Signal Integrity Analysis for High-speed PCB
Introduction:
As large-scale and ultra-large-scale integrated circuits are more and more applied to power circuit systems, the requirements of users are higher and higher. As a result, the integration scale of chips is larger and smaller, the number of pins is larger and higher, and the frequency is higher and higher. However, the rapid development of the circuit brings about such a problem: the chip size is getting smaller, and the pins are generally increasing, leading to a higher circuit density, which makes the signal integrity problem caused by the correct processing of the high-speed signal become a key factor for the success of the design.
How are high-speed circuit defined? There are usually two solution, narrow and broad. In the narrow sense, the working frequency of the circuit is no less than 30MHz, and a third of the circuit high-speed circuit. But this is one-sided. Therefore, the generalized explanation is more reasonable, considering the time of the signal’s rise and fall. If the time of the circuit signal driving end, we consider this as a high-speed signal. At the time, the factors considered are irrelevant to the specific frequency of the signal. Such a circuit becomes a high-speed circuit.
Signal Integrity:
Signal integrity refers to the quality of the signal on the transmission path, which can be ordinary metal wire, optical device or other media. Signal integrity refers to the value of voltage level required to be achieved when required. Poor signal integrity is not caused by a signal factor, but by multiple factors in board design. The signal integrity of high-speed circuits mainly includes delay, reflection, oscillation crosstalk and so on.
1. Definition of Delay and Influencing Factors
Delay is the transmission delay of the signal on the transmission line, which is related to the transmission line length anf signal transmission speed. The delay of the signal has a great influence on the timing of the system.If the delay is too large, it will affect the operation of the circuit at the next moment. In serious cases, it will also lead to the disorder of timing, resulting in the failure of the circuit to work normally.
2. Definition and Solution of Reflection
Reflection refers ti the incompelete absorption of signal energy due to impedance mismacth when the signal is transmitted on the transmission line. Reflection can be caused by a variety of factors, such as the sudden change of corner, through hole and wire width, or the incorrect connection of the wire ends and transmission through the connector. If the load impedance is less than the source impedance, the reflected voltage is negative. The reverse reflection voltage is positive.
In order to minimize the negative effects of reflexes, we must take steps to suppress reflexes. In general, the most feasible method is to match the characteristic impedance of the transmission line with the source impedance and load impedance of the signal, that is to say, we can connect to a resistor at the sending end or the receiving end to make the reflection coefficient zero to eliminate refection. There are many ways to connect an end as following:
1) Parallel terminal matching. The advantage of this technique is that it is simple to design, easy to apply, and requires only one additional resistor, while the drawback is that the matching resistance will bring dc power and degrade the logic high output level of the signal. Parallel terminal matching is more used in analog circuit design to meet the requirement of the high accuracy at the expense of power consumption.
2) Serial terminal matching, also known as source impedance matching. The advantage of this method is that few additional components are needed, only one resistor element is needed for each driver, and the power consumption of matching resistance is very small. However, the impedance of TTL device varies with the logical state of the device, which makes design requiring low power consumption, serial terminal matching is more commonly used.
3) David south terminal matching---double resistance form. The advantage of this matching technology is that the matching resistance does not only have single effect, it can be used as the pull-down resistance in addition to the pull-up resistance, which can restrain the signal overshoot and reduce unnecessary power consumption, thus strengthening the noise tolerance of the system. But there is also a drawback to this matching: there is a large dc current flowing through the partial pressure resistance.
4) AC terminal matching, also known as RC terminal matching technology. The advantage of this method is that the capacitance has the function of resistance low frequency and transmission high frequency, thus reducing the power consumption. Proper all stages of the design, design and simulation throughout the entire design process, and realize the controllability of the process and the quantification of specific indicators. Only in this way can the problem of signal integrity be avoided and efficient design be achieved.
