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20120521 Photodiode Linearity

Need to measure the linearity of the pre-ip photodiode behind the last turning mirror just before the LWIP. Just before the last shift, paper was added to the front of the photodiode to attenuate to the signal sufficiently so that the photodiode wasn't saturating. This linearity will also apply relatively (but not absolutely - due to different ADC pedestal) for the data from last shift.


Photodiode has around 6 - 8 layers of normal copier paper in front of it. This was added until the scope trace didn't look saturated with a flat top on the scope when the laser was at full power. A BNC RG58 cable connects the photodiode to an SMA BPM cable (teal coloured ones) that runs to the LW hut. An inverter is then used before passing it straight to the ADC.

The gate of the ADC was investigated before the last shift and it is not possible to generate a gate short enough to only sample the peak of the pulse. However, the integral of the pulse and the integral of only the rising peak showed the same dependency but the amplitude of the rising edge integral was obviously a lot lower. Therefore the full integral was chosen.

Background Subtraction

The ADC pedestal had changed since last shift and in addition the gate was slightly out of time - presuming the laser timing has changed. Used /run/ADC_pedestal_subtraction.py with laser blocked (and main LW detector switched on but no voltage) for pedestal subtraction.

  • 100 Pulse background log with laser shut 20120521_1300_log.dat


  • 100 Pulse logs for each flash lamp timing

Flash Lamp Delay (us) Data Filename
200 20120521_1244_log.dat
190 20120521_1245_log.dat
180 20120521_1248_log.dat

  • and at this point the cursed energy meter bombed and is unreachable.
  • it doesn't externally trigger or work at all - useful.


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Topic revision: r1 - 21 May 2012 - LaurieNevay

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