A spectrum analyzer is a device that measures and displays signal amplitude (strength) as it varies by frequency within its frequency range (spectrum). The frequency appears on the horizontal (X) axis, and the amplitude is displayed on the vertical (Y) axis. It looks like an oscilloscope, and in fact, some devices can function as either oscilloscopes or spectrum analyzers.

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Importance of spectrum analyzers

Radio frequency (RF) signals and wireless communications are ubiquitous today thanks to Wi-Fi, mobile networking and communications, wireless internet of things device sensors, traditional radio, RADAR and more. To test and design such circuits and systems, it is useful to see the entire signal spectrum and other factors like spurious signals, noise, modulated signal width, etc.

Devices like swept spectrum analyzers and vector signal analyzers do not provide such capabilities. Also, they provide only snapshots of the signal in either the frequency or modulation domain, which cannot describe the dynamic and transient nature of modern RF signals.

This is where spectrum analyzers are valuable.

What do spectrum analyzers measure?

A spectrum analyzer displays a spectrum of signal amplitudes on different frequencies. It enables analysis that determines whether signals fall within required limits. It displays spurious signals, complex waveforms, rare short-duration events and noise. Spectrum analyzers can also analyze transient signals, capture burst transmissions and glitches, and show if stronger signals are masking weaker ones.

These devices are frequently used to examine the dynamic frequency spectrum of modern RF and audio signals that change over time. They display the individual elements of signals and the performance of the circuits producing them. Organizations also use them to determine what modifications may be needed to reduce interference and so improve the performance of Wi-Fi systems and wireless routers.

A spectrum analyzer can be used for measuring frequency response and noise distortion, occupied bandwidth and interference sources, and pre-compliance testing.

Types of spectrum analyzers

There are several different kinds of spectrum analyzers.

Swept-tuned or superheterodyne

A swept-tuned, or superheterodyne, spectrum analyzer down-converts part of the input signal to the center frequency of a bandpass filter by running a voltage-controlled oscillator across a range of frequencies. This enables 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 bandpass filter. A smaller bandwidth yields a higher spectral resolution, and vice versa.

Fast Fourier transform

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

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.

Audio

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.

Spectrum analyzers: Advantages and disadvantages

With swept-tuned analyzers, there is a tradeoff between how rapidly the display can update a full frequency span and resolution. With weak signals, a pre-amplifier is needed before analysis.

FFT analyzers can strain the capabilities of analog-to-digital converters. They also require significant processing power, limiting the possible frequency range that can be analyzed.

Real-time FFT analyzers offer good resolution and reduce sampling gaps, however.

Spectrum analyzer visualizing airwaves

How spectrum analyzers work

With most spectrum analyzers, users can set a start and stop frequency, as well as a center frequency halfway. These are also the axis for the frequency used to determine the span, aka passband -- for example, the range between the start and stop frequencies. The spectrum analyzer measures the radio noise floor and analyzes how close two signals can be while still being resolved into two separate peaks.

It uses a linear scale for signal frequency and a logarithmic or decibel scale for amplitude. The latter enables the device to see and display signals with large amplitude differences on the same screen.

A spectrum analyzer's interface can be connected to a wireless receiver or personal computer to enable visual detection and analysis of electromagnetic signals over a defined band of frequencies. Called panoramic reception, this can help determine the frequencies of interference sources to wireless networking equipment, like Wi-Fi and wireless routers.

Real-time spectrum analyzers examine signals using real-time digital signal processing (DSP), which is done prior to storage in memory. This enables users to dive deeper into signals and discover events that are invisible to other kinds of devices. They can also perform signal conditioning and calibration and extensively analyze the data in memory using batch processing.

In real-time spectrum analyzers, DSP and a control processor that controls the device display provide advanced capabilities that swept-tuned spectrum analyzers and vector signal analyzers lack. They also enable the automation of several routines.

People often use spectrum analyzers to examine the dynamic frequency spectrum of modern radio frequencies and audio signals that change over time.

Applications of spectrum analyzers

On the display of a spectrum analyzer, output signals at frequencies other than the intended communications frequency appear as vertical lines (pips). This makes them useful to determine whether a wireless transmitter is working according to government-defined standards for emissions purity, i.e., whether the transmitter is staying within its allocated frequency band and not causing interference in other bands.

In addition to RF design and testing, spectrum analyzers are also useful for electronic circuit design and electronics manufacturing and repair.

A real-time spectrum analyzer is particularly helpful for the following:

• Wi-Fi testing
• voice and data communications, e.g., cellular radio
• digital video
• radar test analysis
• interference hunting
• signal spectrum management

Spectrum Analyzer How-To Guide: What They Are, What They Measure, & How to Use Them

A spectrum analyzer measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals.  Given the challenge of characterizing the behavior of today’s RF devices, it is necessary to understand how frequency, amplitude, and modulation parameters behave over short and long intervals of time.

