How do I systematically isolate a failure in a multi-stage RF transmitter chain?

Systematically isolating a failure in a multi-stage RF transmitter chain follows a divide-and-conquer approach: inject a known signal at the input, measure the output at each stage, and compare the measured performance against the expected performance (from the design specifications) to identify the failing stage. The procedure: verify the input signal first (connect the signal source directly to a spectrum analyzer or power meter to confirm the input signal is correct: correct frequency, correct power level, correct modulation). Then work through the transmitter chain stage by stage: measure the output power at each stage's output (using a directional coupler or by temporarily disconnecting the stage-to-stage connection and inserting the power meter or spectrum analyzer). At each stage, calculate the gain: output power - input power. Compare the measured gain against the expected gain. The failing stage will show: significantly lower gain than expected (suggesting the active device has degraded or failed), higher distortion than expected (suggesting the stage is compressing or oscillating), unexpected spurious outputs (suggesting oscillation, instability, or component failure), or abnormal DC bias conditions (incorrect supply voltage, excessive current draw, or zero current). The divide-and-conquer strategy: start at the midpoint of the chain. Measure the signal there. If the signal is correct at the midpoint: the failure is in the second half. If the signal is incorrect: the failure is in the first half. Then bisect the suspect half. This locates the failing stage in log2(N) measurements instead of N measurements.