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Some of my student and I are designing a Physics experiment involving a high power LED, a square wave from function generator (50 ohm output impedance) and a digital oscilloscope (1 MOhm imput impedance). The output of the function generator has two BNC cables. One goes into the oscilloscope channel and then other goes into the LED. The LED is coupled with a light sensor (ThorLabs DET36A/M and the output is connected to the another channel on the oscilloscope. Both these channels are plotted on the same channel.

We gave a square tooth wave of amplitude 0.5 V with a off-set of 1.5 V (so it fluctuates between (1.5-2 V). I was expecting that the light pulse from the oscilloscope would follow the same pattern as the square wave (of course with a smaller amplitude). But instead we got the waveform in the attached picture. enter image description here

The confusing part is the decaying peak even when the voltage is 1.5 V. This trend is similar even if I set the minimum voltage to zero in the cycle. If I extend the duty cycle of the function generator I get the picture attached above.

I am unable to understand this pattern like a RC circuit. I understand some capacitor is involved somewhere, very possibly in the photosensor but I am not able to understand what is going on physically and how to fix this.

There is a known impedance mismatch between the oscilloscope and the function generator.

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    \$\begingroup\$ First thought: Is your second channel set to AC-coupled on your oscilloscope? The picture is too blurry to tell. \$\endgroup\$ – Hearth Jan 25 at 16:40
  • \$\begingroup\$ I did not change the setting. By default DC coupling is selected as far as I can remember. \$\endgroup\$ – Shaz Jan 25 at 16:48
  • \$\begingroup\$ Perhaps double check? Many scopes remember the setting they were on last. \$\endgroup\$ – Hearth Jan 25 at 16:56
  • \$\begingroup\$ So I did go into that that setting to check out ground coupling and what it does (as I am fairly novice in electronics). I remember seeing that it was set to DC and after experimenting I do remember changing it back to DC. Any other potential source? \$\endgroup\$ – Shaz Jan 25 at 17:06
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    \$\begingroup\$ The manual for the detector explicitly states that it is AC-coupled, with a -3 dB bandwidth of 0.2 Hz to 10 kHz. I can't tell what your pulse rate is from your awful photograph, but if it's slow enough, this would explain your results. \$\endgroup\$ – Dave Tweed Jan 25 at 17:14
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Following up on the discussion in the comments, it turns out that the battery of the sensor was indeed weak. However, upon changing the battery the sensor still gave the same response. On speaking with Thor lab tech support they pointed out, among other possibilities, that the sensor might be saturating. When I fixed the problem, the signals on the oscilloscope was it was expected. See picture attached. enter image description here

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