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# The difference between PCB high speed and low speed design

To determine whether a signal is a high-speed signal, you must first distinguish between several misunderstandings.

Misunderstanding 1: The signal cycle frequency FCLOCK is high and belongs to high-speed design.

In fact, the highest frequency we consider when designing often depends on the effective frequency of the signal (also known as the corner frequency) Fknee.

The signal cycle frequency and effective (turning) frequency are defined as:

FCLOCK Tclock = 1/(10% 90%) 0.5/ Fknee = Tr − (for most signals in practice)

Myth 2: Capa citive and Inductive Ideal Devices

In the low-speed field, the working frequency bands of capacitors and inductors are relatively low, and they can be considered as ideal devices. However, in the high-speed field, the capacitive inductance on the PCB can no longer be simply regarded as a pure capacitive inductor. For example, in the low-speed field, we can regard the capacitor as an open circuit. In the high-speed circuit, assuming that the operating frequency is F, the reactance value of capacitor C on both sides is 1/ 2πF × C, at a high operating frequency. In the case where the reactance value becomes small, the capacitance appears as a short circuit. Recognize the above two misunderstandings to discuss the problem of low-speed signals and high-speed signals. For low-speed signals, the levels of the points on the transmission path are approximately the same. We can use lumped thinking to treat the transmission path, that is, the states of the points on the transmission path are the same; for high-speed signals, on the transmission path The level of each point is different, and it needs to be viewed in a distributed way, that is, the state of each point is different. Therefore, the high speed and low-speed distinction depend on the length of the signal transmission path.

In general, when the length of the signal transmission path (ie, the length of the signal line) is less than 1/6 1/6 1/6 1/6 of the effective wavelength of the signal, the level of each point on the transmission path can be considered. The states are approximately the same. From the relationship between signal wavelength and frequency λ = c / F, we can distinguish between high speed and low speed according to the following steps;

1 Obtain the effective frequency of the signal Fknee and the length L of the trace;

2 Using Fknee to calculate the effective wavelength λknee of the signal;

3 Determine the relationship between LLLL and 1/6 1/6 1/6 1/6λknee. If LLLL>>>>1/6 1/6 1/6 1/6λknee, the signal is a high-speed signal; otherwise, it is a low-speed signal.

Note: The first way to obtain Fknee is to measure directly. The second is that the empirical value can assume that the rising edge of the signal is 7% of the signal period. At this time, the effective frequency Fknee is about 7 times of the periodic frequency FCLOCK. For example, the periodic frequency FCLOCK is 100MHz. The cloc k signal can be estimated to have an effective frequency of approximately 700 MHz.

In summary, we can also get the concept of transmission lines and non-transmission lines. As with the judgment of the high-speed low-speed signal, the high-speed signal is regarded as the transmission line, and vice versa.

example:

Signal 1111: FCLOCK is 100MHz, the rising edge time is 2ns, and the trace length L is 6in (inches);

Signal 2222: FCLOCK is 100MHz, the rising edge time is 0.5ns, and the trace length L is 6in (inches).

For signal 1:

For signal 2:

Of which: 1mmmm=39.37 in

The trace length of signal 1 is 6in, which is less than 1/6 of λknee, so it can be regarded as a non-transmission line;

The trace length of signal 2 is 6in, which is greater than 1/6 of λknee, so it should be regarded as a transmission line.

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