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Lena/Oct 2016: Difference between revisions
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We can make DACs out of the digital FPGA pins with PWM http://www.allaboutcircuits.com/technical-articles/turn-your-pwm-into-a-dac/
== 10/20/2016 ==
=== Testing the new differential polarimeter ===
inally completely debugged the differential polarimeter v0.1. Prototyping in the electronics shop turned out to be a bad decision because of the low board quality. Through-hole pads kept falling off, and most of the vias were busted.
The device itself performs as good as the SRS current preamplifier. We took several magnetic field measurements with both current amplifiers:
==== SR570 ====
[[File:MagRun2PSN 2016.10.20 R02N000102 Chan1Y.png|300px]]
==== Differential polarimeter v0.1 ====
[[File:MagRun2PSN 2016.10.20 R08N000102 Chan1Y.png]]
==== 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 9 dB.
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. The next review of the board will have grounded pins in the photodiode header to connect the cable shielding.
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