This set of measurments turned out to be bad. The OP27 (±15V supplies) with 50 Ω series at the output + 50 Ω load impedance seemed to be saturated.
The rest of this post is a stream of conciseness.
As noted in previous post (ATF:2316) the PD unit B in the homodyne had a slightly reduced HF response, possibly due to the use of a foil cap in the PD bias supply low pass. I removed this and resmeasured.
~~Unit B TF: measured NF1611 DC voltage of 312 mV (@ 50Ω impedance), PD DC output voltage was - 566 mV (@ 50 Ω 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 xxx. Here I ignore the overall phase (at ~1 MHz as the pi phase shift length is 150 m, so negligible). ~~ This was a bad measurment, something was up with the circuit saturating way too low.
Changed 100 Ω series on output to 50 Ω to make it more strait forward to understand how it works with 50 Ω load.
~~Unit A TF: measured NF1611 DC voltage of 658 mV (@ 50Ω impedance), PD DC output voltage was 1.42 mV (@ 50 Ω impedance). Here the photodetector has transresistance of 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 0.3151. Here I ignore the overall phase (at ~1 MHz as the pi phase shift length is 150 m, so negligible).~~ This measurment could have be saturating the current output of the OP27, likely that numbers here are not right.
After examining the output of detector B it became apparent that the output was saturated for powers of more than ~20 mA laser diode drive.
~~Unit B TF: measured NF1611 DC voltage of 312 mV (@ 50Ω impedance), PD DC output voltage was -566 mV (@ 50 Ω 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 -0.3748. Here I ignore the overall phase (at ~1 MHz as the pi phase shift length is 150 m, so negligible).~~ 100 Ω (50Ω + 50 Ω) loading is too low, OP27 open loop gain and voltage swing range slew starts to sag below 150Ω loading. These results are bogus.
This time I measured with 50 Ω on the oscilloscope for apples-to-apples comparison with what the Aglient 4395A was seeing. Turns out I think I was saturating the photodetector current wise in the first round of measurments. All of the above is probably not right and should be disregarded. Also, later I switched to a AD829 as the op amp of choice so the above is not much use for characterizing the detector performance anymore. The OP27 isn't really spec'ed to operate with loads this low and only has its full band with for output loads of >2 kΩ. |