So, in spite of my apprehension, I asked that we look at the power supply again. Reluctantly but respectfully, my colleagues obliged. They connected a digital oscilloscope to the oscillator power supply, and they were right. It was a nice, flat DC supply. They increased the vertical gain, and we measured the ripple, which was practically nonexistent.
Then I noticed that the sweep time on the scope was quite long. I shortened the sweep time, incrementally searching for any AC anomalies riding on the oscillator power supply. Lo and behold, a 2Vpp sine wave showed up looking quite anomalous by taking up most of the oscilloscope's display.
Where was that coming from? We measured its frequency and found that it matched the spectral separation between the oscillator's carrier and each sideband exactly! The telltale 2Vpp signal was traced back to an IC regulator that was oscillating and superimposing its instability on our "flat" DC.
We had the problem on the run. While checking the regulator installation, we found that the newly installed regulator IC was, in fact, the wrong part, and, more importantly, it had been replaced a few days earlier. As usual, hindsight is 20/20. If I had originally checked to see which components had been replaced, I would have put two and two together as soon as I suspected the power supply.
This entry was submitted by John Mitchell and edited by Rob Spiegel.
Tom Machell is an electronics engineer who has worked for Northrop Grumman and L3 Communications. He is presently an avionics/electrical engineer at IMP Aerospace in Halifax, Nova Scotia.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.