Network Analysis Principles

With respect to network analysis, a network is a group of interconnected electrical components. A network analyzer can quantify the impedance mismatch between two RF components to maximize power efficiency and signal integrity. Each time an RF signal leaves one component and enters another, portions of the signal are reflected and transmitted, similar to the light through the lens, shown in the following figure.

In this figure, light from a source directs an incident signal at an optical device, such as a lens. As light hits the lens, depending on the lens properties, some light is reflected back to the source, and some light is transmitted through the lens. The law of conservation of energy requires that the sum of the reflected and transmitted signal equals the source or incident signal. This example ignores any loss due to heat, which is usually negligible.

We can define a reflection coefficient (Γ), a vector quantity with both magnitude and phase, as the ratio of light being reflected to the total (incident) light. Similarly, the transmission coefficient (T) is the vector ratio of transmitted light to the incident light.

The ratio of reflected or transmitted light to the incident light is similar to the performance of the device under test (DUT). Thinking back on the light analogy, if the DUT were a mirror, you would want high reflectivity. If the DUT were a camera lens, you would want it to be highly transmissive. Similar practical measurements can be made in electrical networks. A network analyzer generates a sine wave signal, typically across a range of frequencies. The DUT response includes the incident signal being transmitted through the DUT and reflected back from it, as shown in the following figure.

In the preceding figure, T is defined by B/R and Γ is defined by A/R.

The response of the DUT to the incident signal is affected by the DUT properties as well as any discontinuities in the characteristic impedance of the system. For instance, a bandpass filter is highly reflective out of band, but it is highly transmissive in band. The amount of transmitted and reflected signal usually changes with the frequency. If the DUT slightly differs from the characteristic impedance, resulting in an impedance mismatch, the DUT could generate additional unwanted responses. When using a VNA, the goal is to develop a measurement methodology that accurately measures the DUT response while minimizing or eliminating uncertainties.