Originally posted Nov 6, 2013
Taking time resolution to the extreme in your measurements
Previous posts explained the fundamentals of real-time analysis and described the difference between real-time analyzers and real-time analysis. The simple concept common to any version of real-time analysis is results are calculated from every sample of the selected frequency span, with no gaps.
However, as we saw in the recent post on understanding different types of real-time spectrograms, the displays generated from real-time spectra can be quite different and can be used to analyze specific phenomena or problems. In particular, the setting for acquisition time changes the time scale of the spectrogram over a relatively wide range and thus sets the time resolution of the results.
The ability of high-performance analyzers to generate several hundred thousand spectra per second has an important effect on displays such as spectrograms and density. Because neither displays nor the human eye can process spectra at this rate, each spectrogram line must represent many separate spectra. The analyzer’s display-detector function—peak, negative peak, average, and so on—determines how the copious spectra are combined to form a single line. The effect, even for very short acquisition times, is to limit the time resolution of each spectrogram line or slice.
If you need very fine time resolution and greater measurement flexibility, it’s best to switch from real-time spectrum analysis to another family of real-time measurements: gap-free signal capture and post-processing. Time capture and flexible post-processing (playback) are provided by the 89600 VSA software. Its analysis capabilities are available on many hardware platforms including the PXA and MXA X-Series signal analyzers that provide the real-time capabilities we’ve discussed previously.
Let’s look at a spectrogram of the ISM band produced using capture and playback. Compared to the results described in a previous post about real-time analysis, the spectrogram shown below not only has a different appearance but also provides different benefits.
This is a real-time spectrogram from capture/playback of 100 MHz ISM band at 2.45 GHz. WLAN and Bluetooth® signals are prominent, along with leakage from several microwave ovens. Individual hops and frames are shown clearly and the time-slice markers identify the repeat rate of the wandering signals as 1/60 Hz from microwave ovens.
The benefits of flexible post-processing apply to analysis and troubleshooting. For example, playback speed is adjustable over a very wide range by adjusting overlap. A large overlap provides extremely fine time resolution in the spectrogram, and this can help you see exactly how signals change—or relate to others—over time.
In the spectrogram above, fine time resolution reveals a few instances of interference and many of spectrum sharing, which is vital to effective use of this band. Note that two WLANs are successfully sharing the center channel (in this timeframe, at least). Also note that most, but not all, Bluetooth hops are clear of the WLAN bursts and microwave oven activity.
The capture/playback approach also provides complete flexibility in changing analysis types and parameters. In the 89600 VSA software, center frequency and span can be changed after the capture, enabling analysis to focus on specific signals and time intervals. The analysis mode can be changed from spectrum to time domain or demodulation, and several different types of demodulation can be done at different center frequencies or spans to evaluate interference.
No single spectrogram method is best for all applications. The continuous spectrogram of a real-time spectrum analyzer is an excellent way to monitor signals or bands over longer periods. The capture/playback approach is a powerful way to examine short-term behavior in detail, especially when paired with magnitude or frequency-mask triggers and pre- or post-trigger delays.
For more information on these displays and measurements, please see the application note Real-Time Analysis Techniques for Wireless Measurements.