Keysight Technologies N5511A, User Manual

Page: 64 / 369

64 N5511A Phase Noise Test System User’s Guide
Absolute Measurement Fundamentals
What Sets the Measurement Noise Floor?
Figure 5-7 shows the relationship between the R (signal) input level and the
system noise floor.
Figure 5-7 Relationship between the R input level and system noise floor
The Noise Level of the Reference Source
Unless it is below the system’s noise floor, the noise level of the source you are
using as the reference source sets the noise floor for the measurement. When
you set up your measurement, you want to use a reference source with a noise
level that is at or below the level of the source you are going to measure.
Figure 5-8 demonstrates that as the noise level of the reference source
approaches the noise level of the DUT, the level measured by the System
(which is the sum of all noise sources affecting the system) is increased above
the actual noise level of the DUT.
Figure 5-8 Reference source noise approaches DUT noise