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  • What is high speed circuit?
     
    Date :2017-8-1

     

    1. What is high speed circuit?

    It is generally assumed that if the frequency of the digital logic circuit reaches or exceeds 45MHZ ~ 50MHZ, and the circuit operating above this frequency already accounts for a certain amount of the entire electronic system (say 1/3), it is called high-speed circuit. In fact, the harmonic frequency of the signal edge is higher than the frequency of the signal itself, the rising edge of the signal and the rising edge (or signal transition) caused the signal transmission of the unexpected results. Thus, it is generally agreed that if the line propagation delay is greater than the rise time of the 1/2 digital signal drive end, such signals are considered to be high-speed signals and produce transmission line effects.
    The signal transmission takes place at the moment the signal state changes, such as rising or falling time. The signal passes from the drive end to the receiver for a fixed period of time. If the transmission time is less than 1/2 rise or fall time, the reflected signal from the receiver will reach the drive before the signal changes. Conversely, the reflected signal will reach the drive end after the signal changes state. If the reflected signal is strong, the superimposed waveform is likely to change the logic state.

     

    2. determination of high speed signals

    We have defined the prerequisites for the transmission line effect, but how do we know if the line delay is greater than the signal rise time of the 1/2 drive side? In general, the typical value of the signal rise time can be given by the device manual, and the propagation time of the signal is determined by the actual routing length in the PCB design. The figure below shows the correspondence between the signal rise time and the permissible wiring length (delay).

    The delay per unit inch on the PCB is 0.167ns. However, if the multi-hole, the device pin more, set the network line constraints, the delay will increase. Usually high-speed logic device signal rise time is about 0.2ns. If the board has GaAs chip, the maximum wiring length is 7.62mm.
    Let Tr be the signal rise time and Tpd be the signal line propagation delay. If Tr ≥ 4Tpd, the signal falls in the safe area. If 2Tpd ≥ Tr ≥ 4Tpd, the signal falls in the uncertain area. If Tr ≤ 2Tpd, the signal falls in the problem area. For high-speed routing methods for signals falling in uncertain areas and problem areas.

    Simple layout design can be done manually, and complex layout designs need to be implemented with computer aided design (CAD). Excellent layout design can save production costs, to achieve good circuit performance and thermal performance.

     

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