- Work this week
- This week I debugged several issues with our PCB. One of these is that the receivers do not work. This is because our comparators are not behaving as specced. They are specified as low as 20 mV overdrive with no minimum spec, but we experimentally found that they only work starting at 50 mV overdrive. Our TIAs do not create a large enough voltage swing for this, so they do not output digital values. To fix this, I ordered 2 new comparators that have several specs well below this, as low as 5 mV overdrive. They still do not have a minimum spec but appear to be better. I also made sure to get chips from different manufacturers in case this is a manufacturer issue. These will arrive next week for testing.
- Another board issue detected this week was laser power. The lasers have visibly inconsistent power output, mostly weaker than expected. I did some experiments and determined that this is because the threshold voltage of our laser diodes is not consistent. To fix this, I connected lasers to external power and used experimental and datasheet values combined to calculate the feedback current that equals our expected optical powers, 2.5 mW and 0.5 mW. I then tested how much input current would create that feedback current for individual lasers. I did this for both lasers on one board and replaced the resistors to equal this power, which makes the board now have the correct power. I now know each laser needs this calibration individually, so will need to do it on the other board too.
- Another issue detected this week was power related. I managed to program the FTDI chip on one board to the correct communication and power settings, but I discovered that the board only successfully draws 9V from a laptop charger, not from my laptop. This is despite Apple customer support explicitly stating that this would work. It seems Macs do not support being a USB PD source above 5V. To fix this, we will need to splice wires or find some device capable of high voltage USB PD and data combined.
- I also received new USB connectors this week, and attached one to our second board. This was much faster and easier than the first, since I had to figure out a method the first time. This time, it worked on the first try with no damage to anything.
- I tested various methods to see what the best way to test a single board is, since we need both a transmitter and receiver to test anything. The best method I found was using a small reflective piece of metal to reflect the unfocused laser back towards the receiver on the same board. This is strong enough to trigger the photodiode and TIA, so this is how I have been testing the receiver.
- Schedule
- Due to the specified board issues, I did not 3d print our lens housing this week. This will be pushed once again to next week. I have made a plan for exactly what shape needs to be made, but do not have CAD for it yet.
- Deliverables
- Next week, I plan to deliver at least one housing for the lens. However, the important deliverables are fixing the circuits. I plan to have fixed the laser power on the second board and have fixed the receivers to properly output digital signals using one of the new comparators. Additionally, I want to have settled on a solution for the USB power, even if it is not implemented yet.
Anju Ito, Roger Lacson, and KJ Newman