Dear Christoph, if I understand your problem correctly, you are trying to visualize the output pulse of your thin film hybrid diode on a light pulse of 337 nm. Do you have access to the driver circuit of the light pulse, where you could take the trigger signal for the DSO? In the set-up as depicted, you essentially monitor the voltage that is the result of the light pulse. I think that for current, a ladder network (two resistors in series, e. 33Ohms and 18 Ohms, with the lower close to GND) would be better. Now you also introduce a large parasitic capacitance, that may (strongly) affect the shape of the pulse. Most DSO have 10pF parallel to the 1M. With a ladder this problem is less.

Dear Harrie!

Yes there is a trigger output (the blue line) to give the start signal. The main problem is to clearly "grab" the resulting pulse since I have to carry the device bias and current (~5 V) and the pulse seems to "drown" in the background noise .

You suggest replacing the 50 Ohm with the ladder? I wonder if it is possible to splice the coax or something?

Thank you for your help!

Yours,

Chris

Dear Chris,

in your schematic you show the DSO at high impedance input. In that case you can increase the size of your sense resistor to get a larger signal. Depending on the expected current you should select a size where the voltage drop is still not that large, because otherwise the applied voltage to your sample will probably differ to much when current is flowing. Depending on the size of the resistor the actual input impedance of your probe is also important. As long as the sense resistor is small it can be neglected. The time constant of your current setup is 500ps. Increasing the resistance by a factor of 10 should be ok without too much influence on the measured signal.

A 50Ohms sense resistor is only needed if the other end (DSO input) is set to 50Ohms too. Than both ends are terminated with cable impedance.

If you are concerned about the signal quality a fast amplifier close to the sense resistor converting the high impedance signal to 50Ohms output resistance could help. Here setting the DSO to 50Ohms input impedance is required together with a standard 50Ohms coax cable. 

Best regards,

Oliver

Christoph, how are you providing the bias to the device? By the same coax cable? If so, how is this cable connected to what type of power supply? I get the impression that you are looking at the noise injected by the power supply from a low output impedance power source, thus effectively shorting the output pulse. You would have to insert an inductor in the power line, and use a capacitor for extracting the pulse. See attached diagram. Please let me know if this is correct as outlined by me. For this you would have to split the coax, or use a coax t-splitter.

now with diagram!