Initial measurment of PD TF

I realize I never really measured the signal transfer function for each of these Photo Detectors (PD).  This post summarizes measurements of the optical to electrical signal output signal transfer function. Here I used the Jenne rig at the 40m and took a transfer function using an Aglent 4395A from 30 kHz to 5 MHz.  

I've labeled the two PD in the homodyne as unit A and unit B.  Below (first attached)  is the raw TF as measured by the Aglient taking the ratio of the detector to the reference NF1611 detector:

Fixing differences between output impedance unit A and B

After looking a bit at the circuit I realized that I had added 100 Ω in series in detector B (to limit current draw when driving 50 Ω loads).  This had not been added to the detector A, which means that with more power than 300 µW on this detector the op amp would be drawing more than 30 mA when driving a 50 Ω.  This shouldn't have been an issue when driving the summing circuit, but is good practice.  

I added 100 Ω in series with op27 output (thin film 1206 size) in detector A to match the output to that of detector B.  I remeasured the TF and get a much better match between the paths.

Calibrating TIA Measurment

The following is used to calibrate the  transfer function (See PSL:2247):

where Z_AC,PD is Calculated RF Transimpedance, Z _AC,Ref is the known RF transimpedance of the reference photodiode, 𝛿ϕ is arbitrary phase delay due to light and cable length and R_PC  is the photocurrent ratio at DC of reference PD to RFPD under test given by

For these measurements the calibration factors were as follows:

Unit A TF (after adding 100 Ω series​ output): measured NF1611 DC voltage of 780 mV (@ 1 MΩ impedance), PD DC output voltage was -1.20 V (@ 1 MΩ impedance).  Here the photodetector has transresistance is 6.8 kΩ. The NF1611 detectors have a DC path gain of 10 kΩ and an AC path gain of 700 Ω. From this the R_PC is 0.4496.  Here I ignore the overall phase (at ~1 MHz as the pi phase shift length is 150 m, so negligible).  

Unit B TF: measured NF1611 DC voltage of 791 mV (@ 1 MΩ impedance), PD DC output voltage was -1.18 V (@ 1 MΩ impedance).  Here the photodetector has transresistance is 6.8 kΩ. The NF1611 detectors have a DC path gain of 10 kΩ and an AC path gain of 700 Ω. From this the R_PC is 0.4558.  Here I ignore the overall phase (at ~1 MHz as the pi phase shift length is 150 m, so negligible).  

Results of calibrated transimpedance measurements are plotted below*. I also added the LISO estimated noise curve.  The 3 dB of unit A and B were 1.93 MHz and 1.72 MHz respectively.  There is some discrepancy with the LISO model here, some of this might be artifacts in the reference detector and some is likely dirt effects in the proto board circuit that I'm not going to debug for now.  The biggest discrepancy between the two detectors starts around 900 kHz.  When I looked again at the detectors I realized I had used a Wima foil cap in detector B for the bias LPF. It should be fine for these frequencies, but I switched the foil cap out and replaced it with a 1 µF + 100 nF ceramic cap to match the unit A.  I haven't gotten a chance to remeasure this since as Gautam is still using the Jenne rig to do some stuff. I'll remeasure later, we should then find the two responses match.

*I didn't do the many IRIS measurement​ uncertainty analysis, its not necessary at this stage.