Ten Questions And Answers For PCB Design

In PCB design, engineers will inevitably face many problems. This article gives an example of ten common questions and answers in PCB design, hoping to play a role in circumventing some problems when design PCB.

1. What is the "signal return path"?

Signal return path, ie return current. When high-speed digital signals are transmitted, the signal flow is from the driver along the PCB transmission line to the load, and then the load returns to the driver side through the shortest path along the ground or power supply. This return signal on the ground or power supply is called the signal return path. Dr. Johson explained in his book that high-frequency signal transmission is actually the process of charging the dielectric capacitor sandwiched between the transmission line and the DC layer. SI analyzes the electromagnetic characteristics of this enclosure and the coupling between them.

2. In a system, dsp and pld are both included. What issues should be paid attention to when wiring?

Look at the ratio of your signal rate to wiring length. If the time delay of the signal on the transmission line and the time along the signal change are comparable, the signal integrity issue must be considered. In addition, for multiple DSPs, clocks, and data signal routing topology will also affect the signal quality and timing, which needs attention.

3. Why should copper be laid?

There are several reasons for copper laying.

(1) EMC. For large areas of ground or power supply copper, it will play a shielding role, and some special grounds, such as PGND, play a protective role.

(2) PCB process requirements. Generally, in order to ensure the electroplating effect, or the laminate is not deformed, copper is laid on the PCB layer with less wiring.

(3) Signal integrity requirements, give a high-frequency digital signal a complete return path, and reduce the wiring of the DC network. Of course, there are reasons for heat dissipation, copper installation for special devices and other reasons.

4. What are the termination methods?

Termination (terminal), also known as matching. Generally, there are active end matching and terminal matching according to the matching position. Among them, the source matching is generally resistor series matching, and the terminal matching is generally parallel matching. There are many ways, including resistor pull-up, resistor pull-down, Thevenin matching, AC matching, and Schottky diode matching.

5. How to perform SI analysis on connectors?

In the IBIS3.2 specification, there is a description of the connector model. The EBD model is generally used. If it is a special board, such as a backplane, a SPICE model is required. You can also use multi-board simulation software (HYPERLYNX or IS_multiboard). When setting up a multi-board system, enter the distribution parameters of the connector, usually from the connector manual. Of course, this method will not be accurate enough, but as long as it is within the acceptable range.

6. What are the rules for termination (matching)?

The most critical thing about digital circuits is the timing problem. The purpose of adding matching is to improve the signal quality and obtain a definite signal at the moment of decision. For the level-effective signal, the signal quality is stable on the premise of ensuring the establishment and holding time; for the delay-effective signal, the signal change delay rate meets the requirements on the premise of ensuring the signal delay monotonicity.

7. In addition to protel tool wiring, are there any other good tools?

As for tools, in addition to PROTEL, there are many wiring tools, such as MENTOR's WG2000, EN2000 series and powerpcb, Cadence's allegro, zuken's cadstar, cr5000, etc., each has its own advantages.

8. In a system where digital and analog coexist, there are two kinds of processing methods. One is to separate the digital ground from the analog ground. The beads are connected, and the power is not separated; the other is that the analog power and digital power are separated by FB connection, and the ground is unified. Are these two methods the same?

It should be said that it is the same in principle. Because power and ground are equivalent to high-frequency signals.

The purpose of distinguishing between analog and digital parts is to resist interference, mainly the interference of digital circuits on analog circuits. However, segmentation may cause the signal return path to be incomplete, affect the signal quality of the digital signal, and affect the EMC quality of the system. (Myway Technology's high-speed PCB design training has started! First-line engineer lecturers are hand-in-hand professors to help students quickly learn Cadence ORCAD / Allegro design basic skills from scratch)

Therefore, no matter which plane is divided, it depends on whether the signal return path is increased and how much the return signal interferes with the normal working signal.

There are also some hybrid designs, regardless of power supply and ground. In the layout, layout and wiring are separated according to the digital part and the analog part to avoid cross-region signals.

9. Can the device's IBIS model be used to simulate the logic function of the device? If not, how to perform board-level and system-level simulation of the circuit?

The IBIS model is a behavioral model and cannot be used for functional simulation. For functional simulation, a SPICE model or other structural model is required.

10. What factors determine the method of termination (matching)?

The matching method is generally determined by the BUFFER characteristics, topography, level type and decision method, and the signal duty cycle and system power consumption must also be considered.