Reading S-parameters: Sharp Dips

Figure 1. A resonant cavity composed of two interior layers
on a four-layer printed circuit board.  The return current from
the signal path couples into the plane cavity and excites its
resonance modes.

The last pattern we'll cover in this series on reading S-parameters is sharp dips. These dips result from coupling to high-Q resonant structures and represent very narrowband absorption in S21 or S11.

Where do you see resonant coupling? Resonant structures can include coupling to an interconnect that is floating and not terminated. The structure does not have to be a uniform transmission line, it can also be a cavity made up of two or more adjacent plates as shown in Figure 1. Commonly, when a signal goes through a cavity and we hit the resonances of the cavity, it absorbs energy and results in narrow dips in the S-parameters.

Reading S-parameters: Broad Dips

Figure 1. Quarter wave stubs exhibit resonance which
impacts S21 as broad dips.  The resonant frequency
can be computed based on the characteristics
of the transmission lines and its length.

Besides ripples and monotonic drop offs, the third pattern commonly seen in plots of S-parameters are broad dips due to stub resonances. Figure 1 shows an interconnect which includes a stub.  The interconnect is composed of 50 ohm uniform transmission lines which incorporate a branch point with an open at the end. 

Not surprisingly, the open stub is a discontinuity. What's going to happen? At the open, it's going to reflect and head back. At the junction, it's going to branch. Some of it's going to go back toward the source and some of it's going to go forward toward the receiver.

Reading S-parameters: Monotonic Drop Offs

Figure 1. As a signal propagates, the amplitude drops off
exponentially with distance due to signal losses.

Last week, we showed how ripple in S-parameters relates to the length of the interconnect. The second common pattern we see when reading plots of S-parameters is the monotonic drop off in the amplitude of the transmission coefficient (S21), commonly referred to as the insertion loss. If we apply a sine wave and look at its amplitude as it moves through that interconnect, we see that it drops off exponentially as shown in Figure 1.

V_Out is shown as V_In times a decaying exponential. (Note that attenuation is usually described using base 10 rather than base e).

Reading S-parameters: Ripples

Figure 1. Ripple patterns commonly seen in S11 and S21.

When we look at plots of S-parameters, we can observe four classic patterns that affect the S11 reflection coefficient (aka. return loss) and the S21 transmission coefficient (aka. insertion loss): ripples, monotonic drop offs, broad dips and sharp dips. In this post, we’ll look into how the characteristics of the interconnect affect the ripple pattern, which will help you better understand your own measurements. 

The ripples in S11 and sometimes in S21 in Figure 1 are due to reflections in the interconnect caused by impedance discontinuities. This phenomenon has been investigated in an earlier post, What S-parameters Reveal About Interconnects (Part III)