Keysight Technologies Using De-embedding Tools for Virtual Probing

Transcription

1 Keysight Technologies Using De-embedding Tools for Virtual Probing Application Note S-Parameter Series When direct measurement is not possible, use Keysight Technologies EM field solvers to determine the S-parameters of the interconnect...

2 02 Keysight S-parameter Series: Using De-embedding Tools for Virtual Probing - Application Note Using De-embedding Tools to Gain Virtual Access to Difficult Measurement Points With today s high levels of component and platform integration, measuring signals that can t be physically accessed is becoming the norm. Determining S-parameters with Keysight Technologies, Inc. EM field solvers can provide virtual measurement capability when direct measurement is not possible So far, this series of papers have shown how S-parameter models can be used to de-embed the response of the interconnect between the oscilloscope probe and the point of interest: virtual probing. Let s now consider the question of how to obtain that S-parameter model in the first place. The obvious method connect a VNA to each end of the interconnect and measure the S-parameters works in some cases. Subtracting the effects of a long cable is an example. But what if one end is inaccessible? After all, the main premise of virtual probing is exactly that: it s hard to place a probe on the point of interest. For these cases, the solution is modeling based on an electromagnetic (EM) field solver, EM-based modeling for short. This situation is becoming increasingly common. The pressure on the electronics industry towards miniaturization and integration shows no signs of abating. In fact, new technologies such as dense ball grid array (BGA) packages, high layer-count printed circuit boards (PCBs), 3D integrated circuit (ICs), non-planar, multi-level wafers, <15-nm gate technologies, and ultra-low power designs will continue to eliminate direct point measurements. In many cases, the only practical way to acquire transmission line parameters is by simulation. As explained in the previous papers in this series, Keysight offers flexible of solutions that will make addition of this technique to your repertoire as painless as possible. For this discussion, the configuration utilized will be Keysight s InfiniiSim de-embedding software, coupled with the Infiniium real-time oscilloscope, and an EM-based model generated by an appropriate electromagnetic field solver from Keysight EEsof EDA: ADS Momentum Element, EMPro finite element method (FEM) Element, or EMPro finite difference time-domain (FDTD) Element. The Predicament Making a measurement by attaching the probe directly to an accessible component or circuit is reliable and straightforward. With inaccessible measurement points the opposite is true. For example, the signal ball at the interior of a soldered down BGA cannot be reached without damaging the IC. The same goes for a copper trace on an internal layer of a circuit board. The Infiniium real time scope with InfiniiSim de-embedding software allows the engineer to measure signals from any point on the device under test (DUT), even those inaccessible to a physical probe. The DUT can be anything from a fully populated printed circuit board (PCB) or a highly integrated field-programmable gate array (FPGA), to a single transmission line or via in multi-layer layer substrates. The measurement is accomplished by stand off or virtual probing at a convenient point (an SMA connector on the edge of the PCB, for example) and then subtracting off the time domain response of the intervening connection. As we ve seen, this process is known as de-embedding. This paper shows how to use 3D geometry, material properties, and electromagnetic simulation to create the de-embedding information even for points of interest that are inaccessible.

