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I pulled the beatbox from the 1X2 rack so that I could try to hack in some output whitening filters. These are shamefully absent because of my mis-manufacturing of the power on the board.
Right now we're just using the MON output. The MON output buffer (U10) is the only chip in the output section that's stuffed:
The power problem is that all the AD829s were drawn with their power lines reversed. We fixed this by flipping the +15 and -15 power planes and not stuffing the differential output drivers (AD8672).
It's possible to hack in some resistors/capacitors around U10 to get us some filtering there. It's also possible to just stuff U9, which is where the whitening is supposed to be, then just jump it's output over to the MON output jack. That might be the cleanest solution, with the least amount of hacking on the board.
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I modified the beatbox according to this plan. I stuffed the whitening filter stage (U9) as indicated in the schematic (I left out the C26 compensation cap which, according to the AD829 datasheet, is not actually needed for our application). I also didn't have any 301 ohm resistors so I stuffed R18 with 332 ohm, which I think should be fine.
Instead of messing with the working monitor output that we have in place, I stuffed the J5 SMA connector and wired U9 output to it in a single-ended fashion (ie. I grounded the shield pins of J5 to the board since we're not driving it differentially). I then connected J5 to the I/Q MON outputs on the front panel. If there's a problem we can just rewire those back to the J4 MON outputs and recover exactly where we were last week.
It all checks out: 0 dB of gain at DC, 1 Hz zero, 10 Hz pole, with 20 dB of gain at high frequencies.
I installed it back in the rack, and reconnected X/Y ARM ALS beatnote inputs and the delay lines. The I/Q outputs are now connected directly to the DAQ without going through any SR560s (so we recover four SR560s). |