User`s guide

Absolute Measurement Fundamentals 5
Agilent E5505A User’s Guide 111
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 74 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 73 Relationship between the R input level and system noise floor
-140 -160 -170 -180-150
Expected phase noise floor of phase detector and LNA (dBc/Hz)
f 10kHz
R Port signal level (dBm)
-15
+15
-5
+5
E5505a_r_input_sys_noise
26 Feb 04 rev 1
L Port level +15dBm
Figure 74 Reference source noise approaches DUT noise
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0.5
1.0
1.5
2.0
2.5
3.0
Amount expected DUT noise exceeds reference noise (dB)
Increase in measured noise due
to reference noise (dB)