3 03 Keysight S-parameter Series: Using De-embedding Tools for Virtual Probing - Application Note EM 101 EM is an extremely important topic that analog engineers spend years learning about. Digital engineers historically have not needed to understand it until recently when clock speeds ramped up to the GHz range. As a rule of thumb, EM must be used when the propagation delay across an impedance discontinuity is larger than the rise/fall time of the digital signal edge. The EM equations are called Maxwell s equations. The fundamental set (called the total charge formulation) involves, charges, their motion, the E field and B field they generate, and the Lorentz force so caused. The equations involve two fundamental constants (vacuum permittivity ε 0 and vacuum permeability µ 0 ) whose values depend on the system of units in use. Although the total charge formulation is fundamental, it has limited use in practice because any human scale system will have a gigantic number (on the order of Avogadro s number) of charged particles, making the equations too complex to solve. Luckily there is trick called the free charge formulation that rolls up most of the charge into linear macroscopic material properties: ohmic conductivity, relative permittivity, and relative permeability: Permeability B = µ 0 µ r H Permittivity: D = ε 0 ε r E Ohmic conduction: J free = σε The equations to be solved are: Gauss with free charge neutrality:.d =ρfree = 0 No monopoles:. B = 0 Faraday: E = db/dt Ampère/Maxwell: H = J free + dd/dt Today, modern high-speed computers and sophisticated programs have the ability to solve these equations by keeping track of some bounded part of space and how the fields influence each other within this space. Δ Δ Δ Δ One popular method is based on a grid of so-called Yee cells. These are comprised of two staggered grids of points in space where loops of E surround a B point and vice versa. With this technique, the evolution of the fields over time can be tracked. This is the basis of many finite difference time domain solvers, although modern codes like EEsof s EMPro refine the grid somewhat for efficiency (see Figure 1). E B Figure 1. Yee Cell Diagram

4 04 Keysight S-parameter Series: Using De-embedding Tools for Virtual Probing - Application Note The solution modern computational mathematics There are several solutions available for computational electromagnetics. Finite difference time-domain (FDTD) analysis has been popular but finite element methods (FEM) and method of moments (MoM) are very effective with today s fast computational hardware and powerful software. Depending on the problem any or all of these approaches will work. However, each has advantages and disadvantages. For example, if the 3D structure is a multilayer assembly such as a PCB or chip package, the first choice would be MoM, because of its speed. However, and although MoM is being extended outside of the pure multilayer constraint certain bond wire models for example it doesn t handle arbitrary 3D structures as well as FEM and FDTD. If the target is a connector, dielectric brick or ball grid array (BGA) break out, the choices are between FEM and FDTD. To determine which is the best method simply depends upon the characteristics of the target. For example, a high-q structure favors FEM, while for an electrically large device FDTD is the better choice. If the structure has a large number of ports, FEM is favored because it solves all ports in parallel, in contrast to FDTD which has to solve them sequentially. Ultimately the method chosen is determined by the number and type of characteristics of the structure. Keysight EEsof EDA offer all three types: ADS Momentum Element, EMPro FEM Element, and EMPro FDTD Element. All three solvers are fully integrated with the ADS design flow. ADS layouts can be exported to EMPro and EMPro can import parameterized 3D structures into ADS. The FEM engine can even be run directly from ADS, without opening the EMPro user interface for a smooth design flow (see Figure 2). Import and draw schematics and multilayer structures ADS Parameterized 3D EM components EMPro Import and draw 3D structures ADS layout export Direct simulation: no export required Momentum Simulator 3D Planar EM Method of moments multilayer structures+ FEM Simulator 3DEM Finite element method arbitrary structures FDTD Simulator 3DEM Finite difference, time domain arbitrary structures Figure 2. EMPro functionality is completely integrated with ADS and Momentum

5 05 Keysight S-parameter Series: Using De-embedding Tools for Virtual Probing - Application Note EM analysis Now that the groundwork has been laid, it is time to do the actual EM analysis. First, a look at the design flow. If a structure is already available in one of the common mechanical computer-aided design (MCAD) formats, it is a simple task to import it into EMPro. It can be edited in the user interface (UI) if desired. If the structure doesn t exist, you can create it. The first task in a new structure is to set up the ports. If at all possible use waveguide ports since EM waves are distributed in nature and bidirectional in flow. Waveguide ports give the best accuracy and mimic the way S-parameters are defined in the sense of incident, reflected, and transmitted waves. The tool can automatically divide space into a mesh or grid similar to the Yee cells mentioned above.(see Figures 3a and 3b). Figure 3a. EM flow block diagram part 1 Sensors Simulation Setup Post-Processing FDTD Done FDTD Result window FEM Figure 3b. EM flow block diagram part 2

