Inadvertent Waveguide - John Dunn, Consultant, Ambertec, P.E., P.C.

There was this amplifier constructed on a circuit board and mounted in a metal chassis which worked fine if the chassis cover wasn't attached, but which would go into parasitic oscillation at 1 GHz when the cover was put on.   

 
My assigned task was to fix it without changing anything. (My boss at the time may have suspected that I dabbled in the occult arts. I was never sure with that guy.)

The reason for the oscillation was that with the chassis cover in place, the chassis itself formed an unintended waveguide. It's width was just right to support the parasitic oscillation frequency.    

 
I violated the boss's order and made a change. I added 10Ω resistors in series with the base leads of each 2N918. The amplifier bandwidth wasn't harmed by that, but the 2N918 frequency responses were diminished to below oscillation thresholds.

Comments

[George Storm]

"My boss at the time may have suspected that I dabbled in the occult arts. I was never sure with that guy."
I had several bosses who appeared to believe that all microwave engineering was black art. Being innocent I never thought to utilise this in salary negotiations.
BTW, I had a similar problem with cavity resonance - that one was solved by sticking a bit of carbon-loaded foam in the middle of the lid (so no "circuit change" or board modification - does that count?)

[John D.]

George, the foam idea was great! I'll never know if it would have counted though, that boss' whereabouts today are quite unknown.

[eric]

same problem back in the 80's at x-band in a MIC module. used George's solution but silicone sheet loaded with iron powder (magram) mounted to the inside of the lid temporarily until circuit was redesigned.

[Mattias F]

Oscillations due to cavity resonance are a true classic! As a curiosity it is in some cases possible to reduce the problem with a simple pencil by sketching on the lid. The graphite dampens the modes.