LIGO Document D1300671-v6

Homodyne Detector

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D - Drawings
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These are the Altium files for a homodyne detector design.

Some notes on the circuit and upgrade ideas:

1. The RF gain is a bit too high. The current LMH6624 amp is not unity gain stable. Swap with LT6200 and remove R46 to drop gain 10x. This has been tested in S1800638 and S1800639.
1.1: One can also lower the RF gain by adding a shunt resistor to L11/C69/C71 or replacing one of the capacitors with a resistor. This shunt can be ~500Ohms to get ~2k total transimpedance. See

2. PD1 and PD2 are not shot-noise limited. They are relatively low noise though at 20nV/rtHz. Increase the PD1/PD2 transimpedance to 10kOhms (more or less depending on intended LO strength). This is done on S1800638 and S1800639.

3. PD1 and PD2 are noise limited from the johnson noise in the 5V bias resistor ladder and the noise-gain 2 of the subtraction. Use better opamps and lower the resistances of that ladder.

4. Power handing. The OP27s actually can handle some ~15mA so this isn't a huge issue, but OPA209 can handle ~40mA. These work and do not increase noise in S1800638. They could increase noise at high power handing due to thermal load though.

5. The differential transimpedance could be higher since there is no dynamic range issue with it. It is limited by current noise of opamp. Use a super low noise FET amp such as ADA4625 (4fA/rtHz) and increase to 100kOhm or more. (Not tried yet).

6. Differential amps for the D15 do not preserve noise. Use OPA2209 or other low-noise dual amp to improve them. Perhaps also have them perform shaped-whitening.

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