HP 3048A Operating Manual page 196

Using a Similar Device
The 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 HP 3048A 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. (Refer to your
HP 3048A Reference Manual for details on performing the
Three Source Comparison procedure.)
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 will represent the
sum of the noise of both devices.)
Using a Signal Generator
When using a signal generator as a reference source, it is
important that the generator's noise characteristics are
adequate for measuring your device. The specified and
typical noise characteristics of the various HP signal source
options are provided in the HP 3048A Reference Manual.
3-144 Signal Source Applications