The spectrum analyzer has been a staple of the RF engineering group for over sixty years. Its evolution has stored tempo with technological improvements similar to mobile networks, wi-fi protocols, and complicated modulation methods. As analyzers adopted new functionalities and capabilities, the standard terminology and classification of those gadgets turned extra numerous, typically resulting in confusion. On prime of this, evaluation within the frequency area has grown past conventional spectrum evaluation to incorporate further forms of devices similar to RF recorders, vector community analyzers, and – as I’ll spotlight on this weblog – oscilloscopes. Let’s have a look at how the spectrum analyzer has developed into the sign analyzer and the way it pertains to the trendy oscilloscope.
SWEPT-TUNED SPECTRUM ANALYZERS
The swept-tuned spectrum analyzer operates on the principal of adjusting (or tuning) a decision bandwidth filter by means of a spread of frequencies and measuring the amplitude of the sign at every step. The block diagram seen in Determine 1 illustrates the trail from enter to show. Chances are you’ll discover it appears to be like similar to the super-heterodyne strategy of an AM radio, besides the sweeping is automated as an alternative of manually achieved with a dial, and the output is a show as an alternative of a speaker. Any such design leverages a standard analog RF structure and requires a comparatively small manufacturing value. Nevertheless, attributable to its nature of “sweeping” by means of the spectrum, this kind of analyzer is simply appropriate for steady, unchanging indicators.
Determine 1: Swept-tuned spectrum analyzers used a super-heterodyne design to seize and show amplitude of assorted frequencies throughout the vary of the instrument.
FAST FOURIER TRANSFORMS AND THE JOURNEY TO REAL-TIME
The primary try and cowl a wider spectrum over a single evaluation acquisition concerned designing a number of filters into the instrument, every masking a novel frequency vary. The instrument carried out parallel analyses over the vary of the filters, eradicating the variability of the sign modifications over the course of a sweep. Whereas this system was efficient, the price of putting in many parallel filters warranted a extra possible strategy.
Over time, many parts utilized in a spectrum analyzer shifted from the analog to the digital realm. With the incorporation of analog-to-digital converters (ADCs) and digital processors, spectrum analyzers may now carry out operations on digitized information, offering extra freedom and adaptability. What as soon as required a number of acquisitions now required just one by sampling sign information within the time area with a single acquisition and making use of a Fourier rework (FFT) to the information. This led to the trade calling these devices “FFT analyzers”. This new technique enabled a further functionality – the flexibility to seize each magnitude and section information (vectors), which is important for analyzing complicated, modulated RF indicators. This vector evaluation functionality led to some producers to refer to those devices as “vector sign analyzers,” although once we say “VSA” at Keysight, we’re referring to our Vector Sign Evaluation software program (extra on that later).
Regardless of the numerous developments made by incorporating digital FFTs into the spectrum analyzer, it nonetheless suffered from “blind time” between acquisitions because of the time it took to post-process the FFTs after gathering the information. With the appearance of environment friendly digital sign processing (DSP) within the Nineties, the FFT analyzer developed into the “real-time spectrum analyzer,” which obtained its title attributable to its speedy, steady spectrum show updates. Determine 2 illustrates an instance structure incorporating digital processing.
Determine 2: Availability of processors and superior digital sign processing superior the capabilities of FFT-based spectrum analyzers.
In the present day, fashionable analyzers fall into two classes: gadgets that solely measure magnitude in opposition to frequency (spectrum analyzers or fundamental spectrum analyzers), and gadgets that may measure magnitude and section data in opposition to frequency (sign analyzers), permitting evaluation and demodulation of complicated indicators. Both sort of machine might embrace swept-tuned and FFT-based modes of operation, and if an instrument can present a steady evaluation of the spectrum with none gaps in time, similar to Keysight’s PXA sign analyzer in Determine 3, it’s thought of to have “real-time spectrum evaluation” (RTSA) functionality.
Determine 3: Keysight’s N9030B PXA sign analyzer incorporates a 50 GHz frequency vary with the flexibility to investigate as much as 510 MHz of real-time spectrum evaluation.
IT’S A SCOPE! IT’S AN ANALYZER! IT’S… BOTH!
Fashionable oscilloscopes use an structure that circumstances, samples, processes, and shows a sign within the time area. Usually, scope customers are involved with time-based measurements similar to rise time, eye peak, and overshoot, or they could wish to correlate analog and digital indicators. So, why have oscilloscopes historically been restricted to the time area?
The information obtained proper after the ADC is principally the identical whether or not it’s digitized in an oscilloscope or in a sign analyzer — information samples are within the time area and await additional processing. An oscilloscope historically continues to function within the time area, whereas a sign analyzer converts the information to the frequency area. Due to the similarity in methods, a possibility introduced itself for us so as to add a performance to the oscilloscope that converts the information to the frequency area in a lot the identical method as a sign analyzer.
Keysight is proud to supply oscilloscopes with spectrum evaluation functionalities, however earlier than you throw out your sign analyzer, it’s best to know that the frequency-domain capabilities of an oscilloscope will fluctuate from mannequin to mannequin. Once you’re contemplating a scope for spectral evaluation, make sure to consider these necessary traits:
- FFT methodology – how is the FFT being carried out? Nearly each scope has an FFT perform residing inside its listing of “math” capabilities. Nevertheless, it typically takes devoted {hardware} to course of the incoming information quick sufficient to repeatedly monitor a spectrum.
- Frequency vary – what vary of middle frequencies is the oscilloscope able to analyzing? Some oscilloscopes might function middle frequency ranges past the bandwidth of the oscilloscope.
- Evaluation bandwidth – what frequency span is accessible across the middle frequency, and the way does that fluctuate with the variety of channels getting used?
- Actual-time spectrum evaluation – does the instrument provide gapless, real-time evaluation with a 100% level of intercept (POI), guaranteeing transient occasions are captured?
- RF traits – how does the entrance finish carry out by way of sensitivity, amplitude accuracy, section noise, error vector magnitude (EVM), and spurious-free dynamic vary (SFDR)?
- Triggering – does the instrument assist time-domain triggers and/or frequency-domain triggers?
- Evaluation instruments – does the instrument have the potential to carry out vector and digital demodulation? Can it carry out superior evaluation on mobile, wi-fi, and digital video indicators? Can information be exported to exterior software program for additional evaluation?
Keysight’s MXR-Collection real-time oscilloscope (Determine 4, left) contains choices for spectrum evaluation by means of digital down-conversion (DDC) and real-time spectral evaluation (RTSA). In DDC mode, the scope will seize spectral information within the type of IQ streams, permitting you to save lots of, export, and analyze the information with superior instruments similar to Keysight’s 89600 Vector Sign Evaluation software program (Determine 4, proper). In RTSA mode, the information is introduced visually, however not saved, to make sure gapless monitoring and 100% chance of intercept – an effective way to catch uncommon, intermittent occasions on the display screen.
Determine 4: Keysight’s MXR Collection oscilloscope (left) options spectrum evaluation as much as a 6 GHz middle frequency with each real-time spectrum evaluation of as much as 320 MHz of bandwidth and IQ information streaming as much as 2 GHz of bandwidth. IQ information will be exported to 89600 VSA software program (proper) for superior demodulation, vector evaluation, and standards-specific measurements.
