40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
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Entry  Wed Jun 26 21:52:55 2013, Charles, Update, ISS, CTN Servo Prototype Characterization TF_Mag-CTNServo_v2_Prototype.pngTF_Mag-CTNServo_v2_Prototype_AD829s.png
    Reply  Thu Jun 27 18:24:25 2013, Charles, Update, ISS, CTN Servo Prototype Characterization - Done Correctly TF-CTNServo_v2_Prototype-Individual_Stages.pngTF-CTNServo_v2_Prototype-Calc_vs_Meas.pngTF-CTNServo_v2_Prototype-Individual_Stages.figTF-CTNServo_v2_Prototype-Calc_vs_Meas.fig
Message ID: 8759     Entry time: Wed Jun 26 21:52:55 2013     Reply to this: 8771
Author: Charles 
Type: Update 
Category: ISS 
Subject: CTN Servo Prototype Characterization 

Following the circuit design in elog 8748, I constructed a prototype for the servo portion of the ISS (not including the differential amp) to be used in the CTN experiment. The device was built on a breadboard and its transfer function was measured with the Swept Sine measurement group of an SR785. For various excitation amplitudes, the transfer function (TF) was not consistent.

TF_Mag-CTNServo_v2_Prototype.png

Recall the ideal transfer function for this particular servo and consider the following comparisons.

  • The unity gain frequency is consistent, and the measured TFs all exhibit some amount of 1/f behavior up to this point, but there is no zero around f~10^3 and individual low-frequency poles/zeros are not visible.
  • For each of the inputs, there is a feature that is not exhibited in the ideal TF. We see a large drop in gain a little past 10^3 Hz for a 100mV input, just past 10^2 Hz for a 10 mV input and around 10^1 Hz for a 1 mV input.
  • The ideal TF also goes as 1/f for f < 10 Hz, so I believe the low-frequency behavior of each of the above transfer functions is simply a physical limitation of the breadboard or the SR785, although I don't think this is caused by the circuit elements themselves. I used OP27 op-amps in the prototype as opposed to AD829 op-amps which are must faster and end up amplifying noise. To ensure that these op-amps were not the source of the gain limitation, I also tried using AD829 op-amps. The resulting transfer functions are shown below.
  • Both the frequency at which we see the anomalous feature and the maximum gain increase nearly proportional to the increasing input excitation amplitude.

This gain limitation is problematic for characterizing prototypes as my particular servo has very large gain at low frequencies. 

TF_Mag-CTNServo_v2_Prototype_AD829s.png

At the risk of looking too deeply into the above data,

  • It appears there is a slight change in slope around f ~ 10^3 Hz which would be consistent with the ideal TF.
  • For f > 10^3 Hz, one can easily see the TF goes as 1/f. The slope for f < 10^3 Hz is not as clear, although it obviously does not show 1/f^2 behavior as we would expect from the ideal TF.
  • We see the same gain limitation around G ~ 55 as we did with OP27 op-amps.

Unfortunately, the noise was too large for lower excitation amplitudes to be used to any effect. I'll try this again tomorrow, just as a sanity check, but otherwise I will proceed with learning Altium and drawing up schematics for this servo.

 

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