Table of contents
1. Definition of high-speed signal:
2. Concentration constant and distribution constant:
3. What is a high-speed circuit?
4. Problems to be paid attention to when studying high-speed circuits
2) Signal reflection, overshoot, ringing
3) Signal delay and timing error
1. Definition of high-speed signal:
(1) Any signal with a frequency greater than 50MHz is a high-speed signal;
(2) Whether the signal is high-speed or not is not directly related to the frequency , but when the rising edge (or falling edge) time of the signal is less than 50ps , it is considered a high-speed signal;
(3) When the length of the transmission path where the signal is located is greater than 1/6 times the wavelength of the transmission signal , the signal is considered to be a high-speed signal;
(4) When the signal is transmitted along the transmission path and has serious skin effect and ionization loss , it is considered to be a high-speed signal.
The above definitions are all reasonable to some extent. The author's point of view is: "For an electronic system design engineer, when you do not have enough confidence and evidence to ensure that the signal works normally in the system, special processing must be performed, and simulation is required to determine layout, wiring, Design constraints such as matching and shielding should be handled as high-speed signals at this time.”
For 2, a higher signal frequency is a sufficient condition for high-speed signals, but not a necessary and sufficient condition; of course, the rising and falling edges of the signal are also very important criteria for judging: generally speaking, the rising and falling edges of the signal The smaller the value, the more high-frequency components of the signal, the larger the signal bandwidth, and the integrity of the signal must be considered .
2. Concentration constant and distribution constant:
(1) Concentration constant : The so-called "concentration" means that on the circuit diagram, each component and its transmission line with actual dimensions are represented by a single circuit symbol or lead that does not contain dimensions. That is to say, an element having an actual size and its transmission line are handled as "a case where no size is concentrated at one point". Low frequency circuits can be regarded as lumped constant circuits.
(2) Distribution constant : The so-called "distribution" means that each component and transmission line are treated as components with actual dimensions, and a component and transmission line are integrated as tiny components for processing. That is, the transmission line is treated as a circuit element. High frequency circuits can be regarded as distributed constant circuits.
Note: The lumped constant circuit is a special circuit of the distributed constant circuit, which is used in the occasion where the amplitude and phase of the signal in the transmission line are approximately constant .
Original link: https://blog.csdn.net/weixin_40877615/article/details/93623029
3. What is a high-speed circuit?
In general, when we discuss the characteristics of a circuit, a basic common sense is that the voltages (or signals) everywhere on a wire are equal at the same time.
The above conclusions are no problem in low-speed circuits, but in fact, the transmission of electrical signals is also limited in speed. When the frequency of the signal in the circuit is high to a certain extent, the change of the signal has not been transmitted from the source end of the wire to the destination end, and the signal at the source end has a new change, and the voltage of each point on the same wire will be different. At that time, we could not use the basic theories of digital circuits and analog circuits to analyze, which involves some characteristics of high-speed circuits.
There is no clear boundary between high-speed circuits and low-speed circuits. The difference between a high-speed circuit and a low-speed circuit is that the circuit is studied with the thinking of a distributed system or a lumped system. The low-speed signal can be regarded as concentrated into one point when analyzing, while the level of each point on the transmission path of the high-speed signal is different. . Some literature considers a signal as a high-speed signal when the transmission path length of the signal is greater than λ/6 (1/6 wavelength). In fact, when the wavelength of the signal in the circuit is on the same order of magnitude as the length of the trace (or when the wavelength is longer than the trace), it should be considered as a high-speed signal.
For example, the communication line of the 100M network card has a maximum rate of 100MHz, then its wavelength λ=c/100M (c is the speed of light), and λ=3m is obtained. If this signal is connected to the computer through a router, the wavelength and the path length are equivalent Now, it should be regarded as a high-speed signal; if the length of the wiring on the printed board does not exceed a few centimeters, it can be considered a low-speed signal.
It should be noted here that high-speed signals are not necessarily high-frequency signals . For some RS-485 signal lines, when the transmission length is 1km, even if the signal baud rate is only a few hundred k, it should be regarded as a high-speed signal. Also note that high-speed circuits are a concept in digital circuits, because digital signals are square waves, which actually contain high frequency components. Instead of using the cycle of digital signals to calculate the wavelength, use the rise time/fall of digital signals Actually estimate the highest frequency and then get the wavelength, as shown in the figure below, use tr and tf to calculate:
4. Problems to be paid attention to when studying high-speed circuits
1) Resistors, capacitors, and inductors are not ideal devices, and there are equivalent circuits at high frequencies
In the two types of high-speed circuits, devices such as resistors, capacitors, and inductors are no longer ideal characteristics, which are equivalent to the characteristics of the combination of resistors, capacitors, and inductors.
Such as the high-frequency equivalent circuit of a resistor:
The figure below depicts the relationship between the absolute value of the impedance of the resistor and the frequency. The impedance of the resistor is R at low frequencies. However, when the frequency increases and exceeds a certain value, the influence of parasitic capacitance becomes the main factor, which causes the impedance to drop; when the frequency continues As it increases, the total impedance rises due to the effect of the lead inductance, which at very high frequencies represents an open line or infinite impedance:
The high-frequency equivalent circuit of a capacitor:
The relationship between the absolute value of the impedance of the capacitor and the frequency is shown in the figure below. Due to the dielectric loss and the finite length of the lead, the capacitor shows the same resonance characteristics as the resistor. When the frequency is higher than the resonance frequency, it shows the inductance characteristic:
The high-frequency equivalent circuit of an inductor:
When the frequency is lower than the resonance point, it shows the characteristic of inductance; when it is close to the resonance point, the impedance of the inductance increases rapidly; when the frequency continues to increase, the characteristics of the parasitic capacitance become obvious, and the impedance gradually decreases:
On the path of high-speed signals, the wire is no longer ideal. It can be equivalent to a circuit composed of resistance, inductance, and capacitance:
2) Signal reflection, overshoot, ringing
On the transmission line, if the impedance is not matched, the signal will be reflected when it is transmitted to the impedance discontinuity point, which will distort the signal. When the distortion becomes severe, signal recognition failure will occur, and the signal distortion will also increase the sensitivity of the circuit to noise.
The overshoot and ringing of the digital signal jump edge are also caused by signal reflection in many cases, which is caused by the superposition of the signal after multiple reflections and the original signal. As shown in the example below:
3) Signal delay and timing error
Because the rise time of high-speed signals is very short relative to the transmission path, if the two signals that need to be synchronized arrive at the destination at different times, it may cause the read level to be wrong, or read an intermediate level state.
4) Signal crosstalk
Crosstalk is manifested by the fact that when a signal passes through one signal line, the related signal will be induced on the adjacent signal line on the PCB. In high-speed circuits, the rising edge of the general signal is also relatively steep, and it is more likely to cause crosstalk between signal lines. Crosstalk is generally caused by distributed capacitance and distributed inductance, as shown in the following figure:
5) Electromagnetic radiation
High-speed circuits generally have a higher frequency than low-speed circuits. When the system is powered on, it will radiate more electromagnetic waves to the surrounding environment, thereby interfering with the normal operation of electronic equipment in the surrounding environment.
At the same time, high-speed circuits are more susceptible to external electromagnetic interference due to their own sensitivity to impedance, crosstalk, delay, etc. Special consideration should be given when designing.
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Original link: https://blog.csdn.net/little_grapes/article/details/129134258