A multi-channel phase detector using programmable logic devices

Recent developments in programmable logic devices have led to in-system programming capabilities for FPGA (Field Programmable Logic Array) and CPLD (Complex Programmable Logic Device) devices using inexpensive programming heads and PC-type computers. This development has major advantages for electronic equipment used in a research setting. Many of the devices are electrically erasable and re-programmable hundreds or thousands of times, allowing fast in-system circuit changes or error-correction in the field. This paper describes the successful application of a programmable logic device to a Multi-channel Phase Detector for the HHFW RF systems on NSTX. The Multi-channel Phase Detector is part of a closed-loop phase-control system designed to hold the antenna phase constant regardless of plasma reflections or antenna tuning. The Multi-channel Phase Detector Is a twelve-channel phase measuring device for the High-Harmonic Fast Wave (HHFW) antennas. It is implemented on a circuit board with a CPLD logic device, twelve analog line drivers and optical inputs. Multiple styles of phase detectors have been implemented within the CPLD and their characteristics studied for application to the NSTX HHFW system. Results are shown for a closed-loop phase test using the Multi-channel Phase Detector.