ohm Plane impedance measurement with VNA 9

S 1 Conversion to Self Impedance ii = S 1 1 1 1+ jωτ S1 5 1 + 1 S 1 S1 1 p Where 1 = 50+jωL p1 = 50+jωL p τ p = L p /50 Plane impedance measurement with VNA 30

S 1 Conversion to Transfer Impedance Where S ji 1 = S 1 5 1 = 50+jωL p1 = 50+jωL p τ p = L p /50 1 + 11 1 1 + 1 S11 1 1+ 1+ 1+ 50 jωτ p S 1 1 + 11 1 1 + Plane impedance measurement with VNA 31

Recommended Resources Hewlett Packard Vector Network Analyzers: HP 870 VNA HP4396 VNA Circuit simulator software: Avant! HSPICE Plane impedance measurement with VNA 3

Conclusions Power-distribution network is characterized by self and transfer impedances One-port measurements cannot handle low impedances -port VNA measurement introduced Probes: Dual semirigid coax with soldered pigtail Transmission-line grid is used to simulate parallel planes Good agreement between measured and simulated self and transfer impedances was found Plane impedance measurement with VNA 33

References [1] Dale Becker, Larry Smith, "Power Distribution Design for High Performance Systems," Short course at the 1998 Topical Meeting on Electrical Performance of Electronic Packaging, October 6-8, 1998, West Point, NY [] Understanding the Fundamental Principles of Vector Network Analysis, Hewlett Packard Application Note 187-1. [3] I. Novak, "Reducing Simultaneous Switching Noise and EMI on Ground/Power Planes by Dissipative Edge Termination," Proceedings of the 1998 Topical Meeting on Electrical Performance of Electronic Packaging, October 6-8, 1998, West Point, NY, pp.181-184. [4] E. Leroux, Peter Bajor, Modeling of Power Planes for Electrical Simulations, Proceedings of the 1996 Wroclaw EMC Symposium, June 5-8, 1996. [5] Henry Wu, Jeffrey Meyer, Ken Lee, Alan Barber, "Accurate Power Supply and Ground Plane Pair Models," Proceedings of the 1998 Topical Meeting on Electrical Performance of Electronic Packaging, October 6-8, 1998, West Point, NY, pp.163-166. [6] Personal communications with Rich Hoft, Hewlett Packard, Burlington, MA. Plane impedance measurement with VNA 34