noise in cmos vco

The ideal VCO output is an impulse in frequency domain so the mixing is just a convolution of RF with LO does not affect the spectrum of RF signal. However, the actual output spectrum is noisy and the received noise power is raised. The noise in VCO can be divided to amplitude variation and phase noise. Amplitude variation is a common mode noise so that differential circuit can handle it. But, phase noise is differential mode noise and nothing helps for this. Therefore, a better VCO design and better passive component such as high Q. The phase noise is seriously degrade the performance in direct conversion receiver because the phase noise will up-convert the IF data to RF signal without any filtering. So, the phase noise would bring the receiver in a tough situration at DC. As mentioned in previous section, the system is able to avoid the DC component by high pass filtering.