R&D (FilterCavity)
EleonoraCapocasa - 16:26, Friday 10 July 2020 (2111)
QPD2 noise measurement -- part2

Matteo, Eleonora

We put a reflective mirror after green FI so that the light is reflected back to the QPD2 using the standard path. The power was changed acting on the AOM modulation amplitude. 78MHz and 87.6 MHz modulation are switched off.

Pic 1: RF output of QPD2 Seg 1 in with different power level. 

Pic 2: Same of Pic 1 but zoomed around 78 MHz.

Pic 3: Peak height at 15.2MHz and 30.4 MHz as a function of the green power. (This is a real signals come from PDH of SHG)

Pic 4: Noise at 20 MHz as function of the green power. 

Measured data are reported below:

Green Power [mW]

Current [mA] 

Noise @ 20 MHz [dBm] Noise @ 20 MHz [uV/sqrt Hz] Peak @15.2 MHz [dBm] Peak @ 15.2 MHz [mV/sqrt Hz] Peak @ 30.4 MHz [dBm] Peak @ 30.4 MHz [mV/sqrt Hz]
5.2 0.6 -63.18 0.28 -12.53 0.097 -28.20 0.0159
11.7 1.8 -60.77 0.37 -4.57 0.24 -21.00 0.0364
15.2 15.2 -60.20 0.40 -0.40 0.39 -18.55 0.0482
20.8 20.8 -58.81 0.47 1.52 0.49 -14.88 0.0736
25.5 25.5 -58.14 0.51 1.34 0.48 -12.73 0.0943
 

 

to convert from dBm to V/sqrt[Hz] we used sn_v = sqrt(10^(sn_dB/10)*1e-3*R/RBW) with R = 50 Ohm and RBW = 300 kHz
 
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We used the following setting :

RBW: 300kHz.  VBW: 10kHz

Average: 10

Internal amplifier: OFF

Spectrum analyzer attenuator: 10 dB

When measuring the peak hight we used different attenuation levels (20/30  dB) to avoid saturation

Images attached to this report
2111_20200710092429_107tot.png 2111_20200710092503_zoom.png 2111_20200710092508_pick.png 2111_20200710094205_shotnoise.png