By combining the output of two synthesizers that are in-phase, a theoretical 3-dB phase noise benefit is possible due to the output power being 6-dB higher while the noise power is only 3-dB higher.

To measure the combined phase noise, some modification to the EVM is needed. See Section 7 for details.

In Figure 3-5, the blue trace shown the phase noise of individual LMX2594 device while the black trace respresented the phase noise of the combined signal.

Figure 3-5 Combined Phase Noise