Originally posted Oct 4, 2016
Sometimes the answer is not in the first place you look
Real-time spectrum analyzer (RTSA) capability has gradually moved from a specialized measurement made by specialized instruments to an option available for many spectrum and signal analyzers. Keysight recently added an RTSA option to FieldFox handheld analyzer models that include RF/microwave spectrum analyzers and combination analyzers (spectrum analyzer, vector network analyzer and cable/ antenna tester).
We’ve come a long way from the early 1990s, when RTSA was available only in surveillance-focused RF spectrum analyzers with purpose-built signal processing hardware. A decade later RTSA became a type of RF/microwave analyzer for more general use, though still with a dedicated architecture to meet the signal processing demands.
A few years later, the increasing power of ASICs and FPGAs allowed RTSA to be folded into mainstream signal analyzers as an option that, in many cases, can be retrofitted to existing ones. The FieldFox models provide real-time bandwidth to 10 MHz and frequency coverage to 50 GHz, accurately measuring signals as brief as 12 µs. They can detect—but may not accurately measure—signals as brief as 22 ns. Impressive performance for a general purpose tool that is handheld, battery powered, and environmentally sealed.
This “density” display from a FieldFox microwave combination analyzer with the RTSA option represents the dynamics of a number of different digitally modulated signals with different colors signifying frequency of occurrence.
To catch these brief events, the FieldFox handhelds calculate 120,000 spectra per second. The benchtop signal analyzers are even faster, covering real-time bandwidths to 510 MHz and detecting signals as short as 3.3 ns.
However, detection and basic measurement of a transient is sometimes just the first step in solving a problem, and I thought I’d describe additional tools and techniques that will tell you what you need to know.
After power and frequency, timing is the element that can help you assess the significance of a signal and begin to understand cause and effect. Spectral sequences such as spectrograms put power, frequency, and timing in a single display that dramatically enhances understanding. A simple process of saving and displaying successive spectra has great intuitive leverage.
Some of the most problematic signals and events are both brief and highly intermittent. In these situations the essential element of the measurement solution is triggering, from basic magnitude triggering to frequency-mask and time-qualified triggers. These techniques take advantage of real-time calculations of signal magnitude and spectrum to set a customized trap for the specific signal or event you’re chasing. The analyzer can do the tedious work while you get a cup of coffee.
One of the most powerful and intuitive tools for elusive signals is signal capture and playback. This is generally performed with 89600 VSA software, operating on the same benchtop signal analyzer platform as RTSA. The entire signal is streamed to memory, without gaps, for playback and flexible post-processing. Since the signal is captured in the time domain, any type of analysis—in any domain—can be selected after the fact.
Capture and playback is a sort of time machine for the RF engineer. The signal analyzers and VSA software support negative trigger delays, allowing analysis before the trigger event. They also implement signal resampling, so you can go back in time and change your mind about center frequency and span. Magic! You can even change the speed of playback to see events in all their subtle detail.
When the signal or behavior seems to be aggressively hiding from you, the most effective approach is to compound these measurement techniques: Real-time analysis enables frequency mask triggering, which enables signal capture with a negative trigger delay. You can lay in wait, missing nothing while you enjoy your coffee or tea, and capture the most elusive problem for later analysis of any type you choose.