The 250 kHz Metrology system is used to detect any motion of the telescope to a reference point. Currently, 12 laser channels run several meters into the earth and are reflected from the telescope mirror. Any motion is detected as a phase shift between the 16 signal channels and one reference channel. Currently, the system can detect 1/256th lambda of a 633nm laser. The data of all 16 channels is synchronously transmitted into a Linux system for further processing. The metrology data is essential for the exact position measure that occurs at the NPO.
Here is a small screen-shot of the 6 layer PCB. Most of the processing occurs through high-speed CPLDs. I had to use 4 CPLDs to accommodate all 16 metrology channels. FPGAs can handle more complex logic designs, but are more expensive and timing prediction is easier to guaranty using CPLDs. A little controller is used for fault protection and communication purposes.