Originally posted Mar 18, 2016
“OK, Jamie, let’s go to the high-speed”
There are times when understanding an event or phenomenon—or simply finding a problem—demands a view using a different time scale. I’m a fan of the Mythbusters TV series, and I can’t count the number of times when the critical element in understanding the myth was a review of high-speed camera footage. I’m sure the priority was mostly on getting exciting images for good TV, but high-speed footage was often the factor that really explained what was going on.
Another common element of those Mythbusters experiments was their frequent one-shot nature, and high-speed cameras were critical for this as well. Single events were trapped or captured so that they could be examined over and over, from different angles and at different speeds.
Time capture, also called signal capture, came to the general RF analyzer world in the early 1990s with the introduction of vector signal analyzers (VSAs), whose block diagram was a natural fit for the capability. While it was primarily a matter of adding fast memory and a user interface for playback or post-processing, significant innovation went into implementing a practical magnitude trigger and achieving trigger-timing alignment.
The block diagram of a VSA or a signal analyzer with a digital IF section is good foundation for time capture, and it required the expansion of just two blocks. Capture/playback is especially useful for the time-varying signals that VSAs were designed to handle.
Over the years, I’ve used time capture for many different measurements and think it’s really under-used as a tool for RF/microwave applications in wireless, aerospace/defense, and EMI. It’s an excellent way to leverage the knowledge of the RF engineer, and it’s easy to use: first select the desired frequency and span and then press the record button.
The insight-creating and problem-solving magic comes during playback or post-processing. Captures are gap-free, and playback speed in VSA software can be adjusted over a huge range. Just press the play button and explore wherever your insight leads. You can see the time, frequency, and modulation domains at the same time, with any number of different measurements and trace types. You can easily navigate large capture buffers with numeric and graphical controls, and even mark a specific section to replay or loop continuously so you can see everything that happened.
A simple capture/playback of a transmitter switching on shows a transient amplitude event in the bottom trace. The top two traces use variable persistence to show the signal spectrum and RF envelope as playback proceeds in small steps.
Today’s signals are highly dynamic, the RF spectrum is crowded, and design requirements are stringent. You often need to optimize and troubleshoot in all three domains—time, frequency, and modulation—at once. You have the skill and the knowledge, but you need a total view of the signal or system behavior. In my experience, there’s nothing to match the confidence and insight that follow from seeing everything that happened during a particular time and frequency interval.
I’ll write about some specific measurement examples and techniques in posts to come. In the meantime, feel free to try out time capture on your own signals. The 89600 VSA software includes a free trial mode that works with all of the Keysight X-Series signal analyzers and many other Keysight instruments, too. Just press that red record button and then press play. It’ll make you feel like an RF Mythbuster, too.