A new phase-feedback digital oscillator for HHFW transmitters

Traditional variable-phase, multiple-output oscillators have been constructed using phase-locked loops synched to a master oscillator. Phase control is achieved by introducing a DC offset into the VCO, thus forcing the loop to compensate by shifting its phase. Such systems have limited phase shift range and often have transient locking problems leading to loss of phase control. A new variable-phase, multi-channel oscillator has been developed that directly synthesizes a variable phase output using techniques borrowed from direct digital synthesis. The oscillators are under control of a simple digital signal processor (DSP) that implements a phase feedback system. The present system achieves simultaneous real-time phase control of six HHFW transmitters following a pre-programmed phase profile. Future plans include real-time reaction to plasma parameters. The sample rate of the system is considerably faster than commercial "canned" solutions based on rackmount computers and graphically-programmed processes. The new oscillator is a component of a phase-feedback system for the HHFW transmitters on NSTX.