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All Places > Keysight Blogs > Better Measurements: The RF Test Blog > Blog > 2017 > September
2017

  Avoiding extra functionality in your circuits

A common saying among electrical engineers is that if you want an oscillator, try building an amplifier. This is all too often accurate, and unwanted oscillations in amplifiers can be a problem for designs at almost any frequency, from very low to high. Of course, if you design an oscillator it probably won’t, at least not at first, but that’s a topic for another day.

Although not part of the saying, it’s important to realize that if you do get an unintentional oscillator, it’s likely to be a terrible one, both unstable and unpredictable. That has consequences for RF and baseband measurements, and we’ll get to that in a bit.

My first taste of these parasitic oscillations outside of a college lab was working with a manufacturing engineer to troubleshoot a 13 MHz function generator. Some samples of the generators were found to be oscillating at frequencies over 80 MHz. The oscillations were found almost by accident, because none of us expected significant output at over six times the generator’s highest frequency!

In today’s wireless environment, unintended oscillations can be a potent source of interference. They’re also more consequential because the spectrum is so crowded, and vital or high-profile services are more likely to be affected. And, as with that function generator, undesirable output signals may be overlooked for a while because they are so far outside of the normal operating frequency range.

Years after the function generator adventure, I heard from a Keysight (then Agilent) systems engineer about a much more serious case of parasitic oscillations. Engineers in the Moss Landing area on Monterey Bay had been using an HP/Agilent signal analyzer, a handheld radio, and a directional antenna to track down signals that were intermittently but persistently jamming GPS signals. The jamming extended well outside the harbor entrance, so it was a clear hazard to navigation.

The interference was maddeningly inconsistent and the directional antenna was of limited help due to strong reflections. As described in an article by the investigators, they eventually tracked the problem to parasitic oscillations in an active TV antenna on a boat at the marina.

Surprisingly, eliminating the first interferer did not completely fix the problem, and instead revealed two more accidental jammers. At least two of the three used the same amplifier board, where a design change had provoked the oscillations.

The instability that made these jammers so hard to find provides some lessons for RF engineers. The principal one is that the oscillations may not be there when you happen to be looking for them, so to ensure their absence you’ll need to explore a wider-than-usual range of frequencies and operating conditions.

Though self-exciting, the oscillators at Moss Landing were sensitive to a variety of factors, some predictable and others capricious: temperature; power supply voltage and its fluctuations; fluorescent lights; antenna configuration; building wiring; and nearby metal objects. Even the motion of the hand of a researcher 10 feet away could alter the frequency by several megahertz. Tellingly, the handheld radio could demodulate the fluctuating output to reveal the distinctive sound of a bilge pump!

It’s clearly essential to test your designs with real-world variations, and these days you have the added challenge that desirable signals and interference are both time-varying. Fortunately, you can take advantage of the signal processing and display capabilities available in signal analyzers to catch virtually everything. For example, spectrograms and real-time spectrum processing can digest and display vast amounts of data, highlighting even the briefest or most agile signals.

Simulated narrowband interference in a group of satellite channels. Real time spectrum analysis (density) display and spectrogram display, showing spectrum vs time

Intermittent, low duty cycle interference in satellite channels is clearly revealed in a real-time spectrum display (top), and the time-varying nature of the interference is shown by the spectrogram display (bottom).

Signal analyzers such as Keysight’s X-Series can also be equipped with VSA software for in-depth analysis and demodulation, and gap-free signal recordings can be made to allow flexible post-processing of transient events.

Of course, amplifiers aren’t the only source of spurious and troublesome oscillations. I once built a FET speed control for a remote-controlled car, and its interference disabled the car’s own radio. The switching frequency was only 1 kHz, but during each brief transition, the FETs oscillated wildly, with powerful harmonics reaching nearly 1 GHz. If only my intent had been to produce an unstable, high-power comb generator!

  A pre-filter to manage an excess of information

In the first two decades of the modern spectrum analyzer—say from the 1960s to the 1980s—it was arguably possible for an RF engineer to know almost everything about making spectrum measurements. One reason: the signals and the analyzers were relatively simple.

Signals were generally assumed to be CW or pulsed CW. Noise and signal-to-noise measurements were straightforward adjustments from spectrum results. Pulsed signals were similarly measured by interpreting conventional spectrum results.

RF spectrum analyzers used a superheterodyne architecture with fundamental mixing, and microwave analyzers used harmonic mixing to cover the higher bands. A semiautomatic technique was adequate to identify images or other false responses, if preselection was not available.

Things have changed immensely in the past 25 or 30 years: the transition from analog to digital modulation; the development of advanced radar and EW systems; and the overcrowding of the airwaves we all share. In lockstep fashion, the corresponding measurement standards have grown in size and complexity.

All these advances have driven a process of mutual bootstrapping, one that has transformed spectrum analyzers into signal analyzers. Built around a wealth of digital technologies, today’s analyzers offer options for modulation analysis, vector signal analysis, and real-time spectrum analysis.

Thus, software has become a vital part of these signal analyzer solutions, frequently in the form of measurement applications. Some, such as PowerSuite, are broad and general purpose. Others are highly specific, designed to make complex sets of measurements in compliance with a particular standard such as LTE or 802.11ac.

These synergistic tools—hardware and software—are now essential for RF engineering. However, if success depended only on starting an app and pressing the right buttons, there would be no need for clever and dedicated engineers. In the real world, successful design and troubleshooting require myriad measurements and setups, and a deep understanding of the results.

If we can no longer know everything about our signals and tools, how do we ensure that we know the right things? I can make no guarantees, but I can offer a few suggestions to help you stay current while keeping the time and effort reasonable.

Discussion forums and blogs: Those that focus on test equipment, such as the one Keysight hosts, are a way to explore common issues and interact with other users, including experts from the manufacturers. Test and measurement blogs are often a companion to the forums, providing news and commentary in specific areas.

Webcasts, both live and recorded: Because I’m rarely in complete control of my schedule, I especially appreciate recorded webcasts. They’re a source of the specific information I need, just when I need it, even late at night or on a weekend. Search engines and webcast collection pages can help you find the one you need.

Articles on common problems and errors: A surprisingly useful type of article is an expert explanation of the most common measurement errors or problems in a given area. At their best, these articles can be a pre-filter that distills unmanageable amounts of measurement knowledge into actionable advice. Such articles also tend to be relevant across time and evolving technology, as this one from Keysight’s Bob Nelson demonstrates. For example, he explains how the log of an average is not the same as the average of a log, and how display detectors yield different answers from the same measurement data.

A single measurement data set is processed by three display detectors to produce three measurement traces, signified by different colors. The detectors are peak, minimum, and "normal."

Three display detectors produce three different measurement traces from the same data set. The “correct” answer depends on the purpose of your measurement.

Serendipity: I can’t count the number of times I’ve learned something important and useful just by chance. The source may have been an offhand comment, an article stumbled upon, a random encounter with another engineer, or the intersection of a search engine and my inherent curiosity. While I can’t rely on these happy accidents, I must confess to feeling slightly uncomfortable with how often they occur.

I realize that for most of you, measurements are a means to an end, enabling your real job: doing the engineering that drives the next waves of ever-advancing technology. It is often an uphill trek that leaves precious little time for simply keeping up. If you have any additional tips for success, please chime in with a comment.