Voltage Controlled Oscillator (VCO) is one of the most sensitive analog circuits that suffer from the harmful effects of noise which is generated by the switching of digital circuits residing on a common substrate. This paper studies the effect of substrate noise on the performance of a 5.2 GHz LC-Tank VCO in a lightly doped substrate. It will be shown that the noise can couple into different components in the VCO, leading to spur generation at the side-bands of the oscillation frequency which will cause phase noise degradation of LC-Tank VCO. This work uses the results of substrate modeling in high frequency region. It supposes the substrate not as a purely resistive network but a resistive-capacitive network to account for the behavior of the substrate in high frequency and RF regimes.
