What differentiates Absolute phase noise measurements from residual phase noise measurements?
Absolute phase noise has the units of dBc/Hz. It is a measure of power spectral density of noise normalized to a 1Hz bandwidth referenced to a carrier power. An example of this is the noise of a oscillator at 10KHz offset from the carrier. Specifications for absolute phase noise indicate the spectral image you should be able to acheive coming out of the device if you provide a sufficiently clean reference input.
Residual phase noise could also be called Additive Phase Noise. Additive phase noise is the amount of phase noise that is attributable to the device or subsystem being measured. The phase noise of any external oscillators or clock sources is subtracted. This makes it possible to predict the degree to which the device impacts the total system phase noise when used in conjunction with the various oscillators and clock sources, each of which contributes its own phase noise to the total. In many cases, the phase noise of one element dominates the system phase noise. When there are multiple contributors to phase noise, the total is the square root of the sum of squares of the individual contributors.
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