A spectrum analyzer does what the name suggests: it detects the signals present in a selected range of spectrum. The basic function is to represent the signals in a graphical display as amplitude—or power level—on the y-axis, against frequency on the x-axis; the amplitudes of detected signals are represented in the frequency domain. An RF spectrum analyzer covers radio and microwave frequencies. The maximum frequency range with preselection currently available is from 2 hertz to 85 GHz; higher frequencies are possible with external mixers. Usually, linear scale is used for frequency on the x-axis, and a logarithmic or decibel scale (also logarithmic), for amplitude on the y-axis, so that signals of widely varying amplitude can be seen at the same time. Spectrum analyzers are widely used in RF test to display not only properties of wanted signals, such as whether a signal is occupying the designated bandwidth, but also to search for unwanted signals. For RF test, a pure spectrum analyzer for detecting the level of wanted and unwanted signals by displaying the spectral components in a frequency range hardly exists any more. The nature of many modern pulsed signals, plus the necessity to detect and investigate transient signals, means that the classical spectrum analyzer using the same super heterodyne principle as radio receivers cannot reliably detect all signals intermittently present as transients, or measure the phase of a signal. As the frequency range of interest (the frequency span) exceeds the capability of the spectrum analyzer to process data simultaneously, the frequency span is scanned (swept) from low to high. If a transient signal is not present while the frequency is being swept, it is not detected. Digital processing using Fast Fourier Transformation (FTT) from the time domain to the frequency domain has greatly extended both the signal detection and analysis capabilities of the super heterodyne spectrum analyzer. FFT provides much faster capture and analysis of the frequency span: using FFTs in parallel results in a wider instantaneous bandwidth so that, with suitable filters, pulsed and transient signals are also detected. Many spectrum analyzers will also offer zero span mode to analyze phase as well as amplitude of a signal, and demodulate the signal at the selected frequency. Apart from the simple representation of the detected signals on a screen, measurements of noise, gain, phase, occupied signal bandwidth, and adjacent channel power are all possible. The digital signal can be exported for post-processing by software tools providing additional analysis.