Thanks very much.
These circuit could be considered as two independent LC circuit, coupled with 78pF capacitor. A combination of these two LC will produce two overall circuit oscillations. One is both oscillation at the same phase, the other is at a 180 phase difference. That could be somehow interpreted as two 'normal mode oscillation'. I could only reach this explanation.
More generally, if more LC circuit are coupled, in order to predict its possible numbers of frequencies, it is better to first find out how many numbers of 'normal mode oscillation' exist. So, I take a 3 LC coupled circuit to test. But the hypothesis doesnot work! There should be 3 frequencies, but what I observed is only 2. Could you please help point out what's wrong with my explanation?
Actually I am a chemist try to build such a circuit for research purpose. I know through mathematics, one could deduce those two frequencies. But what I more interested in is to figure out a more qualitatively way to understand that circuit rather than complicated formula.
Think of it as two independent tuned circuits with energy transferred from one to the other via the common impedance, i.e., the 75 pF shunt capacitor. That's somewhat of a simplification, but it's a useful aid in understanding how the two interact.
It will become clearer if you redraw the circuit as a two loop mesh with the 75 pF being the common element and it will become clearer.
Try a Google search with the terms "under coupled" "critical coupled" and "over coupled" tuned circuit for the classic treatment of coupled tuned circuits. These are also known as "double tuned" networks.
This has the look of a homework problem to me so I will limit my observations to these comments.