Bode plot problems photodiode characterization

[gyfarius]

I am trying to do frequency characterization of the optical detectors together with amplifier to fully take into account properties of the photodetector. I made an optical test system that consistst of current control (0-0.1V OUT1= 0-100mA) of the LED (works up to ~5MHz, higher frequencies probably require laser diode), reference photodetectors 1kHz-1GHz or DC-1MHz. Light from the LED is splitted with beamsplitter in 2 channels, and focused on reference photodetector and sample photodetector. In case of big difference in the amplification, sample detector has additional possibility to add neutral density filter to reduce the optical signal level (e.g. ND1-ND6 = 10x - 1e6x reduction of the light). Out1 is connected to current source modulation, IN1 connected reference photodiode (after amplifier), IN2 connected to sample photodiode amplifier.

I take an PD and amplifier that supposed to have DC-500kHz bandwidth. When I look at the system at the oscilloscope, it works as it should, showing no damping of the signal up to 100kHz, and at 500kHz In2 signal oscillations are halfed. At 1 MHz, PD averages all the signal and only certain "DC" level shown ~50mV. However, Bode measurement (Factory default tool, was never able to access the market) does show some small change at 500kHz but no decay afterwards (max 3 dB). Unfortunately, I am not able to attach screenshots 150kB "Sorry, the board attachment quota has been reached." Please excuse, if I dont use right terms or making stupid mistakes, I am laser physicist and never got propper electrical engineering background.

I use 1.04-18-27498667c sw version. STEMlab 125-14 v1.0

[redpitaya]

Hello Gyfarius,

Thank you for writing on the forum.

I would suggest upgrading the OS to 1.04-28 (The latest 1.04 Version of the OS for STEMlab 125-14), before the Unified OS. There were some fixes to the bode analyzer application in that update. We recently discovered minor bugs with how the signal is calculated with lower value signals, which we plan to fix with the next few OS updates. Instruction on the link below:
https://redpitaya.readthedocs.io/en/lat ... re-sd-card

Please make sure that you do not use oscilloscope probes that are connected to fast analog OUT ports (the probes can be used for inputs, but they will disrupt the output signal (they contain a resistance and a capacitance), use standard cables if possible.


If you have pictures, please send them to support@redpitaya.com.

To reach the marketplace, your Red Pitaya must have internet access. This means that it must be connected to a router with an internet connection (a direct ethernet connection to a laptop or computer will prevent that, as the computer does not share the internet access with the board).

If you have any more questions, please let me know.