Five minutes to tell you what a high-speed circuit is

"High-speed circuits" have become a common term used by electronics engineers today, but what exactly is a high-speed circuit?This is indeed a "familiar" and "fuzzy" concept.As a matter of fact, the industry of high speed circuit does not have a unified definition, usually on the definition of high-speed circuit has the following a variety of views: some people think that, if the frequency of the digital logic circuit to achieve or exceed 45 MHZ ~ 50 MHZ, and work on the frequency of the circuit have accounted for the entire electronic system of a certain quantity (such as 1/3), is called a high-speed circuit;There are also those who think that there is no big link between high-speed circuits and frequencies, and whether the high speed circuit depends only on their rising time;Some people think that high speed circuits are the circuits that we haven't had in the past year, or can generate and take into account the skin effect;More and more people are on the high speed of quantitative definition, namely when the digital signal on the transmission line circuit delay is more than 1/2 rise time, is called high speed circuit, this article also continue to use this definition as a view of the standard of high speed problem.

What's more, one of the more confusing is the concept of "high frequency circuits", and what's the difference between "high frequency" and "high speed"?For high frequency, the understanding of many people is the high frequency signal, although can't say that this view is wrong, but for high-speed electronic design engineers, should understand more deeply, we care about in addition to the natural frequency of signal, signal emission should also be considered when accompanied by high order harmonic impact at the same time, generally we use the following the launch of this formula to define the signal bandwidth, sometimes referred to as the EMI emission bandwidth:

F=1/(Tr* PI), F is frequency (GHz);Tr (nanosecond) refers to the time of rise or decrease of signal.

Usually when F BBB 0, 100 MHZ, you can call it a high-frequency circuit.So, in a digital circuit, whether it is a high-frequency circuit, not the frequency of the signal, but mainly depends on the rising edge and the falling edge.According to this formula, when the rise time is less than 3.185 ns, we think it is a high frequency circuit.

For most PCB electronic circuit hardware design engineer, no need to dwell on differences in concept, should have a broad definition of "high speed" in the heart, that is: if on the premise of ensure the correct electrical connections, circuit still can't work stable performance, and the need for special layout, wiring, match, shielding, such as processing, so, this is the "high speed" design.