It’s RF interference again…

Blog Post created by benz on Jul 10, 2017

  The case of the troublesome garage door opener


Note from Ben: This is the first in a series of guest posts from Jennifer Stark of Keysight. As discussed here earlier, our increasingly crowded RF environment will result in more interference, and a higher likelihood of it causing problems. To stay ahead of them, you’ll need your creativity, deductive skills, and persistence


Interference is everywhere. And often from an unlikely source.

Let's take the case of an engineer (we’ll call him Mike) who recently had a problem with his garage door opener. Mike had recently installed a new garage door opener. Frustratingly, the garage door remotes that our Mike and his wife carried in their cars intermittently failed to activate the garage door opener.

As a first step, Mike called the support line for the manufacturer of the garage door opener to report the defective product. The installation support person walked Mike through a troubleshooting procedure over the phone. The procedure did not identify any reason that the hardware should be defective. At that point, the installation support person gravely pronounced “You have something called RF interference. That’s your problem, not ours.”

It turns out that Mike is an RF engineer, so he took this as an interesting challenge.

Mike used his N9912A FieldFox handheld RF analyzer in spectrum analyzer mode. He cobbled together a homemade antenna for the input connector and started sniffing around the house for RF interference. He identified the target frequencies by pressing the garage door remote button while looking at the RF spectrum.

Waterfall and spectrogram displays are a way to visually understand the time domain behavior and frequency of occurrence of signals and interference. This display is from a handheld spectrum analyzer with additional software that helps in detecting and visualizing interference.

Waterfall and spectrogram displays are useful for spotting interference and understanding its behavior in the time domain. The N9918A-236 Interference Analyzer and Spectrogram software for FieldFox analyzers adds these displays to spectrum measurements.

Armed with this information about the frequency range of interest, Mike set out looking for any signals that were near the frequency of the garage door opener. A diligent engineer, he went all over the house looking for clues. He looked in the garage. He looked upstairs in the house, above the garage. He looked in corners of the house.

Eventually, he discovered a small but significant signal in the kitchen. It appeared to be coming from the refrigerator. This puzzled Mike, but his engineering discipline compelled him to investigate. Unplugging the refrigerator did not eliminate the signal. Checking at a different time of day, Mike discovered that the interfering signal was absent even when the refrigerator was operating. It was a mystery.

Leaning on the kitchen counter to collect his thoughts, Mike took stock of what he had learned:  intermittent garage door issues, signal coming from the kitchen. Then, Mike had an insight. The garage door only failed when his wife was home, so the issue was related to the comings and goings of his wife.

At this point, Mike noticed his wife’s purse in its normal spot on the counter by the refrigerator. Mike investigated his wife’s purse with his FieldFox. Sure enough, the interfering signal was coming from the purse (not from the refrigerator). Inside the purse was the remote key fob for the car. Mike removed the battery from the key fob and the interfering signal immediately went away.

The solution was simple—replace the troublesome key fob. Now the garage door is working properly, Mike is happy, and Mike’s wife is happy. And, the pesky RF interference is no more.