A spectrum analyzer is a device that displays signal amplitude (strength) as it varies by signal frequency. The frequency appears on the horizontal axis, and the amplitude is displayed on the vertical axis. To the casual observer, a spectrum analyzer looks like an oscilloscope, and in fact, some devices can function either as oscilloscopes or spectrum analyzers.
The electronics industry uses spectrum analyzers to examine the frequency spectrum of radio frequency (RF) and audio signals. These devices display the individual elements of these signals, as well as the performance of the circuits producing them. Through the use of spectrum analyzers, organizations can determine what modifications may be needed to reduce interference and thus improve the performance of Wi-Fi systems and wireless routers.
How spectrum analyzers work
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.
Uses for spectrum analyzers
A spectrum analyzer can be used to determine whether or not a wireless transmitter is working according to federally defined standards for purity of emissions. Output signals at frequencies other than the intended communications frequency appear as vertical lines (pips) on the display. A spectrum analyzer can also be used to determine, by direct observation, the bandwidth of a digital or analog signal.
Types of spectrum analyzers
Swept or superheterodyne
A swept-tuned, or superheterodyne, spectrum analyzer down-converts part of the input signal to the center frequency of a band-pass filter by running a voltage-controlled oscillator across a range of frequencies. This allows for the full frequency range of the device to be analyzed. In this case, the resolution bandwidth is closely related to the minimum bandwidth detectable by the device and is controlled by the band-pass filter. The smaller the bandwidth, the greater the spectral resolution.
Fast Fourier transform, FFT
Some digital spectrum analyzers use Fourier transforms -- a way of decomposing a signal into its individual frequencies. These analyzers need a sampling frequency at least twice the bandwidth because frequency resolution is the inverse of the time over which the wave is measured and Fourier transformed.
Real-time analyzers collect real-time bandwidth and sample the incoming RF spectrum in a limited span of time, converting the information using the fast Fourier transform (FFT) algorithm. Because it's real-time data collection, there is no "blind time," and there are no gaps in the calculated RF spectrum.
Spectrum analyzers can also be used in the audio spectrum, displaying volume levels of frequency bands audible to humans. This method is aimed at analyzing the harmonics of an audio signal. Once known as wave analyzers, these types of spectrum analyzers are widely used by sound engineers and can run on almost any computer equipped with a sound card.
Advantages and disadvantages
Swept-tuned spectrum analyzers face tradeoffs between how rapidly the display can update a full frequency span and the resolution. Sometimes, if engineers are working with a very weak signal, a preamplifier is needed before analysis can begin. On the other hand, FFT analyzers can strain the capabilities of analog-to-digital converters and require significant processing power, limiting the possible frequency range that can be analyzed. Real-time FFT analyzers can offer good resolution while reducing potential gaps in sampling.
Spectrum analyzer interface
A spectrum analyzer interface is a device that can be connected to a wireless receiver or a personal computer to allow visual detection and analysis of electromagnetic signals over a defined band of frequencies. This is called panoramic reception, and it can be used to determine the frequencies of sources of interference to wireless networking equipment, such as Wi-Fi and wireless routers.