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.

 

Are you saying that a voltage divider does not reduce the dc input current? ... Or are you saying that you have tried series resistors at the DC input?
... There is also the current divider. That would be two parallel resistors, with the load circuit in series with one of them. ... Just try some experimental values if you don't know the exact input input impedance of your circuit.