High Speed PCB

2021-02-26 14:35:32 Edward Qin 23

What exactly is a high-speed circuit? In the past low-speed era, the signal rise time during level transitions was longer, usually a few ns. The interconnection lines between devices will not affect the function of the circuit, and there is no need to care about signal integrity issues. But in today's high-speed era, with the increase of IC output switching speed, many are at the picosecond level. Regardless of the signal period, almost all designs have encountered signal integrity problems. In addition, the pursuit of low power consumption makes the core voltage lower and lower, and DDR4 1.2v core voltage has become very common. Therefore, the noise margin that the system can tolerate is getting smaller and smaller, which also makes the signal integrity problem more prominent.

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The root of the signal integrity problem lies in the reduction of the signal rise time. Even if the wiring topology does not change, if an IC chip with a small signal rise time is used, the existing design will be in a critical state or stop working at all.

 1. Definition of high-speed circuits

In a narrow sense, it is generally considered that the frequency of digital logic circuits reaches or exceeds 50MHz, and the circuits operating above this frequency have accounted for a considerable part of the entire electronic system, such as one-third, which are called high-speed circuits. In fact, the harmonic frequency of the signal edge is higher than the frequency of the signal itself. It is the rising and falling edges of the signal that cause various problems in signal transmission. Therefore, when the transmission path length of the signal is greater than 1/6 times the wavelength of the transmitted signal, the signal is regarded as a high-speed signal; when the signal propagates along the transmission line, serious skin effect and ionization loss occur, it is regarded as high-speed signal. Therefore, it is generally agreed that if the propagation delay of the signal on the circuit board is greater than half of the rise time of the digital signal driver, it is considered that such signals are high-speed signals and produce transmission line effects. Such circuits are high-speed circuits.

 2. The difference and connection between high speed and high frequency

Most signal integrity books generally introduce the two basic concepts of high-speed and high-frequency before conducting signal integrity research. From this point, we can know the importance of distinguishing these two concepts, because through these two This concept can clarify the object of signal integrity analysis. The concept of high frequency is relatively simple to understand. It is only a description of frequency. As we all know, frequency is the reciprocal of the period, and high frequency is the expression of high frequency and short period.

Let's talk about the concept of high speed, returning to the concept of speed. Speed is a physical quantity that characterizes the speed of movement. In physics, it is the differential of displacement versus time, which is dS/dt. It can also be applied to circuits to refer to the differentiation of electrical displacement with respect to time, which is characterized by the speed of voltage change, that is, dV/dt, which is usually expressed as rise time. Therefore, a high-speed circuit is an expression of rapid voltage change and short rise time. In the circuit system, the size of the rise time has a great influence on the signal integrity, and it is also the source of the signal integrity problem. So that the signal integrity analysis is basically around dV/dt to analyze and discuss, rather than discussing the cycle, which is also the essential difference between high speed and high frequency.

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Therefore, from the description of the two concepts above, it can be understood that there is no direct relationship between high frequency and high speed. A few examples can be cited pros and cons. For example, when a clock signal has a frequency of 50 MHz and a rise time of 90 ps, it is not a high-frequency signal, but it is a high-speed signal, that is, the frequency is not high, but the rising edge is fast. For another example, the frequency is 500MHz and the rise time is 0.8ns. Compared with the signal in the above example, the frequency is much higher, but the speed is much lower than the signal in the above example. Therefore, we say that there is no relationship between the high frequency of the signal and the high speed.

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From another perspective, is there really nothing to do with high-speed and high-frequency? In fact, this is not the case. People usually confuse these two concepts precisely because they are inextricably linked. Specifically, as the frequency increases, the period decreases. As a result, we must increase the speed because we must ensure sufficient setup time and hold time. With the compression of the cycle, if you want to have enough setup time and hold time, you can only shorten the rise time and fall time to meet the requirements of signal timing validity. For example, the frequency of a signal is 100MHz, that is, the period is 10ns, and the rise time and fall time are 1ns respectively, so the effective sampling time window of the signal is: (10-1-1)ns, which is 8ns. If the frequency of this signal is increased to 200MHz, keeping the rise time and fall time unchanged, the sampling window becomes (5-1-1)=3ns, and as the frequency continues to increase, the sampling window will continue to decrease , Extreme conditions will lead to inability to sample correctly, so the rise time and fall time are forced to decrease to meet the higher and higher sampling clock frequency. To sum up in one sentence, the increase in frequency will inevitably force the increase in speed. The evolution of high-frequency circuits has led to high-speed circuits. The relationship between high-frequency and high-speed is a sufficient condition, not a necessary condition. In addition, in the process of signal integrity analysis, high-speed circuits are generally emphasized.

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