Studying the characteristics of an oscillator with a phase noise analyzer, but I don't quite understand the difference between absolute and residual noise. Any help would be greatly appreciated
Absolute phase noise is the phase noise measured on the output signal of a one port device. Generally, when phase noise plots or tables are shown on the data sheet of an oscillator or signal generator, it is the absolute phase noise measurement data. Residual phase noise is a measure of the noise added to an input signal by a 2-port device. Some signal generators specify the residual phase noise in their data sheets. Think of the signal generator as a two port device (in this case, a frequency multiplier) where the “input port” is the external frequency reference (generally 10 MHz) and the “output port” is the RF output.
Having residual phase noise data enables users to calculate the absolute noise of their signal generator at the RF output when using an external 10 MHz signal as the reference input. ref
Absolute phase noise is the phase noise of the entire system.
Residual phase noise is the phase noise of the system excluding the contribution of the oscillator.
In measuring absolute phase noise, you can use a spectrum analyzer, for example and connect it directly to the output of the DUT. The DUT and analyzer can use separate oscillators.
In measuring the residual phase noise, the instrument and the DUT need to share the same oscillator.