From the experiment we had done, we had got an open loop transfer function of the SHG control loop. To optimize this control loop, we changed the parameters of the servo (corner frequency and gain) monitoring the open loop transfer function, its unity gain frequency, and phase margin, using the network analyzer (Agilent 36540A).
I show several pairs of parameters below. On the 7th column we have 224Hz unity gain freq. and 30 degree phase margin, it seems somewhat better than the others.
With a gain greater than 2000, system became unstable.
Still, there need to be more investigation.
| Corner freq.[Hz] | Gain[dB] | Unity gain freq.[Hz] | Phase margin[degree] |
| 10 | 1000 | 1000 | 1.5 |
| 10 | 200 | 512 | 11 |
| 10 | 100 | 352 | 14 |
| 10 | 50 | 224 | 23 |
| 3 | 1000 | 672 | 0.6 |
| 1 | 1000 | 384 | 15 |
| 0.3 | 1000 | 224 | 30 |
| 0.1 | 1000 | 96 | 42 |
| 0.03 | 1000 | 32 | 59 |