User`s guide

112 Agilent E5505A User’s Guide
5 Absolute Measurement Fundamentals
Selecting a Reference
Selecting an appropriate reference source is critical when you are making a
phase noise measurement using the phase lock loop technique. The key to
selecting a reference source is to compare the noise level of the reference with
the expected noise level of the DUT. In general, the lower the reference
source’s noise level is below the expected noise level of the DUT the better.
(Keep in mind that you only need to be concerned about the reference source’s
noise level within the frequency offset range over which you plan to measure
the DUT.)
As shown by the graph in Figure 75, the further the reference source’s noise
level is below the noise level of the DUT, the less the reference source’s noise
contributes to the measurement results.
Using a Similar Device
The test system performs best when you are able to use a device similar to the
DUT as the reference source for your PLL measurement. Of course one of the
devices must be capable of being voltage tuned by the system to do this.
To select a similar device for use as the reference source, you must establish
that the noise level of the reference source device is adequate to measure your
DUT. The Three Source Comparison technique enables you to establish the
actual noise levels of three comparable devices when two devices are available
in addition to the DUT.
If only one device is available in addition to the DUT, you can perform the
Phase Noise Using a Phase Locked Loop Measurement using these two devices
and know that the noise level of each of the devices is at least as good as the
measured results. (The measured results represent the sum of the noise of
both devices.)
Figure 75 DUT noise approaches reference noise
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0.5
1.0
1.5
2.0
2.5
3.0
Difference between DUT and reference noise levels (dB)
Increase in measured
noise due
to reference noise (dB)