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Unintended Oscillations and Interference Problems

Blog Post created by benz on Sep 20, 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!

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