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

In summary:

• ASAIR_A has a smaller transimpedance gain at 45.5 MHz than that of the test document.
• ASAIR_B looks healthy.

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.

• DC laser power = 120 mV / 100 Ohm / 0.67 A/W = 1.79 mW
• Expected AM signal when driven by 0.1 Vdrivepp = 5.13 mW x (1.79 mW / 1 mW) x 0.1 Vpp x 0.67 A/W x 803 Ohm = 494.0 mVpp
• Measured AM signal at the SMA jack of the RFPD enclosure = 248 mVpp
  • This is smaller by a factor of two.
  • If we blame the transimpedance, it must be 403 Ohm rather than 803 Ohm.
• Hereafter, I ignore the PD response and use the 248 mVpp signal to check the electronics chain in the downstream.
• Expected IF signal at I-mon = 248 mVpp x 10.9 x 1/2 (single-ended) = 1.35 Vpp
• Measured IF signal at I-mon = 1.42 Vpp
  • good agreement with a small discrepancy of 5%.
• Expected ADC counts = 248 mVpp x 10.9 x 2^16 counts /40 V = 4428 counts pp
• Measured ADC counts = 4060 counts pp
  • This is within 10% deviation. The signal chain after the PD is healthy.

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.

• DC laser power = 823 mV / 2k Ohm / 0.4 A/W = 1.03 mW
• Expected AM signal when driven by 0.1 Vdrivepp = 5.13 mW x (1.03 mW / 1 mW) x 0.1 Vpp x 0.4 A/W x 1.6k Ohm (see the plot from alog 9719) = 338.0 mVpp
• Expected IF signal at I-mon = 338 mVpp x 13.8 x 1/2 (single-ended) = 2.33 Vpp
• Measured IF signal at I-mon = 2.8 Vpp
  • this is too big by 24 %.
• Expected ADC counts = 338 mVpp x 13.8 x 2^16 counts /40 V = 7642 counts pp
• Measured ADC counts = 8577 counts pp
  • This is bigger by 12%. Good enough.

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.

• DC laser power = 1.03 mW
• Expected AM signal when driven by 0.1 Vdrivepp = 5.13 mW x (1.03 mW / 1 mW) x 0.1 Vpp x 0.4 A/W x 1.2k Ohm (see the plot from alog 9719)= 254 mVpp
• Expected IF signal at I-mon = 254 mVpp x 11.4 x 1/2 (single-ended) = 1.45 Vpp
• Measured IF signal at I-mon = 1.32 Vpp
  • this is 9% smaller.
• Expected ADC counts = 254 mVpp x 11.4 x 2^16 counts /40 V = 4744 counts pp
• Measured ADC counts = 4210 counts pp
  • This is smaller by 13%. Good enough.

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.