Originally posted Jan 15, 2016
Trespassing is harder to miss in a densely occupied country
The 802.11ah wireless standard mentioned in my last post is promising, but it highlights a challenge that’s facing many engineers in the wireless space: out-of-band or out-of-channel emissions.
In an article from Electronic Products via Digi-Key’s article library, Jack Shandle writes: “Two significant design issues in the 915-MHz band are: 1) The third, fourth, and fifth harmonics all fall in restricted bands, which imposes some design constraints on output filtering. 2) Although it is unlicensed in North America, Australia and South Korea, the band is more strictly regulated in other parts of the world.”
Of course, the higher allowed transmit power and improved propagation of the 915 MHz band—compared to the 2.4 GHz band—adds to the potential for interference. But these days, your harmonic and spurious emissions don’t have to fall in restricted bands to be a concern. Compared to previous eras, the modern wireless spectrum is so crowded that excess emissions are far more likely to cause someone a problem and be noticed. Wireless standards are correspondingly stringent.
For RF engineers, the interference challenges exist in both the frequency and time domains, and this can make problems harder to find and diagnose. The time-domain concerns are not new, affecting any TDMA scheme—including the one used in my 20-plus-year-old marine VHF handheld. Using Keysight vector signal analyzers, I long ago discovered that the little radio walked all over a dozen channels in the first 250 ms after each press of the transmit key. A newer handheld was actually worse in terms of spectrum behavior, but settled down more quickly.
Back then, that behavior was neither noticed nor troublesome, and I don’t suppose anyone would complain even today. However, that quaint FM radio is nothing like the vast number of sophisticated wireless devices that crowd the bands today. Even a single smartphone uses multiple radios and multiple bands, and interference is something that must be discovered and fixed at the earliest stages to reduce cost and risk.
Given the dynamic nature of the signals and their interactions, gaining confidence that you’ve found all the undesirable signals is tough. Using the processing power of today’s signal analyzers is a good first step.
This composite real-time spectrum analysis (RTSA) display shows both calculated density and absolute spectrum peaks. Real-time spans of up to 500 MHz are available, letting you know you’ve seen everything that happened over that span and in that measurement interval.
Though RTSA makes the task easier and the results more certain, RF engineers have been finding small and elusive signals for many years. Peak-hold functions and peak detectors have been available in spectrum analyzers since the early days and they’re effective, if sometimes time-consuming.
Minimizing noise in the measurement is essential for finding small signals, but the traditional approach of reducing RBW can make sweep times unreasonably long. Fast-sweep features and noise subtraction are available in some signal analyzers, leveraging signal processing to expand the speed/performance envelope. Keysight’s noise floor extension is particularly effective with noise-like signals such as digital modulation.
Of course, finding harmonic and spurious emissions is only half the battle. A frequency reading may be all you need to deduce their origin, but in many cases you need more information to decisively place blame.
In addition to frequency, the most useful things to know about undesirable signals are their spectral shape and timing. That means isolating the suspects and relating them to the timing of other signals. One traditional approach is a zero-span measurement, centered on the signal of interest. It’s a narrow view of the problem but it may be enough.
Far more powerful tools are available using the memory and processing power of today’s signal analyzers.Frequency-mask triggering is derived from RTSA and can isolate the signal for display or generate a trigger for complete capture and playback of signals. Signal recording is usually done with the 89600 VSA softwareand can include capture of events that occurred before the trigger.
For even more complex time relationships, the VSA software borrows from oscilloscopes to provide a time-qualified trigger for RF engineers. Command of both the time and frequency domains is the most effective path to interference solutions.
If you don’t have these advanced tools, you can add them to existing signal analyzers with a minimum of fuss. With your RF intuition and good tools, interference has no place to hide.