Traditional tools like Swept Spectrum Analyzers (SA) and Vector Signal Analyzers (VSA) provide snapshots of the signal in the frequency domain or the modulation domain. This is often not enough information to confidently describe the dynamic nature of modern RF signals. To overcome these evolving challenges, it is crucial for today’s engineers and scientists to be able to reliably detect and characterize RF signals that change over time, something not easily done with traditional measurement tools. 

The Real-time Spectrum Analyzer (RSA) is an instrument that can discover elusive effects in RF signals, trigger on those effects, seamlessly capture them into memory, and analyze them in the frequency, time, modulation, statistical and code domains.

As RF signals have become ubiquitous in the modern world, so too have problems with interference between the devices that generate them. To overcome evolving challenges, it is crucial for today’s engineers and scientists to be able to reliably detect and characterize RF signals that change over time, something not easily done with traditional measurement tools.

One such measurement tool that’s evolved to keep up is the Real-Time Spectrum Analyzer. A real-time spectrum analyzer can help with most modern RF measurement challenges, like: discovery of rare, short deration events; seeing weak signals masked by stronger ones; observing signals masked by noise, finding and analyzing transient and dynamic signals; capturing burst transmissions, glitches, switching transients, to just name a few.

 

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Main Types of Spectrum Analyzers

Spectrum Analyzers can be classified in 3 basic categories in reference to their architecture – Swept Spectrum Analyzers (SA) and Vector Signal Analyzers (VSA) and Real-time Spectrum Analyzers (RSA).

Swept Spectrum Analyzers (SA)

The swept-tuned, superheterodyne spectrum analyzer is the traditional architecture and is best suited for observing controlled, static signals. The SA makes power vs. frequency measurements by downconverting the signal of interest and sweeping it through the passband of a resolution bandwidth (RBW) filter. The RBW filter is followed by a detector that calculates the amplitude at each frequency point in the selected span. While this method can provide high dynamic range, its disadvantage is that it can only calculate the amplitude data for one frequency point at a time. Consequently, measurements are only valid for relatively stable, unchanging input signals Consequently, measurements are only valid for relatively stable, unchanging input signals

Vector Signal Analyzers (VSA)

Analyzing signals carrying digital modulation requires vector measurements that provide both magnitude and phase information. A VSA digitizes all the RF power within the passband of the instrument and puts the digitized waveform into memory. The waveform in memory contains both the magnitude and phase information which can be used by digital signal processing (DSP) for demodulation, measurements, or display processing. While the VSA has added the ability to store waveforms in memory, it is limited in its ability to analyze transient events. The serial nature of batch processing common in means that the instrument is effectively blind to events that occur between acquisitions. Single or infrequent events cannot be discovered reliably, so external triggering may be necessary and requires impractical prior knowledge of these transient events.  Other limitations of the VSA include challenges in isolation of weak signals in the presence of larger ones and signals that change in frequency but not amplitude.

Real-time Spectrum Analyzers (RSA)

The RSA performs signal analysis using real-time digital signal processing (DSP) that is done prior to memory storage as opposed to the post-acquisition processing that is common in the VSA architecture. Real time processing allows the user to discover events that are invisible to other architectures and to trigger on those events allowing their selective capture into memory. The data in memory can then be extensively analyzed in multiple domains using batch processing. The real-time DSP engine is also used to perform signal conditioning, calibration and certain types of analysis.

What is the difference between a spectrum analyzer and an oscilloscope?

Oscilloscopes and spectrum analyzers are two of the most important instruments in any electronic laboratory. Simply speaking, a spectrum analyzer is used to measure frequency information on a signal, whereas oscilloscopes are used to measure the timing information around a signal.  In real life, however, the nature of signals is not known in advance, so having both instruments allows proper characterization of the signal. It is important to note that spectrum analyzer scopes have both instruments in one to allow a more comprehensive measure of a signal or portability in certain environments.  The much more sensitive capabilities or immediate measurements of an RSA are just some of the reasons one might choose a spectrum analyzer as a separate instrument. 

What do spectrum analyzers measure?

Spectrum analyzers are used for many measurements including:

• Frequency Response, Noise and Distortion characteristics of all kinds of radio-frequency circuitry

• Occupied Bandwidth and Interference Sources in Telecommunications

• Basic Pre-Compliance Testing for EMC Testing

Other measurement techniques involve setting up the spectrum analyzer to test harmonics of audio signals by musicians and audio engineers, using reflective or refractive techniques to separate out the wavelengths of light with optical spectrum analyzers, and vibration amplitudes at various component frequencies among many others. The measurement techniques you will use will depend on your application, but hopefully these basics are enough to get started.

