A broadband frequency ramp generator for very fast network analysis based on a fractional-N phase locked loop

Abstract

In this paper, we propose a frequency ramp generator for very fast network analysis in the frequency range from 0.4 GHz to 6 GHz. Highly linear analog frequency ramps are synthesized at high frequencies (8.4 GHz to 14 GHz), using a wideband voltage-controlled oscillator (VCO) stabilized by a fractional-N phase locked loop (PLL), and are mixed down to the desired frequency band using a second PLL-stabilized VCO at a constant frequency of 14.4 GHz. In order to achieve a wideband PLL-stabilization, the loop gain variation is reduced by inserting a voltage dependent damping network between the loop filter and the wideband VCO. This leads to an almost constant transient and phase noise performance in the complete frequency range. The overall phase noise at the output of the ramp generator is below −99dBc/Hz at offset frequencies above 10 kHz. The proposed ramp generator allows for fast and accurate network analysis which is demonstrated by means of measurements with a homodyne prototype system. The sweep time of the system (0.5 ms) is significantly shorter compared to previously published ramp based systems and commercial network analyzers, and allows monitoring of rapidly changing measurement scenarios.