What are Real-Time Spectrum Analyzers?

Real-Time Spectrum Analyzers (RTSA) are a type of spectrum analyzers which provide instant (real-time) visualization of the RF signals in frequency domain (amplitude against frequency). RTSAs are becoming increasingly popular as RF spectrum congestion increases and new techniques – such as frequency hopping, spread spectrum or other pulsed techniques – are being introduced. These new signal processing techniques require transients to be accurately monitored as they provide further understanding of the operation of wireless channels and any underlying issues.

Spectrum analyzers can be categorized in two broad categories:

• Sweep/Superheterodyne Spectrum Analyzers (Analog)
• Real-Time Spectrum Analyzers (Digital)

Sweep spectrum analyzers are traditional analog analyzers which are based on either tuned-filter or superheterodyne principle. These analyzers measure/sweep signal strengths in a frequency band – determined by the resolution bandwidth (RBW) – at each step with hundreds of steps in a single sweep. They measure the signal strengths sequentially – from the lower to higher frequencies – and maps them to generate a plot of signal strength. Sweep spectrum analyzers can perform hundreds of such complete sweeps every second with high accuracy. However, sweep spectrum analyzers usually require a steady-state signal for best results and are not very effective at detecting transient signals.

This is where real-time spectrum analyzers (RTSAs) come in. RTSAs collect signal samples in the time domain and then convert them in frequency domain using the FFT (Fast Fourier Transform) algorithm. At any given time, FFT allows the conversion of the signal into the frequency domain and measure/visualize the signal strengths at any frequency in the RBW (theoretically FFT allows measurement at any frequency). These spectrum analyzers can measure signal strength in real-time and ensures that there is no blind time. This makes RTSAs extremely useful in measuring transient signals, which last for a very short duration of time (usually few ms/µs).

A real-time spectrum analyzer display showing measurements in frequency domain. Image Credits: Keysight

RTSAs have very high processing power to calculate hundreds of thousands of FFTs every second to provide a gap-free analysis of signals. One of the most crucial parameters of an RTSA is the “duration for 100% probability of intercept (POI)” which is the minimum amount of time for which a signal needs to be present for the guaranteed interception and capture of a signal for further analysis. Modern real-time signal analyzers can detect signals with 100% probability of intercept (POI) which last only for a few µs/ns, which falls in line with the general duration of transients in a system. This is achieved by the capability of RTSAs to perform extremely high-speed FFTs. 

Over the past few decades, there has been rapid advancements in wireless technology with usage of higher frequencies and introduction new modulation techniques. This has also led to increased importance and rapid improvements in spectrum analyzers. Real-time spectrum analyzers are now available in various form-factors apart from the traditional bench-top analyzers. RTSAs are now readily available in lightweight and portable enclosures which are USB-powered which are don’t cost as much with excellent functionalities and features. Remote-controlled, rack-mount RTSAs are also available which provide another option for engineers to perform their measurements on-the-go.

To keep pace with the growing requires of the test and measurement industry, RTSAs also include various additional features to assist engineers. Some RTSAs allow users to record the result of a spectrum analysis for further analysis in the future. The option to allow RF recording can be handy when analyzing the performance of radio equipment over time.

Real-time spectrum analyzers are used extensively to measure characteristics in a multitude of RF devices. They are used to ascertain bandwidths in telecommunications along with tracking sources of interference. These analyzers are used for pre-compliance testing in EMC. RTSAs are also useful in diagnosing wireless transmitters and whether or not they are operating as per defined standards.

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