Use electromagnetic field simulation and measurement technology to quickly locate power/ground impedance problems

The input impedance between the power supply and the ground is an important indicator to measure the characteristics of the power supply system. The factors that affect the characteristics of the power supply system include: PCB layering, circuit board wiring, the shape of the power/ground plane, the layout of components, the distribution of vias and pins, the operating frequency of the IC, etc. In order to reduce the impedance between the power supply and the ground, the following design guidelines should be followed: 1. Reduce the spacing between the power supply and the ground layer; 2. Increase the size of the board; 3. Increase the dielectric constant of the filling medium; 4. Use multiple pairs of power and ground layers. For design engineers, an important application of measuring the impedance between the power supply and the ground is to optimize the placement of decoupling capacitors on the board.

The main function of decoupling capacitors is to suppress the resonance of the circuit board itself to reduce noise. At the same time, since EMI or noise distribution is usually closely related to the distribution of power/ground impedance in various areas of the entire circuit board, controlling the impedance between power and ground is one of the important measures to reduce the radiation of the circuit board to control EMI problems. This involves two aspects: 1. How to determine the location of the decoupling capacitor; 2. How to determine the specific value of the decoupling capacitor.

Figure 1 Model structure of power/ground plane

One of the traditional methods for measuring power/ground impedance is to use a vector analyzer to determine the power/ground impedance problems in the layout and wiring of the circuit board. The main problem with this method is that the circuit board must be designed and manufactured and the components must be installed. Once system design problems such as EMI or noise exceeding the standard are found, the possibility of reworking and redesigning the circuit board is relatively high. In addition, it takes a long time to measure the power/ground impedance using this method, the positioning accuracy of the decoupling capacitor is not enough, and repeated experiments are required to finally optimize the layout of the decoupling capacitor.

On the other hand, decoupling capacitor design rules based on experience generally require:

*Connect a 10~100mF electrolytic capacitor across the power input terminal;
*Configure a 0.01mF ceramic capacitor for each integrated circuit chip;
One of the important problems with the rule of thumb is that it is possible to add too much decoupling capacitance.
In order to shorten time to market and reduce costs, system manufacturers need a faster method to observe the area with large power/ground impedance on the circuit board system and accurately optimize the layout and setting of decoupling capacitors. To this end, this article focuses on the application of electromagnetic field simulation and measurement tools in locating large power/ground impedance points and their development trends.

Simulation tool: zero-cost location of power/ground impedance design problems

In many literatures, effective inductance is used to simulate the electrical characteristics of power and ground planes. The effective inductance model at low frequencies (Figure 1a) does not take into account the propagation and resonance of waves in the power and ground planes. Therefore, it is not suitable for simulating high-speed packaging structures and the simulation results are not accurate. The wire antenna model (Figure 1b) is another approximation of the power and ground plane structure. This method can handle wave propagation and the interaction of vias, but it takes a long time to calculate for complex structures. In addition, it is not convenient to directly connect this frequency domain technology with the time domain circuit simulator. Many companies use the popular 2D capacitor/inductor grid model in the circuit simulator to simulate the power and ground planes (Figure 1c). Using this method, the conductive plane is divided into small units, and each unit is simulated by the capacitor and inductor in the unit. The main advantage of this method is that it is suitable for the simulation of transient SPICE type circuits.

The goal of power/ground modeling is to compress power/ground noise, optimize decoupling capacitor layout, and select the correct decoupling capacitor value. The EDA tool selected in this process must have the following basic components:

(1) 2D field solver that can extract the RLGC matrix of transmission lines;
(2) Lossy transmission line simulator;
(3) 3D field solver for bond wires, vias, and metal planes;
(4) Behavioral models of ICs, driver circuits, and receivers.

For example, using Sigrity's SI simulation tool, and through a series of "what-if" simulations, the appropriate coupling capacitor value can be determined. Figure 2 shows a 4-layer board with signal, power, ground and signal layers, and the chip is located in the center of the board. Figure 3 shows the spatial distribution of the historical peak noise voltage between the power plane and the ground within 10ns. From Figure 4, it is easy to identify the location of the power and board decoupling capacitors. In addition, it can be seen that there is a large power/ground noise fluctuation in the upper corner, which is where the clock line via is located. Obviously, when the clock line transitions from the top layer to the bottom layer, the transition hole between the power and ground couples the power/ground noise. Figure 5 shows that the clock line via is located at the hot spot of synchronous switching noise.

