What is Electronic/Radio Frequency (RF) Noise

- details and overview of the different types of electronic and radio frequency, RF, noise that can be experienced in RF and electronic circuits.

Noise is a key factor in many electronic and radio frequency or RF circuits.

Noise comes in many forms. It can be generated in many ways and noise can affect electronic and radio frequency, RF circuits and systems.

These different types of noise may affect circuits in different ways, or they may need to be taken into account in different ways.

As a result it necessary to distinguish between the different types of noise and define how they are generated. In this way their effects can be minimised to a certain degree, although it is never possible to eliminate noise.

Nature of noise

Although there are many different types of noise, there are many aspects of noise that are common across all types along with the various forms of description.

Noise by its very definition is random. It extends in various forms across the frequency spectrum, although not always in the same amplitude. Accordingly there are different categories of noise according to the frequency distribution:

•White noise:   White noise is the type of noise that affects all frequencies equally. It spreads up from zero frequency upwards with a flat amplitude.

•Pink noise:   Pink noise gains its name from the fact that it does not have a flat response. Its power density falls with increasing frequency. It gains its name because red light is at the lower end of the light spectrum.

•Band limited noise:   Noise can have its frequency band limited either by filters or the circuit through which it passes.

Effect of electronic / RF noise

Noise can have many effects on a system. Amplitude noise, i.e. variations in amplitude caused by noise can mask out a signal, or it can cause data errors, increasing the bit error rate. For the best performance, obviously the signal should be as clear of the noise as possible, although in many instances, there is an optimum return balancing an acceptable level of data errors or signal to noise ratio against the cost involved.

Signal noise

Amplitude based forms of noise tend to have more effect on amplitude based systems whereas noise from phase jitter tends to affect phase modulated systems more.

Different types of RF / electronic noise

The various forms of electronic or radio frequency, RF noise can be categorised in a number of ways. However the list below details some of the main types:

•Thermal noise:   This form of noise, also referred to as Johnson or Johnson Nyquist noise arises as a result of the thermal agitation of charge carriers - typically electrons - in a conductor. As the temperature, and hence the agitation of the charge carriers increases so does the level of noise.

•This noise is a major form of noise experienced in low noise amplifiers and the like. To reduce it, very high performance amplifiers, e.g. those used for radio astronomy, etc., have been operated at very low temperatures. Figures like noise temperature are used as a measure of the level of this thermal noise.

•Shot noise:   This form of noise that arises from the time-dependent fluctuations in electrical current. This is caused by the discrete nature of electron charges. Shot noise is particularly noticeable in semiconductor devices, such as tunnel junctions, Schottky barrier diodes and p-n junctions.

•Phase noise:   Phase noise is a form of RF noise that is visible on radio frequency, and other signals. It appears in the form of phase jitter or perturbations on the signal. These manifest themselves as sidebands that spread out either side of the signal or carrier.

•Phase noise can affect a signal or system in a variety of ways. One major area is when phase modulation is used to carry digital information. Phase noise can degrade the bit error rate, as the noise can disrupt the phase changes that indicate the state of the data to be transmitted.

•Flicker noise, 1/f noise:   This type of noise occurs in almost all electronic devices. It has a variety of causes, each related to the direct current flow. It has a frequency spectrum that falls off steadily into the higher frequencies.

•Avalanche noise:   This is a form of noise that is generated when a junction diode is operated close to the point of avalanche breakdown. This occurs in semiconductor junctions when the carriers in a high voltage gradient develop sufficient energy to dislodge additional carriers through physical impact. The current generated by this is not even as it is determined by high energy electrons hitting the crystal lattice to generate more hole electron pairs. As a result it is a very noisy process.

Electronic noise or RF noise can take many forms. It is of particular importance within RF applications because it can cause disruption to many systems. However it is also of interest in many other areas such as audio and other areas where signals are processed, it is even important in digital systems where noise can cause data to be corrupted.

As different types of noise affect systems in different ways, it is necessary to adopt different tactics to reduce their effects wherever possible.

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