Simple delay circuit pspice model skin#
Alternatively, since the lower the skin depth the higher the resistance per unit length, then the resistance per unit length, R L, increases with the square root of frequency. Where ω is the frequency in radian per second µ is the conductor's permeability in H (Henries per meter) and σ is the conductor's conductivity in S (Siemens per meter).Įquation 1 shows that the skin depth decreases with the square root of the frequency. (Note that this application note will not rigorously discuss skin-effect losses.) The skin depth, δ, is defined as: There are two main loss mechanisms with cables: skin-effect and dielectric losses.Īt high frequencies, the signal tends to propagate along the surface of the conductor. Moreover, the parameters of interest that are being degraded in the signal path can then be understood. However, based on that model, more intelligent decisions can be made about the type of cable to be used. To understand the effects of cables at all frequencies, the cable needs to be modeled. Why Model a Cable?Īs frequencies start to exceed 500MHz, the cable starts to noticeably impact the bandwidth of the signal path and begins to degrade this path in many ways. Example of cable loss effects (simulated results from Figure 3 below). We will then present a simple method of modeling the cable for use in standard SPICE simulators.įigure 1. This application note will discuss the two main loss effects related to cables: the skin effect and dielectric losses. In the comparator, the low-frequency dribble up primarily degrades delay versus pulse width and propagation delay versus overdrive it also degrades the minimum pulse width.
Simple delay circuit pspice model driver#
In the driver or buffer, the low-frequency dribble up (Figure 1) primarily degrades propagation delay versus pulse-width dispersion it also degrades minimum pulse time and rise time. These cable effects are seen¹ on drivers, buffers, and comparators. Using a simple transmission line model may not effectively model this element because it is difficult to model a cable both in the frequency and the time domains.Ĭable nonideal dispersive effects can affect system performance. Even with this understood, cables are notoriously difficult to model correctly. If not modeled as part of that system, cables can lead to unexpected system performance degradation and to costly time delays in debugging and corrections.
This is especially true for signals that exceed 500MHz. This application note discusses the two main loss effects related to cables (skin-effect and dielectric losses), and presents a simple method of modeling the cable for use in standard SPICE simulators.Ī similar version of this article appears on EDN, July 30, 2012.Ĭables are used in many high-frequency board designs and can become a critical element in the signal path. Nonideal cable dispersive effects can affect system performance.
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