Figure 2. Problematic clock network on a circuit board during synchronous switching outputs

The solution to compressing coupled noise is simple: install a decoupling capacitor near the clock via at the corner of the circuit board, the power/ground noise there is reduced, and the inductive coupled noise on the clock line is also reduced to below the noise threshold.

Generally speaking, through high-precision modeling calculations and full-wave electromagnetic field solution methods, such as the three-dimensional finite-difference time-domain (FDTD) method or the finite element method (FEM), it is always possible to optimize the layout of the decoupling capacitors of the entire circuit board in principle. Therefore, it is a development direction of the EDA industry.

For example, NEXXIM, the latest product launched by Ansoft, is a new generation of time-domain and frequency-domain circuit simulation tools. It has the ability to accurately and quickly simulate ultra-complex and large-scale RF and analog-to-digital hybrid circuits in the time-domain and frequency-domain. With its unique electromagnetic field simulation model, it can accurately model special devices that cannot be satisfactorily solved by traditional simulation, such as some special nonlinear devices and transformers (including asymmetric and various tapped transformers). At the same time, with its powerful simulation capabilities, it supports the increasingly complex systems and simulations.
Agilent has expanded its EDA product line to include complete 3D electromagnetic field (EM) simulation, including direct links with circuit layout and collaborative simulation capabilities.

Figure 3 Spatial noise distribution between power supply and ground at 1.5ns

Juniper Networks' Flomerics FLO/EMC provides an analysis environment for simulating electromagnetic induction inside or around electronic devices. This software is different from general electromagnetic simulation software. It uses the Transmission Line Matrix (TLM) method to solve Maxwell's equations, which can maximize the advantages of EMC simulation. The TLM method realizes that in one simulation cycle, all frequencies of useful signals can obtain the full broadband response of the system through a single operation. Its contribution to EMC analysis lies in the wide range of possible response and radiation changes. In addition, the TLM method establishes an equivalent transmission line matrix and can directly solve their voltages and currents, thereby accurately predicting the frequency and position of electromagnetic radiation.

Figure 4 Spatial distribution of peak noise voltage between power supply and ground within 10ns

The biggest benefit of using simulation tools is that before the circuit board and system design are completed, EMI problems in the system design can be discovered through simulation, and it is possible to quickly optimize the layout and settings of decoupling capacitors, thereby completing the preliminary design of the system at "zero cost".

Use electromagnetic field measurement tools to quickly observe power/ground impedance design problems

High-speed PCB analysis and simulation design tools can help engineers solve some problems in predicting the frequency and location of electromagnetic radiation. However, to accurately simulate EMC problems, SPICE models must be used. Currently, almost all ASICs cannot provide SPICE models. Without SPICE models, EMC simulation cannot take the radiation of the device itself into account (the radiation of the device is much greater than that of the transmission line). In addition, simulation tools often have to compromise between accuracy and simulation time. Tools with relatively high accuracy require a long calculation time, while tools with fast simulation speeds have very low accuracy. Therefore, using these tools for simulation cannot fully discover the problems of EMI and excessive power/ground impedance in high-speed PCB design.

Figure 5 Clock tree affected by synchronous switching noise at the via

Among various electromagnetic radiation measurement methods, near-field scanning measurement methods are often used. The measurement based on the near-field scanning principle is mainly carried out in the active near-field area. Most of the radiation signals emitted by the DUT are coupled to the magnetic field probe, and a small amount of energy diffuses into the free space. The magnetic field probe couples the magnetic flux lines of the near H field and the current on the PCB. In addition, it also obtains some trace components of the near E field. Most of the energy in the active near-field area of ​​the PCB is contained in the near magnetic field. Rongxiang Technology's Emscan scanning system is suitable for near-field diagnosis of these PCBs.

Emscan measurements can provide the following very important information: interference generation points, interference distribution, interference conduction paths covering large areas, PCB areas where interference is located, and coupling between internal structures or adjacent I/O modules. It can also show the effect of separating digital and analog circuits.

In addition, Emscan has spectrum scanning and space scanning functions. The advantage of spectrum scanning is that it allows engineers to have a general understanding of the spectrum generated by the DUT: how many frequency components there are and what is the approximate amplitude of each frequency component. The result of the space scan is for a frequency point, which is a topographic map with color representing amplitude. Engineers can see the dynamic electromagnetic field distribution of a certain frequency point generated by the PCB in real time, so as to optimize the layout and parameter selection of the decoupling capacitor. Therefore, using electromagnetic field measurement tools to observe power/ground impedance design problems is also one of the current trends in the industry.