In-situ frequency calibration of frequency modulated continuous wave reflectometry.

Abstract

Frequency modulated continuous wave reflectometers have been used to measure the plasma density profiles of the KSTAR tokamak. Three reflectometers are operating in an extraordinary polarization mode in the frequency range of Q band (33-50 GHz), V band (50-75 GHz), and W band (75-110 GHz). Each full frequency band is linearly swept in 20 μs. The accuracy of the density profile measurement is dependent on how precisely the frequency is calibrated. Two new frequency measurement techniques are developed to in situ calibrate the instantaneous frequency during the sweep time of 20 μs. First the intermediate frequency (IF) of the receiver is analyzed based on wavelet transform as applying a fixed frequency signal from a synthesizer to the local oscillator (LO) port. The frequency of the transmitted microwave is simply obtained by adding or subtracting the measured IF to the given LO frequency. Once the group delay of the whole system is known, the frequency can be calibrated by measuring the IF. By measuring the reflectometer output without plasma with delay lines of different lengths and subtracting them, the frequency can be calibrated by eliminating the unknown group delay of the system except the known group delay of the delay line. These two techniques are described in detail, and the calibrated frequencies conducted by the two techniques are compared.