still play on the NMR system.

first, track the chopper noise. it is indeed the natural frequency induced by the chopper.

  1. the noise only appear when chopper speed steady
  2. there is no electric connection between chopper and NMR system
  3. the noise does not depend on the chopper frequency

i take out the mount of the chopper, put foam and pack of tissue. the chopper noise gone.

the main noise is from the NMR coil, some noise from the input and some from the tuner. the noise level is about 20mV with raw sample, if take 10 average, the noise reduced to 5mV.

  • i change the tuner, so, now may be not matching impedance
  • the glass holder for the water was burned. i may do something wrong….
  • try to understand the NMR system. what are the meaning of each setting.
  • try to use the network analyzer to find the matching impedance, but don’t know how to operate the analyzer.

Chopper noise: