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Entry  Fri Sep 2 21:16:45 2016, rana, Update, Electronics, Satellite Amplifier 
    Reply  Mon Sep 5 21:05:06 2016, rana, Update, Electronics, Satellite Amplifier osempd.pdf
       Reply  Tue Sep 6 00:14:14 2016, gautam, Update, Electronics, Satellite Amplifier osempdComparison.pdf
          Reply  Mon Sep 19 13:25:03 2016, Teng, Update, Electronics, Satellite Amplifier Ele_measurement.pngEle_noise.png
             Reply  Mon Sep 19 13:57:21 2016, rana, Update, General, never post EPS files in the ELOG. Ever. 
Message ID: 12468     Entry time: Fri Sep 2 21:16:45 2016     Reply to this: 12470
Author: rana 
Type: Update 
Category: Electronics 
Subject: Satellite Amplifier 

In November of 2010, Valera Frolov (LLO), investigated our satellite amplifiers and made some recommendations about how to increase the SNR.

In light of the recent issues, we ought to fix up one of the spares into this state and swap it in for the ITMY's funky box.

The sat amp schematic is (D961289). It has several versions. Our spare is labeled as version D (not a choice on the DCC page).

  1. U13-17 seem superfluous. Why would we need a high current buffer to drive the slow EPICS ADCs ?
  2. The Radd resistors indicated on the B2 schematic have not been added to this board. What would be the purpose of adding them anyway?
  3. The transimpedance resistors are, in fact, 29.4k as indicated on the B1 schematic.
  4. The LT1125: V_n = 3 nV w/ f_corner = 5 Hz. I_n = 0.4 pA w/ f_corner = 100 Hz. So at 1 Hz the current noise is limiting at ~120 nV/rHz or 650 nV/rHz using the 161k that Valera recommends.
  5. Even in that case the ADC noise is higher than the opamp noise.
  6. We should be using metal film resistors instead of the thick film junk that's installed now.

Edit (Sep 6): The purpose of the Radd resistors is to lower the resistance and thus up the current through the LED. The equivalent load becomes 287 Ohms. Presumably, this in series with the LED is what gives the 25 mA stated on the schematic. This implies the LED has an effective resistance of 100 Ohms at this operating point. Why 3 resistors? To distribute the heat load. The 1206 SMD resistors are usually rated for 1/4 W. Better to replace with 287 Ohm metal film resistors rated for 1 W, if Steve can find them online.

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