Electronic – What can the source of 100Hz noise be

The base frequency of auditable noise of a 50 Hz transformer should be 100 Hz since the two half waves (negativ and positive) are giving the same movement in physical space on the moving part of the transformer.

Mechanical fixation and/or sound insulation is the way to go. Apart from screwing the transformer down, use some liquid glue (e.g. potting compound, protective lacquer) to cover coils and iron sheets. Be careful to use non-flamant, temperature resitive materials for potting. Also try to insulate the sound propagation to the housing by mounting the transformer on a foam pad.

1). Confirm output impedance of Analog sensor output by load test for ~50% drop

• complementary emitter followers used in Op Amps or discrete are notorious for resonance with capacitive loads from 100pF/m cables and current limiting causes low out followed by overshoot followed by ringing at loop gain BW of Op Amp
  • This may be what is happening
• this is where current mode converters work better 4-20mA with separate return wire, shielded or twisted pair and CM ferrite sleeve.
• add 100-300 R series at source to verify reduction in edge pulse
• then load destination with 1k to reduce stray noise coupling and verify voltage drop if any. You may need a power op amp buffer with 120 Ohm source and low impedance load to reject stray coupling or whatever cable Z you are using.

• check with proper high speed measurements for RF noise and LPF as required to rule out RF aliasing noise at 8kHz sampling rate

  • consider 10mH CM choke and RF cap across output for 50/60Hz and up noise

  • example of your CM 50Hz noise showing up as a differential mode (DM) signal

  • due to unbalanced source and load Z on 50 Ohm coax.

  • Is a shame to degrade an instrument with > 60dB SNR down to 20 dB with improper analog cable maybe 30~40dB SNR with filter