This is Part 3 in a series of articles which review the basics of conventional swept versus real-time spectrum analyzers and highlight some of the recent advances and instrument form-factors.
Read Part 1, Guide to Real-Time Spectrum Analyzers: Types, here.
Read Part 2, Guide to Real-Time Spectrum Analyzers: Definitions, here.
Swept spectrum analyzers are simpler and less costly than their digital FFT counterparts.
However, they can only display a spectrum one sweep at a time. There will be dead time between sweeps, so it’s very possible they will miss or inaccurately capture fast-moving or brief pulsed signals, such as digitally-modulated or spread-spectrum signals like Wi-Fi, Bluetooth, or a multitude of digital modulations.
This problem of dead time is somewhat alleviated by using the “Max Hold” feature on most analyzers, where multiple sweeps are captured and “built-up” to reveal the envelope of the fast-moving or intermittent signals.
Real-time analyzers, on the other hand, have little, or no, dead time between FFT captures, so are optimized to capture very fast-moving or intermittent signals.
This makes them ideal for capturing and displaying today’s digital communications systems, such as wireless and mobile systems. They are also ideal for spectrum surveillance, with many of the high-end analyzers able to record and play back historical spectrum captures – even demodulating the signals at a later time.
This assumes, however, that the frequency spectrum being monitored is no wider than the real-time bandwidth of the analyzer.
A distinct advantage with swept analyzers is their ability to sweep a broad range of frequencies while maintaining a narrow frequency resolution. For example, they can sweep from near zero to 10 GHz with a resolution bandwidth of 1 kHz in a single sweep and not miss any steady signals.
An FFT-based analyzer would have to step the LO and make multiple measurements to piece together the same 10 GHz span, with the resulting dead time in between frequency bands. This may actually take longer than sweeping…it depends.
However, for most current FFT analyzers, the narrower the RBW the more the speed advantage of FFT because it simultaneously calculates all the measurement points within the FFT analysis BW. Therefore, the smaller the RBW the faster the speed advantage of FFT.
Another big advantage of real-time analyzers is the fact that the persistence mode can display multiple signals on the same frequency or band channel. For example, multiple Wi-Fi signals on a single channel may be observed and analyzed.
In addition, features like spectrogram (waterfall) displays, showing spectral captures versus time, are very useful for identifying interference or EMI issues. Many have Frequency Mask Triggering (FMT), where a mask is generated around a signal or portion of spectrum of interest. If an intermittent signal penetrates the mask, the data is instantly captured and recorded. This allows post-capture analysis of recorded interference or intermittent EMI.
Finally, the “I/Q” baseband data may be used to display digitally modulated data streams as constellation plots or demodulated digital data versus time. AM, FM, and phase-modulated signals may also be demodulated and analyzed. Power measurements, such as spectral power density (SPD) may also be calculated in both the frequency and time domains.
Most of the higher-end analyzers are really vector signal analyzers (VSA), which preserve the amplitude and phase information. This information may be displayed any number of ways, allowing analysis of high-speed digital data, including error vector magnitude (EVM), a rating of digital modulation.
Benefits of Swept Analyzers
- Generally lower cost
- Good phase noise and minimal spurs and usually superior dynamic range
- Able to sweep a broad range of frequencies while keeping good frequency resolution
- Recent technologies have reduced the physical size
- Some are battery portable
- Familiar user interface and controls
- Some test standards continue to specify swept measurements
Benefits of Real-Time Analyzers
- Very fast capture rate
- Fast “data-dense” displays (density, persistence, spectrogram)
- Able to capture fast-moving or intermittent signals with 100% POI
- Able to display multiple signals within the same frequency channel
- Spectrum-based frequency-mask triggering
- Can view elusive or dynamic signal behavior
- Can demodulate and analyze digital data