Difference between High Frequency Circuit and High Speed Circuit
There is no strict distinction between high-frequency and high-speed circuits. It is only a general category artificially divided for different design problems.
"High-speed circuit" has become a term often mentioned by electronic engineers nowadays, but what exactly is high-speed circuit? This is indeed a "familiar" and "vague" concept.
In fact, the industry does not have a unified definition of high-speed circuits board. Usually, there are many opinions about the definition of high-speed circuits. Some people believe that if the frequency of digital logic circuits reaches or exceeds 45 MHZ to 50 MHZ, and the circuit working on this frequency has already occupied a certain proportion of the whole electronic system. For example, 1/3, is called high-speed circuit; some people think that high-speed circuit and frequency have no great connection, whether high-speed circuit depends only on their rising time; others think that high-speed circuit is the circuit that we have not touched in earlier years, or that can produce and take into account the skin effect; more people think that high-speed circuit is a kind of circuit that can produce and take into account the skin effect. The definition of high-speed is quantified, that is, when the delay of digital signal in the circuit on the transmission line is greater than 1/2 rise time, it is called high-speed circuit. The final definition is accepted by most designers.
In addition, another confusing concept is "high frequency circuit". What is the difference between "high frequency" and "high speed"?
For high frequency, many people understand that the signal frequency is higher. Although this view can not be said to be wrong, for high-speed electronic design engineers, the understanding should be more profound. We should not only care about the inherent frequency of the signal, but also consider the impact of high-order harmonics accompanied by signal transmission. The following formula is used to define the transmission bandwidth of a signal, sometimes referred to as EMI transmission bandwidth:
F = 1/(Tr*pi). F is the frequency (GHz); Tr (nanosecond) refers to the rising or falling time of the signal.
Usually when F > 100MHz, it can be called high frequency circuit. Therefore, in the digital circuit, whether it is a high-frequency circuit or not depends not on the signal frequency, but mainly on the rising edge and the falling edge. According to this formula, when the rise time is less than 3.185ns, we think it is high frequency circuit.
Generally speaking, if a distinction must be made between "high-speed circuit" and "high-frequency circuit", it is that high-speed circuit mainly depends on the relationship between the rise/fall time of signal and transmission line delay. Of course, it may be more than 1/2 rise time, or it may be 1/4, 1/6 or 1/8, depending on different applications. For the definition of high frequency circuit, it mainly depends on the spectrum generated by its edge, which is the characteristic of the signal itself, without considering the factors of transmission line. Of course, the transmission bandwidth is more than 100M, 500M, or 1GHz, which is not strictly defined, depending on the application.