6 06 Keysight S-parameter Series: Using De-embedding Tools for Virtual Probing - Application Note EM analysis At this point there is an option to set up sensors if desired, run the simulation and visualize the resulting fields and currents superimposed on your structure. A lot of insight can be gained from these graphics, which will help explore the design space by suggesting what structure to resize or move. If the result is satisfactory, a multiport S-parameter model of the structure can be imported into any tool (like Infiinsim) that accepts CITI or Touchstone format. There is a demo available that will step through the creation of a new CAD project and obtaining the S-parameter file. If the reader would like to view this before proceeding with this paper, it is available here: maxwells-equations-primer/. Basically the steps are as follows within the EMPro environment: 3D drawing environment: Basic extruded shapes (cube, sphere) Boolean operations: subtract tool from blank Hierarchical editing: Undoing any step, not just the last step by using the hierarchy Add material properties and change appearance Importing a complete structure Add and setup waveguide ports Align simulation boundary with measurement plane Set up simulation Visualize results: Bode plot, Smith chart, fields Export S-parameters Advanced visualization 3D field animation etc. Channel element removal TX Measure here Channel Element Result Probe here TX Figure 4. Getting the real measurement by virtual probing Once the S-parameter file has been generated, it can be used to determine the characteristics of the DUT, without having to place the probe at the exact physical point of the DUT the essence of this discussion (see Figure 8). This process will be detailed using Infiniisim to subtract or factor out (de-embed) the effect of this channel element. The net effect is that this is the data at the desired measurement point minus the effects of the elements between the probe and measurement point

7 07 Keysight S-parameter Series: Using De-embedding Tools for Virtual Probing - Application Note Conclusion This paper has presented the best method for determining accurate measurement parameters via simulation. The discussion involved how to de-embed, or factor out the interconnect characteristics when the physical measurement point is not at the desired measurement point. This method presented will allow the designer to probe in a convenient location on the structure and use the acquired data to observe in real time the actual waveform or eye pattern diagram occurring seen at the desired, inaccessible location. Such methodologies and their related tools are extremely useful to the designer because they help you anticipate problems and save time, money, and bring products to market earlier thereby accelerating your design process. The final paper in this series will present to the designer the concluding process of taking S-parameter files, whether simulated or measured, and view them on an oscilloscope.

8 08 Keysight S-parameter Series: Using De-embedding Tools for Virtual Probing - Application Note mykeysight A personalized view into the information most relevant to you. AdvancedTCA Extensions for Instrumentation and Test (AXIe) is an open standard that extends the AdvancedTCA for general purpose and semiconductor test. Keysight is a founding member of the AXIe consortium. ATCA, AdvancedTCA, and the ATCA logo are registered US trademarks of the PCI Industrial Computer Manufacturers Group. LAN extensions for Instruments puts the power of Ethernet and the Web inside your test systems. Keysight is a founding member of the LXI consortium. PCI extensions for Instrumentation (PXI) modular instrumentation delivers a rugged, PC-based high-performance measurement and automation system. Three-Year Warranty Keysight s commitment to superior product quality and lower total cost of ownership. The only test and measurement company with three-year warranty standard on all instruments, worldwide. Keysight Assurance Plans Up to five years of protection and no budgetary surprises to ensure your instruments are operating to specification so you can rely on accurate measurements. Keysight Technologies, Inc. DEKRA Certified ISO 9001:2008 Quality Management System Keysight Channel Partners Get the best of both worlds: Keysight s measurement expertise and product breadth, combined with channel partner convenience. For more information on Keysight Technologies products, applications or services, please contact your local Keysight office. The complete list is available at: Americas Canada (877) Brazil Mexico United States (800) Asia Pacific Australia China Hong Kong India Japan 0120 (421) 345 Korea Malaysia Singapore Taiwan Other AP Countries (65) Europe & Middle East Austria Belgium Finland France Germany Ireland Israel Italy Luxembourg Netherlands Russia Spain Sweden Switzerland Opt. 1 (DE) Opt. 2 (FR) Opt. 3 (IT) United Kingdom For other unlisted countries: (BP ) This information is subject to change without notice. Keysight Technologies, Published in USA, August 2, EN