Signal Integrity: From High Speed to Radiofrequency Applications

5

The S-Parameters Testing Technique

The interconnection between different circuits in a system is one of the sources of signal degradation due to the multiple reflections and couplings that occur at it. Connectors and hardware are generally characterized by frequency domain parameters such as the input reflection coefficient (return loss) and insertion loss, which are used to define system performance. Most of these parameters are based on the principle of transmission lines, and thus on controlled impedance configurations. The transmission lines introduced in Chapter 4 are easily described in the frequency domain once inductive and capacitive losses or effects become significant.

5.1. Definition of measured parameters

A high-frequency signal arriving on a connector undergoes reflection and transmission depending on the frequency.

5.1.1. Reflection and transmission

The measurement of reflection at the input is based on adaptation, i.e. the transfer of power between the generator and the circuit being measured. From this, we deduce the characteristic parameters of a system, specifically:

– the reflection or reflection loss coefficient (return loss);

– the standing wave ratio (SWR) or voltage standing wave ratio (VSWR).

We can deduce other system characteristics from these:

– the transmission or insertion loss coefficient;

– attenuation.

At high frequencies, the measurements are generally taken with a vectorial network analyzer, which makes it possible to access amplitude and phase. ...