High speed circuit design practice -- Overview

Catalog

1、 Definition of high-speed circuit

2、 Common misconceptions about

3、 How to distinguish between high-speed circuit and low-speed circuit

4、 Hardware design process

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Understand the main points ：

1、 The highest frequency of a signal depends on its effective frequency rather than its periodic frequency ;

2、 Capacitance in high-speed circuit 、 Inductors are not ideal devices ;

3、 High speed signal refers to the signal with large difference in each level on the transmission path . The difference between high speed and low speed , Not only depends on the signal frequency , It also depends on the length of the transmission path ;

4、 The higher the signal frequency , Then the signal line length of the low-speed and high-speed watershed is shorter .

1、 Definition of high-speed circuit

        From a more narrow definition , It is generally considered that the frequency of digital logic circuit reaches or exceeds 50MHz, And the circuit working above this frequency has accounted for a large proportion （ such as 40%） It is called high-speed circuit .

2、 Common misconceptions about

        （1） The highest frequency to be considered in the design often depends on the effective frequency F_knee, Instead of the signal cycle frequency F_clock. The specific definitions are as follows ：

         Pictured ,T_clock Is the clock cycle of the signal ,T_r It's a signal 10%-90% Rising time .

Then the periodic frequency and effective frequency of the signal are ：

        The effective frequency of most signals in reality can be calculated by the above formula .       

         From the knowledge of digital signal processing , Any signal can be regarded as the superposition of multiple sine wave signals . The frequency is F Ideal square wave , By frequency F Sine wave and its odd harmonics （3F,5F,7F etc. ）, various The amplitude of sine wave is                           V_n = 2/(3.14XN), As the frequency increases , The amplitude of harmonic component gradually decreases . For high frequency signals , The amplitude of each frequency decreases faster . Define that the amplitude drops to the corresponding component in the ideal square wave 70%（ That is, the power drops to 50%） when , The harmonic component is the effective frequency of the signal .

         （2） capacitance 、 Inductance is not an ideal component ！（ It can be used as an ideal in low-speed circuits ）

3、 How to distinguish between high-speed circuit and low-speed circuit

        （1）-- Distributed system and centralized system

         For low-speed signals ： Lumped type —— The level on the transmission path is similar , That is, the states of all points on the transmission path are the same ;

        For high-speed signals ： Distributed —— The points on the transmission path cannot be seen as one point , As different points .

         The key ： How to distinguish depends not only on signal frequency , It also depends on Transmission path length . generally speaking , Length of signal transmission path L Less than the effective wavelength of the signal （c/F( Effective frequency )） Of 1/6 when , It can be considered on this path , The states of all levels are similar -- Low speed circuit （ The high speed is greater than 1/6）.

        （2） How to obtain the effective frequency in the actual situation ？（1） measurement 10%-90% The rise time of     （2） Suppose the rising edge time is of the signal period 7%, Then the effective frequency F_knee About the periodic frequency 7 times .（ It is not true for extremely high frequency signals ）

4、 Hardware design process

        Demand analysis -- Outline design -- Detailed design -- debugging -- test -- Change production

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