SR830 lock-in amp measurement problem

[notabird]

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.

 

[f95toli]

I might be missing something here, but why are you DC couplng the SR830? There is no need to apply any DC to the input.
Unless, you are for some reason referring to the 10kHz as "DC"..
Also, "DC-modulated" makes no sense.
.
Note that the input on the SR830 can go into overload if the TOTAL signal is too large; it is not just the amplitude of the 10 kHz component that matters. You could always try putting a low-pass filter (or ven better a bandpass filter centered around 10 kHz) before the input of the SR830.

 

[notabird]

I might be missing something here, but why are you DC couplng the SR830? There is no need to apply any DC to the input.
Unless, you are for some reason referring to the 10kHz as "DC"..
Also, "DC-modulated" makes no sense.
.
Note that the input on the SR830 can go into overload if the TOTAL signal is too large; it is not just the amplitude of the 10 kHz component that matters. You could always try putting a low-pass filter (or ven better a bandpass filter centered around 10 kHz) before the input of the SR830.

Thanks. I am using the "DC coupling" setting on the SR830. If I use "AC coupling", I do not get the DUT signal due to excitation from the 10kHz amplitude-modulated 2 GHz ac current. But seems like a band pass filter might work, shall try that thanks.

 

[f95toli]

Thanks. I am using the "DC coupling" setting on the SR830. If I use "AC coupling", I do not get the DUT signal due to excitation from the 10kHz amplitude-modulated 2 GHz ac current. But seems like a band pass filter might work, shall try that thanks.

That sounds very strange, I can't remember the cut-off for the AC-coupling but it should certainly be below a few Hz. At 10 kHz you should definitely be able to use AC coupling (I've used SR830s for many years so I am quite familiar with the model); in fact, I can't think of any reason to use DC-coupling at all in this case; any DC-offet could easily saturate the input amps (or at least force you to use less sensitivity).
I assume you are using the single-ended input?

 

[Kristian Poulsen]

According to datasheet, the SR830 is limited to 102.4kHz, the 2GHz signal is way beyond that. Looks like you are getting your AC signal demodulated by internal protection circuitry. I believe, you would need to change setup completely.