after last night, my colleague checked the NMR coil and it is in good shape. that, she taught me how to operate the network analyzer by command in GBIP.
after we tuned the tuner such that the impedance matching and further tune the impedance to get minimum reflection wave. we got a very strong polarization signal, and a clear T2* decay. then we repeat the experiment for same setting for 3 more times. we get consistence signal. ( FID area is 142, 141, 141, 157)
then we changed the microwave power to be 3.0W, compare to the previous one 3.5W. the signal get significantly smaller. FID area is 57. we repeat the experiment and start getting BG.
something wrong with the system.
when we check the coil, it blended, in bad shape. we use a plastic tape to wrap and fix the shape. now we can repeatedly getting same impedance.
but still nothing but BG, or not a strong signal.
we reset the magnetic field, to stabilized it. not work.
there is a un-expected signal of 2V from the monitor output of the field sweep circuit, eliminated, not work.
we changed the trigger signal be a laser power detector, not work.
we changed the crystal angle, not work
we changed the input impedance to be 50+i , regardless the reflected wave, not work.
nothing work today. need a fresh mind.