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Entry  Tue Nov 17 21:10:35 2009, rana, Summary, Electronics, Busby Low Noise Box: Photos and Upgrades IMG_0217.JPGbusby-noise.png
    Reply  Wed Nov 18 00:38:33 2009, rana, Summary, Electronics, Voltage Noise of the SR560's OUTPUTs (the back panel) sr560.png
Message ID: 2286     Entry time: Tue Nov 17 21:10:35 2009     Reply to this: 2288
Author: rana 
Type: Summary 
Category: Electronics 
Subject: Busby Low Noise Box: Photos and Upgrades 

IMG_0217.JPG

It looked like the Busby Low Noise Box had too much low frequency noise and so I upgraded it. Here is a photo of the inside - I have changed out the 0.8 uF AC coupling cap with a big, white, 20 uF one I found on Rob's desk.

The Busby Box is still working well. The 9V batteries have only run down to 7.8V. The original designer also put a spare AD743 (ultra low current FET amp) and a OP27 (best for ~kOhm source impedances) in there.

Here's the noise after the fix. There's no change in the DC noise, but the AC noise is much lower than before:

busby-noise.png

I think that the AC coupled noise is higher because we are seeing the current noise of the opamp. In the DC coupled case, the impedance to ground from the input pins of the opamp is very low and so the current noise is irrelevant.

The change I implemented, puts in a corner frequency of fc = 1/2/pi/R/C = 1/2/pi/10e3/20e-6 = 0.8 Hz.

Overall, the box is pretty good. Not great in terms of current noise and so it misses getting an A+. But its easily a solid A-.

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