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Entry  Thu Feb 24 01:29:09 2011, Zach, Electronics, GYRO, RFPD #01 TF rfpd_vs_liso_log.png
    Reply  Thu Feb 24 03:54:29 2011, Frank, Electronics, GYRO, RFPD #01 TF 
Message ID: 1324     Entry time: Thu Feb 24 03:54:29 2011     In reply to: 1323
Author: Frank 
Type: Electronics 
Category: GYRO 
Subject: RFPD #01 TF 

Nice work!

According to your schematic the bias is 5V. If that is the case the capacitance is usually much less than 150pF!
If you check the typical graph in the datasheet the capacitance is ~120pF@5V.
According to the data i measured i expect it to be below 120pF @5V.
Old LIGO diodes had about 105pF@ 7V. The diode you are using is one of those old batches.
If you like we can measure it tomorrow with my pulsing setup. Doesn't take longer than 5 minutes including setting everything up and getting the data


 I chose the "final" component values for the first RFPD as follows. Consult the schematic here.

  • AC coupling
    • L6 = 6.8 uH
  • 66 MHz notch
    • L2 = 1 uH
    • C10 = 5.8 pF (1.5 - 15 pF tunable)
  • 33 MHz readout
    • C2 = 24 pF
    • C4 = 7 pF (1.5 - 15 pF tunable)
    • L3 = 750 nH

I have taken a transfer function with the Jenne Laser setup and the result is reasonable but not exactly what we expect. Below is a comparison of the measured transfer function with the prediction from LISO. To get these in a comparable form, I took the raw output of the spectrum analyzer (the ratio of the responses of the DUT and the New Focus 1611 reference PD), multiplied by the flat AC transimpedance of the reference PD (700 V/A), and subtracted the 10 dB of gain from the Cougar (AP1053).


Some notes:

  • As far as the readout and 2f rejection are concerned, the two transfer functions match up fairly well with the exception of a discrepancy in Q.
  • Though the two phases show the same basic behavior, there is a large overall shift across the measurement band. I have no idea why this is.
  • The most staggering difference is that the peak from the AC coupling resonance is much higher in frequency for the measured data. I have been using C = 200 pF for the diode in simulations, while Perkin-Elmer claims that this should be < 150 pF when biased as we have it, but even decreasing it to this level doesn't account for the higher measured frequency.

I have a suspicion that just subtracting 10 dB does not accurately negate the effect of having the Cougar on the end. Though it says nothing about it on the datasheet, I have heard that they have some sort of internal AC coupling, and so this odd behavior could be explained by that somehow. I think I will take the transfer function again, this time using a probe to sense before the Cougar instead of after it, and hopefully then it will conform with the model. In the end, all we care is that it behaves as expected near where we expect to have appreciable input signals (e.g. 33 MHz and 66 MHz), but it;s nice to be able to say that we know what's going on everywhere.

Another thing I noticed on the Cougar datasheet is that the noise figure goes up pretty quickly as you go below ~50 MHz. Using naked-eye extrapolation, I estimate that it is about 3.7 dB @ 33 MHz. I have taken a noise spectrum of the PD, but I am not comfortable enough with the TF calibration to trust the analysis yet. Stay tuned.


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