A coaxial cable has its own characteristic inductance per length and capacitance per length. Interestingly, when the total impedance is computed, the factors of length from each of these contributions cancel out, leading to a fixed total impedance that is independent of length. For the RG58U type transmission cables used in all our labs, that total impedance is 50 Ω.

An oscilloscope is essentially a voltmeter, and thus is designed with a large internal input impedance, typically $R_{\mathrm{int}} = 1 \;\mathrm{M}\Omega = 10^6\Omega$. This is shown schematically in Fig 1.

Figure 1: Schematic of an oscilloscope explicitly showing the 1 MΩ input impedance.

When an electronic pulse moves through a coaxial cable and enters a scope, the pulse “sees” an abrupt increase in impedance. This change in impedance results in a reflection of the pulse without inversion. The scope “sees” the sum of the incoming pulse and its reflection(s). Depending on the width of the pulse and the length of time that it takes for the reflection to return, this sum might be a gross distortion of the original pulse under study.

To avoid the reflection, one can place a 50 Ω resistor $R_{\mathrm{term}}$ in parallel with the scope's input impedance as shown in Fig. 2.

Figure 2: A 50 Ω resistor in parallel with the scope's input impedance.

This parallel combination has a resistance of close to 50 Ω, eliminating the abrupt change in impedance at the scope's input, and thus eliminating the reflection.

A picture of a so-called feed-thru 50 Ω terminator is shown on the left of Fig. 3. Such an all-in-one attachment can be placed directly on the scope input to place the 50 Ω resistor in parallel. Equivalently, one can place a 50 Ω end-cap terminator on one side of a tee attached to a scope input as shown on the right of Fig. 3.

Figure 3: A feed-thru 50 Ω terminator is shown on the left, whereas an equivalent 50 Ω end-cap terminator on a tee is shown on the right.