at the beginning, i measured about 20 readings to establish the signal statistics. However, there is a tread for the signal. and a computer calculation on the likelihood for normali distribution is almost zero. Thus, the experiment has no conclusion.
after, we move to next phase, to matching the ESR frequency with the µw chamber frequency.
the Hartmann-Hahn condition is:
the µw power and µw chamber frequency was fixed. the only thing can be adjusted is the external field.
since changing the external field will also changed the proton Larmor frequency. we have to adjust the NMR frequency as well, in order to have the 90 degree pulse. on the same time, the NMR coil resonance frequency should also be changed by a tuner. We use a network analyzer to preform the job by setting the input impedance is 50Ω+0iΩ as close as possible. Therefore, when we change the external B-field, we have to change the NMR frequency and retune the input impedance.
One deflect of the system is, we don’t have a hall probe to monitor the real magnetic field. and the current displayed on the current supply is not sensitive enough. This, the magnetic field can have some uncertainty and we believe the uncertainly is large. for same current supply, the FID signal can have different period, that indicated that the magnetic field is change. for example, for NMR frequency is at 12.8MHz, a 0.300247T magnetic field will result 60µs period and 0.299464T will result a period of 30µs. the change is magnetic field is 0.783mT. and this can not be shown on the current display on the current supply.
the experiment result is not clear.