Input signal is 40Hz. It started with high pass filter switched to 100Hz. Then when I switched it to 1000Hz, the noises started showing up, and becoming worse at 3000Hz, 10000Hz, 30000Hz, 50000Hz. Halfway I switched it back from 50000Hz back to 100Hz where the noise vanishes.
Without the 40Hz. Does it mean there are high frequency waveforms at 0V origin? What usually caused such high frequency ripples in any circuit?