Why use a Spectrum Analyzer?

Given the challenge of characterizing the behavior of today’s RF devices, it is necessary to understand how frequency, amplitude, and modulation parameters behave over short and long intervals of time. Traditional tools like Swept Spectrum Analyzers (SA) and Vector Signal Analyzers (VSA) provide snapshots of the signal in the frequency domain or the modulation domain. This is often not enough information to confidently describe the dynamic nature of modern RF signals.

The Real-Time Spectrum Analyzer (RTSA) architecture is designed to overcome the measurement limitations of the SA and VSA to better address the challenges associated with transient and dynamic RF signals. The Real-Time Spectrum Analyzer performs signal analysis using real-time digital signal processing (DSP) that is done prior to memory storage. Real time processing allows the user to discover events that are invisible to other architectures and to trigger on those events allowing their selective capture into memory. The data in memory can then be extensively analyzed in multiple domains using batch processing.

How does a Spectrum Analyzer Work?

Modern RSAs can acquire a passband, or span, anywhere within the input frequency range of the analyzer. At the heart of this capability is an RF downconverter followed by a wideband intermediate frequency (IF) section. An ADC digitizes the IF signal and the system carries out all further steps digitally. DSP algorithms perform all signal conditioning and analysis functions.

For spectrum analysis to be classified as real-time, all information contained within the span of interest must be processed indefinitely without gaps. An RTSA must take all information contained in time domain waveform and transform it into frequency domain signals. To do this in real-time requires several important signal processing requirements:

• Enough capture bandwidth to support analysis of the signal of interest
• A high enough ADC clock rate to exceed the Nyquist criteria for the capture bandwidth
• A long enough analysis interval to support the narrowest resolution bandwidth (RBW) of interest
• A fast enough DFT transform rate to exceed the Nyquist criteria for the RBW of interest
• DFT rates exceeding the Nyquist criteria for RBW require overlapping DFT frames:
  • The amount of overlap depends on the window function
  • The window function is determined by the RBW

 

 

What applications can you use a Spectrum Analyzer for?

Whether you’re in the field or in the lab, a Real-Time Spectrum Analyzer can be used for multiple applications like: voice and data communications (like cellular radio or radio communications); video broadcast distributed via satellite using DVB-S and DVB-S2 formats, and digital video is broadcast using the DVB-T format; radar, like radar transmitter test analysis; and spectrum management and interference hunting.

One of the most popular applications is Wireless local area network (WLAN) testing, also known as Wi-fi testing.

How to use a Spectrum Analyzer for Wi-fi testing

Wireless local area networks have become ubiquitous in the last decade as computing has become pervasive in the office, the home, and in personal communications. From the original specifications for Wi-Fi as the IEEE 802.11 standard in the 2.4 GHz industrial, scientific, and medical (ISM) unlicensed frequency band, Wi-Fi has undergone more than a half dozen revisions and progressed from a 2 Mb/s channel to multiple channels with more than 1 Gb/s throughput. Frequency bands now include 2.4 GHz, 3.6 GHz, 5 GHz, and 60 GHz. Currently the most popular variants are 802.11g (2.4 GHz), 802.11n (2.4 and 5 GHz), and 802.11ac (5 GHz).

Testing WLAN transmission with the newer standards can present some challenges. There are optional implementations for channel bandwidth, modulation type, and the number of spatial streams. A spectrum analyzer should have a real-time bandwidth of 120 MHz for testing an 80 MHz WLAN channel to measure transmitter spectrum emission mask tests, unless there is some external trigger method that allows triggering outside the IF bandwidth.

A test instrument should also have good enough linearity and noise specifications to permit measuring the very low EVM that is required for implementation of 256QAM modulation; since the device EVM should be < -32 dBm for 256QAM 5/6 coding, a spectrum analyzer should be at least 10 dB better than that, or <-42 dBm in order to accurately measure the signal without contributing distortion. Finally, for testing of multiple spatial streams the test equipment should have up to 8 independent capture channels that are synchronized to permit phase accurate measurements from multiple antennas.

Tektronix real-time spectrum analyzers at the low end have a 40 MHz real-time bandwidth, which is adequate for characterization and decoding of 40 MHz WLAN channels. It is possible to synchronize these instruments, so multiple WLAN spatial channels can be captured and analyzed.

Wi-Fi transmitters can also be characterized by spectrum emission mask testing for wider channels than 40 MHz, since wide real-time bandwidth is not needed for this type of test, provided that some method of triggering on signal bursts is available, as discussed earlier. The higher-end Tektronix spectrum analyzers have a real-time bandwidth of 165 MHz, which is adequate for full characterization of 160 MHz WLAN channels. The RSA5100B has a residual EVM spec of -49 dBm, which is more than adequate for testing complex modulation such as 256QAM.

 

 

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