Serial fiber optic links for the lower hybrid current drive control system on Alcator C-Mod

A lower hybrid current drive (LHCD) system operating at 4.6 GHz is being implemented on Alcator C-Mod. It will use twelve 250 kW klystrons driving a 96-element launcher to control and sustain the toroidal current profile. Synchronized digital controllers are required to monitor and adjust the output amplitude and relative phase for each klystron. A vector modulator at the input to the klystron preamp serves as the control actuator. An in-phase and quadrature (I-Q) detector at the output serves as the feedback sensor. The LHCD system will use serial fiber optic links (SFOLs) for the control and feedback signals. The SFOL requirements follow directly from the specifications of the other hardware. The real-time control computer provides ±10 V analog control outputs, and expects ±10 V analog feedback inputs. The vector modulator requires two 12-bit parallel digital inputs for control. The I-Q detector provides ±250 mV analog outputs for feedback. Electrical isolation and noise immunity requirements dictate the choice of fiber optics. The configuration of the SFOL´s (with two analog inputs at the XMTR, and two analog and two parallel 12-bit digital outputs at the RCVR), matches the system I/O requirements. The real-time control loop will operate at 9 kilosamples per second (ksps), The SFOL´s need to run at 5x over-sampling, or 45 ksps. The XMTR will use a two MAX176 serial-output 12-bit ADC´s. These chips are capable of 250 ksps; fast enough to allow us to multiplex both input channels (I and Q) as well as synchronization and clock recovery information onto a single fiber. The RCVR will use two MAX531 serial-input 12-bit DAC´s for the analog outputs and two 12-bit registers for the digital outputs. Both XMTR and RCVR modules will use the 74HCT4046 phase-lock loop (PLL) to generate local clocks and synch signals, and the ATF1508 complex programmable logic device (CPLD) for counters, registers, and glue logic.