Diagnosing motor & D-term noise — filters, not PIDs
Hot motors and a twitchy quad often look like a PID problem — but they're frequently a filter problem. Here's how to tell the difference.
Symptoms: hot motors, but the PIDs look fine
If the motors are hot after a short flight but tracking (step response) looks clean, the cause is rarely the PIDs. Usually unfiltered noise is reaching the motors and burning energy as heat.
Read the FFT: motor noise vs. D-term noise
Sharp peaks that move with throttle are motor noise (imbalance, resonance). Broad high-frequency energy in the D-term spectrum is D-term noise, amplified by the derivative term. PIDoctor shows both spectra separately.
Fix at the source: RPM filter + dynamic notch
The RPM filter removes motor harmonics precisely at the source; the dynamic notch catches the rest. Only once those are working well should you add more lowpass filtering — because every lowpass costs latency.
How PIDoctor separates filter from PID problems
The tune score has separate sub-scores for tracking (PIDs) and noise/filters. If the PID sub-scores are green and the noise score is red, you know immediately: it's the filters, not the PIDs.