Measurement of radio frequency noise factors and parameters

The devices used for mobile telecommunications use electronic circuits that can add noise to the signal they are designed to process. High noise levels mean that signal levels need to be increased. The first consequence of this is an increase in the emission level, which leads to rapid battery discharge for a mobile device, more radiation in the cell phone user's brain, and increased electromagnetic pollution. Noise in a receiver leads to a drop in sensitivity. As a result, either the level emitted by relay antennas needs to be increased, or their number needs to be increased, which is a cause for concern for local residents.

The most noise-sensitive part of a receiver chain is the amplifier at the head of the chain. This is why it is often referred to as a "low-noise amplifier". To design such amplifiers, it's important to know precisely the characteristics that describe how each element contributes to noise. Noise in passive elements, essentially thermal at the frequencies of interest to us, is relatively simple to know from electrical models. Active elements are more complex, and it is often necessary to carry out specific noise measurements.

Noise figure is a useful global concept when associating devices. This notion is not precise enough when dealing with components. Their noise is described by "noise parameters", of which there are four for quadrupoles. Knowing these parameters enables us to build an environment around the component that minimizes the noise it transmits in addition to the signal. This is the design guideline for low-noise amplifiers.

Automatic instruments are available to measure the noise factor under certain conditions of validity, as described below. There are currently no instruments that automatically measure noise parameters. Manufacturers of impedance adapters and noise receivers offer measurement benches for noise parameters. These are packages essentially comprising a vector network analyzer, a noise meter and an impedance adapter. But we haven't yet reached an integrated solution such as exists for noise figure.

The aim of this article is to give you a basic understanding of how these devices work, and to provide the basis for setting up your own measurement bench.