Most spectrum analyzers offer users the opportunity to set a start, stop and center frequency. The center frequency is halfway between the stop and start frequencies and is also the axis for the frequency used to determine the span -- the range between the start and stop frequencies. With an RF spectrum analyzer, the analyzer measures the radio "noise floor" and measures how close two signals can be while still being resolved into two separate peaks.
Today, as spectrum analysis software and digital or spectrum analyzer app offerings have become more common, more analyzers are able to do analog-to-digital conversion and sample a significant input signal and frequency range. A modern spectrum analyzer may be able to show displayed average noise level, calculating the average noise level detected by the device. These detectors are typically capable of sample detection, peak detection or average detection.