Recently, one of my new designs has been plagued by a very nasty parasitic oscillation. Anyone who has ever worked on this kind of problem knows how difficult it can be.
Parasitic oscillations have several different causes. In one category, they arise from an unintended resonance in the circuit. Real world circuits contain stray capacitance at almost every node. Perhaps even more important, the circuit traces between the components introduce stray inductance, and mutual inductance with other traces. Although these untended or “parasitic” components do not appear on your schematic, they are real nonetheless! This category of oscillation can usually be cured with the addition of damping resistors or ferrite suppressors.
Parasitic oscillations can also arise from violation of the Nyquist criterion in feedback circuits. (A proper discussion of feedback stability is far beyond the extent of this short note, and the reader is referred to any number of excellent texts on this subject.) In circuits with op-amps, watch out for stray capacitance at the op-amp inputs. Locating power supply decoupling caps too far from the op-amp introduces stray inductance that can provoke a parasitic oscillation in its output stage.
I have found that the best tool for chasing a parasitic oscillation is an RF spectrum analyzer with a custom-built probe. Because the input impedance of RF analyzers is 50 Ohms, one must be very careful about damage from contact with analog power supplies. To solve this problem, I recommend taking a x1 scope probe and soldering a 4.99k resistor in series with a 100 pF capacitor to its tip. This forms a 100:1 attenuator having a shunting impedance of about 5k above 300 kHz and an open circuit at DC. Spectrum analyzers usually have plenty of sensitivity to make up for the probe attenuation.
Don’t worry about compensating this attenuation. Most x1 probes have approximately 30 pF of shunt capacitance in their cable. This causes a low-pass filter with the 50-Ohm input of the analyzer at approximately 100 MHz, which is higher than most commonly encountered oscillations.
Good luck chasing your next oscillation…