1 Introduction

The RF source, the load and transmission line in a radio system can be assumed to have a certain lumped impedance. If these two impedance are equal, all the transmitted power flows into the load. This is known as ideal matching. For example, if the line and load do not match, meaning that the signal traveling on the transmission line faces some discontinuity in impedance, a part of energy gets reflected back into the transmission line. This mismatched impedance can interfere with the signal traveling forward (from source to load) and create a standing wave.

Figure 1-1 Standing Wave Generation

Standing waves are an indication of the quality of transmission. A well-matched transmission line has no reflection and thus no standing wave. The green curve in the Figure 1-1 indicates a standing wave. In most real-world applications, a standing wave is formed on the transmission line which can result in power losses due to heat dissipation. The reflection coefficient (Γ), is also used to denote the ratio of forward and reflected power.

Return loss is the difference in power between the forward and reflected path, when expressed in dB

VSWR, short for Voltage Standing Wave Ratio, is the ratio of the maximum amplitude to the minimum amplitude of the standing wave.

It is not very efficient to calculate VSWR using Equation 3 since it is not straightforward to probe V_max and V_min, we generally demonstrate VSWR as a function of the reflection coefficient as shown in Equation 3.

In practical scenarios, VSWR is typically greater than one due to imperfect matching of transmission line and load. A higher VSWR portrays high reflected signal power, for example a VSWR of value 6 can mean 50% of the power transmitted forward is being reflected back. This can damage the RF signal source and cause degradation of transmission quality. It is thus necessary to keep track of VSWR to understand the reflected power strength to prevent deterioration of the source, transmission line and signal quality.

A VSWR detector is shown in this application note. Two power detectors are used to detect forward and reverse power and their output in log form is subtracted using a difference amplifier. The power values must be divided to calculate, and a log transformation of the power allows implementation of this division by simple subtraction circuits using an operational amplifier. This dual log detector outputs the reflection coefficient as a differential voltage which can be fed into an ADC on a processor or controller/FPGA, from which the value of VSWR can be computed.