The cross correlation technique for noise floor reduction is used widely in the SSA-X. The basic theory of operation for cross-correlation calculation is traditionally well-known as shown in the following figure. The SSA-X implemented this principle by a highly sophisticated manner and enabled its real time operation on display.
The cross correlation technique uses two signal-channel measurement systems. Only the coherent noise present in both channels will average to a finite value. The time average of the incoherent noise will approach zero as M-1/2, where M is the number of average. The measured phase noise of the system is represented as the equation shown in the figure.
The diagram in the effect of Cross Correlation is show in the following figure.
The expression (n0+n1) means the noise (source noise n0 and internal system noise n1) of the channel 1. The expression (n0+n2) means the noise　(source noise n0 and internal system noise n2) of the channel 2. The mark "*" means complex conjugate and all noise parameters are random complex variables. The red arrows show noise phasors of the source noise. The blue arrows show internal noise phasors and assuming n1 and n2 are uncorrelated. After summing up all noise phasors, the internal noise phasors cancel out each other and only the source noise phasors remain.