#Transmission lines series#
For example, at 1 Hz, a circuit that drives a 100-kW load through a long cable sees the cable merely as a 1W resistor in series with the 100-kW load. If the interconnection is long or the signal frequency is high, the interconnection behaves as a transmission line, which has decidedly different characteristics ( Figure 1b).
#Transmission lines Pc#
A typical pc board's interconnect resistance is usually insignificant, so, when you calculate the interconnection delay, you need to consider only the capacitance. The length of the interconnection and the highest frequency signal component are the determining factors.Ī short interconnection or one that operates at a low frequency behaves as a group of lumped circuit elements, such as one capacitor and several resistors ( Figure 1a). What differentiates a pair of wires from a transmission line? The term “wires” includes all types of conductors: traces on pc boards, twisted pairs, ribbon cables, and coaxial cables. Under certain conditions, an interconnection ceases to act as a simple pair of wires and behaves as a transmission line, which has different characteristics. Why is signal integrity poor if an interconnection is unterminated? The low line impedance can overload the driver, and, as a result, the load voltage may never reach the proper value. Sometimes, a designer can think that a wire, which is really a 50W transmission line, is merely a low-frequency connection to a high-impedance load (say, 10 kW). A small noise pulse can carry a signal that is at less than the final amplitude level across the threshold, where it can appear as false data or can cause false triggering.ĭriver overload decreases signal integrity, speed, and component integrity: In certain configurations, improperly terminated lines cause half- or less-than-full-amplitude signal levels to exist during transition periods. Reduced noise margins increases bit-error rate: If the signal is on a clock line, false triggering can result. If the signal retreats too far, the receiving IC can read it as the wrong value. Reflections on an improperly terminated transmission line cause larger voltages and currents, which radiate larger electric and magnetic fields and transfer more crosstalk energy into neighboring wires.Ī signal that exhibits ringing retreats (undershoots or rings back) from its initial maximum-high or minimum-low level. (Latch-up is the result of a parasitic npn transistor that causes a CMOS circuit to cease functioning or even to destroy itself.) Latch-up can cause temporary or permanent damage. When the excessive voltage persists for more than a trivial amount of time, the IC is overstressed and may latch up. Overshoot exceeds maximum-IC-voltage ratings:Ī reflection can cause a voltage to rise above or, because of negative overshoot, fall below the minimum rating an IC's maximum rating. To achieve the extra delay, you can add one or more clock cycles to each operation, or you can reduce the system's clock frequency. One way to avoid the consequences of ringing is to wait for the reflections to subside before allowing the system to process new data. Reflections cause temporary ringing (voltage oscillations above and below the eventual steady-state level).
Ringing delays cause lower system speeds:Ī high-speed signal transition on an interconnection that behaves as an improperly terminated transmission line can generate reflections. Improperly terminated transmission lines can cause the following problems: How can interconnections limit the speed and integrity of high-speed systems?
Therefore, the time that signals spend getting from place to place on pc boards has become a significant factor in system speed. Moreover, although many more functions now fit within the typical IC, the typical pc board now performs so many additional functions that the board size hasn't changed appreciably. Rise and fall times are less than 500 psec.
In the last decade, however, logic-gate delays have decreased to only a fraction of a nanosecond (50 psec in some 0.25-mm, 3.3V ASICs). So, a fraction of a nanosecond or even 2 nsec was not a significant portion of the total signal-propagation time.
A few years ago, when gate delays were 5 to 10 nsec, the propagation time of a signal through an IC might have been tens to hundreds of nanoseconds. (30 cm) of interconnection on a typical pc board. It takes about 2 nsec for a signal to traverse 12 in. You should care about the speed of system interconnections because electronic signals propagate at a significant fraction of the speed of light. It includes the basics on how proper terminations allow you to live with transmission lines. This article explains what transmission lines are and how they function.
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