RF Test Equipment Series: Spectrum Analyzers 101
Spectrum Analyzers are one of the essential RF test and measurement instruments, alongside network analyzers, oscilloscopes, and signal generators. Spectrum Analyzers are one of the most versatile RF test instruments, and are used as dedicated laboratory equipment, integrated within larger RF test assemblies, part of mobile RF signal/interference hunting applications, and are even slung over the shoulder of radio and cell tower technicians. These devices are critical for identifying and locating interference, as well as measuring RF component and system behavior.
Spectrum Analyzers are so extensively used as they are basically highly specialized and configurable receivers that can be used to detect, and measure, continuous wave (CW) and modulated RF/Microwave signals. Typically, the sensing hardware and capabilities of a Spectrum Analyzer are paired with software and control features that enable enhanced signal information gathering and measurement. For example, some Spectrum Analyzers can be used to measure dynamic range, peak power, average power, peak-to-average power ratio (PAPR), and make other useful performance measurements commonly used to characterize RF devices.
The most common and familiar display of a Spectrum Analyzer is the standard frequency versus signal power plot. Some Spectrum Analyzers can also plot frequency and signal power over time, a display known as a waterfall plot, which can be useful for analyzing the behavior of transient signals over time. Other common Spectrum Analyzer displays are modulation/demodulation displays, some including the ability to display IQ data directly samples from a signal.
There are a few main types of Spectrum Analyzers, a swept spectrum analyzer (SSA), a real-time spectrum analyzer (RTSA), and a vector spectrum analyzer (VSA), also known as a Signal Analyzer. A SSA uses a tuning element to sweep along a desired frequency range. Where the original SSAs used analog tuning, filtering, and display elements, modern SSAs digitize the input signal and use FFT methods to translate the time domain input signals into the frequency domain, also known as a Signal Analyzer. RTSAs are similar to SSAs, except that while RTSAs sweep they use overlapping FFTs that allow for the capture of signals down to a very small time duration. RTSAs can also be used to continually capture signal information at a set range of frequencies to the limit of the real-time bandwidth.
Though the majority of the hardware necessary to operate a Spectrum Analyzer is typically contained within the device (or between a PC-driven Spectrum Analyzer and a PC), external RF hardware is needed to connect to device under test (DUT), antennas, and transmission lines. This hardware commonly includes coaxial adapters appropriate for adapting the Spectrum Analyzers coaxial input to whatever device or coaxial cabling is needed. Also, portable antenna are necessary for interference or signal hunting, and are also useful for external testing. Lastly, an inline attenuator may be useful when working with signal levels beyond the threshold of the Spectrum Analyzer, as the internal attenuators of a Spectrum Analyzer are limited to the extent they can attenuate external signals.
References Spectrum Analysis Basics - Keysight