Filter Tuning

Gyro filters remove motor and propeller noise before it reaches the rate controller. Too little filtering sends noise to the motors (hot motors, rough flight); too much adds delay and limits how high the gains can go. Tune filters before or together with the rate loop.

Default Filters

Parameter	Default	Filter
IMU_GYRO_CUTOFF	40 Hz	Low-pass on the gyro data used by the rate loop
IMU_DGYRO_CUTOFF	20 Hz	Low-pass on the D term (angular acceleration)
IMU_GYRO_NF0_FRQ	0 (off)	Fixed notch filter
IMU_GYRO_DNF_EN	0 (off)	Dynamic notch filters from ESC RPM or FFT

The defaults are conservative and suit most mid-size multirotors.

Find the Noise First

1. Fly a short hover with some throttle changes. Adding the High rate logging profile (SDLOG_PROFILE bit 4) gives better spectra.
2. Open the log in Flight Review and look at the gyro spectral density plots.
3. Look for peaks: a band that moves with throttle is motor/propeller noise; a fixed peak is usually a frame resonance.

Options

Low-Pass Cutoffs

Lower IMU_GYRO_CUTOFF and IMU_DGYRO_CUTOFF if noise is broad and high. Raise them (gradually) on a clean, well-mounted vehicle to reduce delay and allow higher gains.

Fixed Notch

For a resonance at one frequency (frame or arm flex), set IMU_GYRO_NF0_FRQ to its centre and IMU_GYRO_NF0_BW to its width.

Dynamic Notch from ESC RPM

The most effective filter tracks the actual motor speed. IMU_GYRO_DNF_EN with the ESC RPM option places notches on each motor's frequency and its harmonics (IMU_GYRO_DNF_HMC).

It needs RPM data from the ESCs. On the 305ap that comes from either:

• Serial ESC telemetry on the motor connector (BLHeli32/AM32), started from the SD card (see ESC Configuration), or
• DroneCAN ESCs reporting status (see DroneCAN).

Serial telemetry updates each motor in turn, so its RPM data is slower than bidirectional DShot (which has not been verified on the 305ap yet).

Dynamic Notch from FFT

Without RPM data, PX4 can find the dominant noise frequency itself with an FFT of the gyro data. Enable it with IMU_GYRO_FFT_EN = 1 and the FFT option in IMU_GYRO_DNF_EN. It works best when one noise peak dominates.

Checking the Result

Compare the gyro spectra before and after: the peaks should be reduced, and motor outputs in the log should look smoother. Then revisit the rate gains, since less filter delay often allows higher D.

See the PX4 filter tuning guide for details.