At low frequencies (slow edge rates), small wires between devices do not affect system performance. You can assume the voltage is the same at every point along the wire at any moment. This is called "lumped" circuit model.

As frequency increases, wire dimensions become comparable to signal wavelengths. In this case, even small inductance and capacitance create significant electrical impedance.

Generally, a significant proporation of the wavelength means that the propagation delay in a wire or interconnect is greater than one-sixth of the rise time of the digital signal. In this case, the "lumped" circuit analysis is incorrect and you must analyze the interconnect as a transmission line.

For calculation purposes, you must understand the concept of electrical length (l). Electrical length is defined as the distance that a signal can travel in an electrical medium during the time that it takes for one rise or fall time, whichever is longer.

Using the concept of electrical length, the general rule for what constitues a significant proportion of the wavelength can be redefined. The system must be analyzed as a transmission line if the physical length of a wire or electrical interconnect is greater than one-sixth of the electrical length of a signal that propagates on that wire.

Velocity is defined as the rate at which an electrical wave propagates in the transmission medium. Using this value you can calculate electrical length in one of the following ways:

  • l(in) = Velocity(in/ns) • trise
  • l(in) = trise(ns)/tpd(ns/in)

where,

trise is the rise/fall time of the digital edge,

tpd is the propagation delay of the edge in the transmission line.

For example, the NI SHC68-C68-D2 shielded cable has a tpd of 165 ps/inch.

On an NI 6551/6552 device, trise for a high-speed digital signal can be as low as 1.5 ns.

Therefore, the electrical length is 9 in. (trise/tpd =  1.5/.165 = 9 in.), and any trace lengths longer than 1.5 in. (9 in./6) should be treated as a transmission line. This cable is significantly longer than 1.5 in., so this cable is considered to be a transmission line and designed the cable to have 50 Ω characteristic impedance.

Note While the propagation delay number in the previous example is specific to the SHC68-C68-D2 cable, if you do not know the specific propagation delays for your interconnects. When using the digital waveform generator and analyzer, assume that you are working with transmission lines for any wire or interconnect longer than 1 to 2 inches.

A voltage source (Vs) generates a digital edge with an impedance of Zs looking "into" the transmission line. The transmission line itself has some low characteristic AC impedance (Z0) to ground, typically 50 Ω for most test systems. The end of the transmission line is most commonly terminated through an impedance (Zt) to ground at the destination.

The following figure shows a simple diagram of a basic single-ended transmission line:

Practically, termination at only one end of the transmission line is often adequate and is more commonly used. However, for high-precision applications, termination at both the source and the load end of the transmission line yields the best results.