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Lena/Oct 2016: Difference between revisions
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→Design problems
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==== Design problems ====
There were several issues with the polarimeter design. The biggest issue is that it picks up and amplifies the heater noise (150kHz). The noise is picked up by the cable that connects the photodiodes to the amplifier. The photoamplifier has a single-pole the filter on the input gain that rolls off at 20kHz. 150kHz signal is only 3 octaves away from 20kHz, so the amplitude attenuation is only
The photodiodes cable has shielding, which works much better when it's connected to the amplifier's ground. When the shielding is connected to the ground, the 150kHz component amplitude drops by an additional factor of 5 (14 dB). The next review of the board will have grounded pins in the photodiode header to connect the cable shielding.
There will be additional 2-pole filters at the circuit output to deal with this problem.
Expected attenuation with fc=20kHz (shielding + input filter + output filter) is 14 + 3 * (6 + 12) = 68 dB
Expected attenuation with fc=10kHz (shielding + input filter + output filter) is 14 + 4 * (6 + 12) = 86 dB
Low-frequency gain of the differential polarimeter seems to be larger than the of SR570. It's unclear if it's a feature or a bug or a poor connection.
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