Characteristics of Arbitrary Ramp Generator: A Tuning Voltage Setup for the FMCW Reflectometer

An accurate measurement of beat frequency in reflectometry enables a detailed electron density profiling of fusion plasma. An ultrawideband voltage-controlled oscillator (VCO), used as a frequency source for reflectometer, has a nonlinear frequency response to the linear tuning voltages. This leads to unintentional variations in the beat frequencies measured by the reflectometer. In this article, we present a remotely configurable custom instrument able to generate the nonlinear voltage ramps (tuning voltage) of 0-20 V with the ultrafast sweep durations of \geq 5~\mu \text{s} . Our instrument, an arbitrary ramp generator (ARG), can be used to drive the VCO to give a linear frequency response. The proposed design is developed using hardware-software codesign techniques and implemented on a system on chip (SoC). Every parameter of the voltage ramp (sweep) can be configured to match the reflectometer application requirements. Our instrument can also be interfaced to a graphical user interface (GUI) offering remote configurability. The modified and unmodified beat frequency response of the reflectometer after frequency sweep linearization using the proposed instrument is also presented. We find that for a constant distance, large variations in the beat frequencies with a range of 1200 kHz have been corrected and reduced to \approx 30 kHz.