Frequency linearization of voltage controlled oscillator (VCO) for 2 µs frequency modulated continuous wave (FMCW) reflectometer

Frequency modulated continuous wave reflectometers have been widely used to measure plasma density profiles in many magnetic fusion devices. The frequency modulation (FM) time of the KSTAR reflectometer was 20 µs, that is, the FM rate was 50 kHz. However, the edge density of the KSTAR tokamak fluctuates typically over the frequency range of 20-50 kHz in the ELMy H-mode plasmas. Therefore, the density profile changes significantly during the FM time, causing significant distortion in the density profile measurements. The FM rate must be increased at least ten times faster than the density fluctuation frequency. A new voltage controlled oscillator (VCO) driver is designed based on the AD8067 operational amplifier to achieve a FM time of 2 µs. The VCO output frequency is linearly modulated by predistorting the tuning voltage of VCO using a 400 MSamples/s arbitrary waveform generator. The resulting output frequency of the VCO is measured by mixing the VCO output with a fixed frequency synthesizer signal and measuring the mixer intermediate frequency using a 2.5 GSamples/s digitizer. The tuning voltage is adjusted to minimize the amount by which the measured frequency deviates from the linearly modulated frequency. A simple linear adjustment proportional to the deviation does not effectively suppress the nonlinearity in FM. The response time of the VCO driver and the VCO input interface should be taken into account by solving the entire circuit equation. In this paper, the developed VCO driver circuit and the frequency linearization method are described.