Originally posted Jul 24, 2015
When you’re playing cat-and-mouse with tricky signals
Hertz-minded RF engineers are becoming more and more comfortable with the time domain and, in particular, with simultaneous operations and signal analysis in the time and frequency domains. Part of the reason is that modern systems—from radar to communications—must be optimized in both domains. Of course, many systems are also frightfully complex in both domains, but that’s a post for another day.
The other part of the reason for this dual-domain focus is defensive: things can go wrong in both domains and engineers will need to find and fix the problems—or at least convincingly point to the guilty party.
Fixing a problem often begins with a clear, unambiguous look at the signal in question. That’s not much of a challenge for a CW signal, and even pulsed or TDMA signals can be handled with proven techniques that have been around for years.
Unfortunately, getting a clear look at the contents of today’s crowded frequency bands is difficult, and getting more so. You’re often looking for one signal among many, and it may be present for only a tiny fraction of your measurement time. To compound the elusiveness, the signal may also be hopping or switching channels.
The challenge is obvious in unlicensed spectrum like the ISM bands, where there are lots of users, minimal supervision, and many different signal types. Even in the licensed bands you may need to track down brief, unintended emissions from users on other bands, including harmonics, spurious or transient switching effects.
As is so often the case, the solution is to take advantage of powerful DSP and borrow something from oscilloscopes, our friendly experts in the time domain: the time-qualified trigger (TQT).
As the name implies, this trigger adds one or more time-qualification parameters to other trigger types such as frequency mask or IF magnitude. Here’s the TQT applied to a magnitude trigger in the 89600 VSA software:
A time-qualification parameter T1 is applied to an IF magnitude trigger on an RF pulse. A data-acquisition trigger is generated only if the pulse stays above the IF magnitude level (dashed horizontal line) for an interval longer than T1. A pre-trigger (negative) delay is used to move the data acquisition earlier and capture the entire pulse.
The simplest time qualification is to trigger when an event lasts longer than a selected time T1. Using the two available time parameters, T1 and T2, provides three additional qualifications:
- Duration less than T1
- Duration between T1 and T2
- Duration less than T1 and greater than T2
Of course, the point in time when the trigger conditions are all satisfied is unlikely to be the point at which you want to begin sampling and measurement. The VSA solutions include adjustable trigger delay to solve this problem, and the negative (pre-trigger) delay is frequently the most useful. It allows you to wait until you’ve found the exact signal you want and then go back in time to see the beginning of it or even earlier events.
Speaking of time travel, triggers such as TQT and IF magnitude can also be used in the VSA software to initiate signal recordings or time captures. Complete, gap-free blocks of frequency and time data can be streamed to memory for any—or multiple—types of post processing. Both center frequency and span can be changed after capture, to examine different signals and to track down cause and effect.
For RF engineers, the frequency domain is also a critical qualifier, and the frequency mask trigger (FMT) in real-time spectrum analysis is a powerful complement to TQT. FMT and TQT can be used together to qualify measurement data blocks in both domains simultaneously, trapping fleeting signals or capturing dynamic behavior with the speed and thoroughness of a hungry barn cat chasing lazy mice.