LAUSR

A voltage controlled oscillator (VCO) is a key component in a frequency modulated continuous wave reflectometer. The linear frequency modulation (LFM) of the VCO has been implemented by predistorting the tuning voltage waveform or by feedback control of the output frequency based on a phase locked loop. The latter has a better linearity but the frequency sweep rate is limited. The former has a relatively poor linearity but has the capability of a fast frequency sweep. In this paper, the predistortion technique is sophisticated to improve the linearity in applications of very fast LFM. The voltage waveform predistorted based on the static characteristics of VCO does not eliminate the nonlinearity completely because VCO’s behavior gets deviated from the static characteristics as the frequency sweep rate increases. The dynamic characteristics of the VCO are obtained by modeling the equivalent circuit inside the VCO as a low pass filter and simulating the circuit response numerically. The calculated voltage waveform is driven by an arbitrary waveform generator, and the output frequency of the VCO is experimentally measured to demonstrate the accuracy of the LFM. The techniques for frequency measurement and frequency linearization are described in detail and the results are presented.A voltage controlled oscillator (VCO) is a key component in a frequency modulated continuous wave reflectometer. The linear frequency modulation (LFM) of the VCO has been implemented by predistorting the tuning voltage waveform or by feedback control of the output frequency based on a phase locked loop. The latter has a better linearity but the frequency sweep rate is limited. The former has a relatively poor linearity but has the capability of a fast frequency sweep. In this paper, the predistortion technique is sophisticated to improve the linearity in applications of very fast LFM. The voltage waveform predistorted based on the static characteristics of VCO does not eliminate the nonlinearity completely because VCO’s behavior gets deviated from the static characteristics as the frequency sweep rate increases. The dynamic characteristics of the VCO are obtained by modeling the equivalent circuit inside the VCO as a low pass filter and simulating the circuit response numerically. The calculated voltage w...