When combining synthesizer outputs, it is not possible to achieve the ideal 3-dB phase noise improvement across all frequency offsets. The amount of improvement is dependent on the noise coherency of the individual contributing factors. Figure 1-1 shows the breakdown of the phase noise plot into three sections. Section-A, bounded by roughly 1-kHz offset, is dominated by the system reference phase noise performance. The system reference is shared between the two synthesizers so this noise is correlated between the two devices. There is no expected improvement in this section due to the combining. Section-B is dominated by the phased lock loop (PLL) performance, but with contributions from the (correlated) system reference and (uncorrelated) VCO noise. Section-B is approximately bounded by the loop filter bandwidth. The relative contribution level of the reference noise to the PLL/VCO noise determines how much improvement is possible by combining. In general, there should be some, but not near the ideal 3 dB case. The last section-C is dominated by the VCO jitter and thermal noise. This area is primarily uncorrelated noise and should achieve near ideal 3-dB improvement.