- This and last weekend we were busy assembling and bringing up the boards
- Assembly and hardware debugging was largely a collaborative effort. My personal contributions were:
- Placing most components (>100) on the imaging boards and doing reflow
- Procuring a reflow oven
- Micro-soldering bodges on the STM32
- Assisting Claire & Sid with debugging the high-speed clock generator and the ethernet peripheral
- Assisting Claire & Sid with x-ray imaging the ethernet circuit to find a loose contact
- Assembling a single-channel version of the LED board (aluminum-core).
- Testing single-channel LED board in DC mode, and testing the bandpass filter with the LEDs
- X-raying the LED board to make sure that the BGA package of the modulation shunt IC is reflowed correctly (picture below)
- Used a differential probe to verify the waveform of the 24 MHz LVDS clock going from the imaging board to the LED board
LED Board Assembly X-Ray
We were concerned that the tiny BGA package of EPC21603 shunt won’t solder well, but it turned out okay! The package can be seen as 6 dots (balls) near the center of the image. One can see the LVDS modulation clock line going from the connector on the bottom to the IC. For the LVDS traces are 0.1mm wide, and the balls are 0.5mm apart.
I assembled the board by hand under a microscope.
Testing & Validation
My primary focus will be validating the LED board and the CC driver.
- I have already done some LED board testing & validation:
- I have tested the LEDs in DC mode, and made sure that the wavelength of the bandpass matches that of the LEDs
- I also determined a maximum LED current for safe operation with no heatsink: 0.7 A through two LEDs results in the LED temperature of 105C as measured by a thermal camera.
- To validate the LED board individually, I will
- use a clock gen and an external current supply to make sure the modulation circuit works
- I will use an oscilloscope to check the voltage on the high side of the LEDs and make sure that the snubber circuit is working correctly
- The snubber circuit is responsible for absorbing high-frequency (100MHz+) ringing when the modulation shunt is opened and closed.
- I may need to tune the snubber zener diode threshold
- The snubber circuit is responsible for absorbing high-frequency (100MHz+) ringing when the modulation shunt is opened and closed.
- I will use a photodiode to evaluate the modulation of the light
- At this point, I should be able to measure the LED bandwidth at different frequencies (By how much does the peak LED brightness decrease when it’s modulated vs DC?)
- To validate the CC driver individually, I will
- use an external PSU and an adjustable electronic load to make sure that the current source can supply the nominal current of 1A
- use a thermal camera to make sure that no components overheat
- use an oscilloscope to make sure that there are no current spikes during on/off transients