Reports until 11:45, Thursday 11 September 2014
H1 ISC
kiwamu.izumi@LIGO.ORG - posted 11:45, Thursday 11 September 2014 - last comment - 19:16, Thursday 18 September 2014(13881)
ASAIR photo diode chain check; ASAIR_A has a smaller transimpedance

I have been checking the whole ASAIR_A and _B electronics chain all the way from the diodes to the ADCs.

In summary:

Note that I used the old calibration coefficient for estimating the amount of amplitude modulation in the AM laser (see alog 9630).

 


ASAIR_A_RF45 (S1300523)

Remarks:

The transimpedance seems smaller by a factor of two than that of the test sheet at 45.5 MHz. All the signal chain after the PD looks healthy.

According to the test sheet from Caltech (S1300523), the transimpedance of the high-rf output should be 803 Ohm. However, it seems that the AM signal I obtained was smaller than the expected by a factor of two. If I we blame the transimpedance for this discrepancy, the transimpedance is smaller by a factor of two.


ASAIR_B_RF18 (S1200242)

Remarks:

The signal chain looks OK. The demodulated signal at the ADC is bigger than the expected by 12%. Good enough.

The demod board has a special one where it has a diplexer upfront. According to the test sheet (S1001003), the conversion gain of the whole demodulation box for 18 MHz is 13.8.


ASAIR_B_RF90 (S1200242)

Remarks:

The signal chain looks OK. The demodulated signal at the ADC was smaller than the expected by 13%. Good enough.

According to the test sheet (S1001003), the conversion gain of the whole demodulation box for 91 MHz is 11.4.


Transimpedance measurement of resonant-type RFPDs

To investigate the too-low-transimpedance in REFL_A(S1300523). I measured the transimpedance of S1300523 and S1300526 (whic is a spare, ) to make a comparison. Of course this measurement relies on the AM laser calibration. I believe that the calibration of the AM laser calibration is better than a factor of two and therefore I claim that the transimpedance is actually lower than 803 Ohm as measured.

I used HP4395A and the AM laser. The calibration of the data was done such that the transimpedance of S1300523 becomes 403 Ohm at 45.50298 MHz. The same calibration coefficient was applied on S1300526. The plot below shows the results. The vertical and hrizontal line indicates 45.5 MHz and 403 Ohm respectively.

From this measurement, I conlude that S1300523 is behaving fine. Also the two RFPDs consistently showed lower transimpedance gain by roughly a factor of two than that reported in the test sheets at 45.50 MHz. A possible explanation I can thinkg of at this point is that the test sheets were somehow consistently overetimsted the transimpedance gains.

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kiwamu.izumi@LIGO.ORG - 19:16, Thursday 18 September 2014 (14025)

This is a follow up of the RFPD calibration.

Since we measured the transimpedance gain of ASAIR_A_RF45 to be lower by a factor of two than that of the test sheet, we were wondering if the AM laser calibrator was still accurate. So today I checked the response of an already-calibrated RFPD to see if the calibration of the AM laser is still right.

  • As a result, I conclude that the AM calibrator is still accurate and the calibration did not change within a uncertainty of roughly 20% or so.
  • Therefore, the transimpedance gain of ASAIR_A_RF45 must be actually low as measured.

 


(Measurement on an already-calibrated RFPD; REFLAIR_A_RF45)

See the previous measuerment on alog 9630. The below are the numbers I newly obtained today.

  • Drive = 0.1 Vp-p at 45.508 MHz
  • Measured RF signal at the SMA jack = 162 mVpp at 45.508 MHz
    • This is lower than the past measurement by 17%. Not too bad.
  • Measured Imon = 880 mVpp
    • This is consistent with the past value shown in alog 9630.
  • ADC counts of the beatnote at 46 Hz = 2886 cnts p-p
    • This is also consistent with the past measurement in alog 9630.
  • DC value measured at the BNC jack = 125 mV.
    • This is higher than the previous measurement by roughly 15 %.