Hello I am currently have difficulties trying to measure dc-modulated voltages while applying a dc current, via Sr830 Lock-in Amplifier (LIA). Basically, I apply an 2 GHz ac current (amplitude modulated@10kHz and referenced to the LIA for lock-in measurement) and a dc current to my DUT (device under test). This is done through a biased tee. The applied modulated ac current generates a dc-modulated current @10kHz which superposes with the applied dc current, both of which goes into the LIA input. I find that when the dc current is too large, the LIA input overloads and I can't measure anything. I have tried connecting resistors to decrease the dc current drawn by the LIA but it does not seem to work. Appreciate if anyone could help! Thank you.