We check modulation depth of the PDH signal to see if we have enough modulation depth or not. The result will be analyzed later.
Based on the design of the RF summing box we got from Peter ,
the TF from 35.5 MHz input to EOM does not have a resonant frequency at 35.5 MHz. Rather, the resonance is ~ 10MHz. We are not sure if we need to have the resonance at 35.5 Mhz or not. So we decide to check if the modulation index is already large enough for the current design, and compare it with that of the modified one. If the modulation depth is already large enough, we can keep it as it is. Basically we just need a high pass for 35.5MHz input to EOM.
By changing the inductance L from 1uH to 80 nH, the peak can be moved to ~ 35.5 MHz.
==Setup & Measurement==
We want to check the slope of the error signal when the RF power changes, so there's a small change in the setup, see fig1.
The input power is 2mW.
Then the slope of the error signal was record, the pk-pk of the sideband and carriers were also record between the input power of 10dBm and 13dBm to the EOM.
Then the RF summing box was modified (changing L), and the measurement was repeated.
coil | input pwr (dBm) | Carrier slope ( MHz/V) | pk-pk value (mV) (carrier/side band)
80nH | 10 | 1.43 || 28/18
| 13 | 1.11 || 38.8/22.6
1 uH | 10 | 0.91 || 49.2/27.6
| 13 | 0.74 || 68.4/37.2
From the result, the best setup is 1uH coil with maximum side band power which provides the best slope ( maximum gain at frequency discriminator). Note that 1uH coil is the original value for the schematic, so modification to 80nH does not help.
-> Modulation depth