In this paper, a low phase noise 28/38GHz dual-band cascaded PLL is designed with a dual-mode voltage controlled oscillator (VCO). The cascaded PLL consists of two stages, where the 1st stage is a charge-pump PLL and the 2nd stage is a sub-sampling PLL. A quadrature dual-mode VCO is designed that is switchable in two frequency bands of 27.5-28.35GHz and 37-40GHz. The circuit is designed using a 130nm BiCMOS technology. The simulated phase noise is -110 and -115 dBc/Hz at 1MHz offset at 28GHz and 38GHz respectively.