How do I identify the source of an oscillation in my RF amplifier circuit?

Identifying the source of an oscillation in an RF amplifier circuit requires systematic diagnostic investigation because oscillations can originate from multiple mechanisms. The most common causes are: insufficient stability margin (the amplifier's S-parameters create a condition where the load or source impedance maps into the unstable region of the Smith chart), feedback through the DC bias network (RF energy couples from the output through shared bias supplies or inadequately decoupled bias lines back to the input), electromagnetic coupling between output and input (through the air, substrate, or poorly isolated signal paths), device parametric oscillation (caused by bias conditions that create negative resistance at specific frequencies, particularly at low frequencies where the transistor gain is very high), and excessive gain at out-of-band frequencies (the transistor may have 20+ dB gain at low frequencies where matching networks provide no selectivity). Diagnostic steps: (1) Observe the oscillation on a spectrum analyzer (identifying the frequency narrows the cause significantly), (2) check if the oscillation frequency corresponds to a known resonance (bias network, cavity mode, or loop delay), (3) probe bias lines with an RF probe to detect RF energy on DC feeds, (4) add lossy material or ferrite absorber to specific areas of the circuit to determine where the coupling path exists, and (5) check the stability circles at the oscillation frequency using the device's S-parameters and the actual circuit impedances.