User`s guide

328 Agilent E5505A User’s Guide
14 Evaluating Your Measurement Results
The reference source
It is important that you know the noise and spur characteristics of your
reference source when you are making phase noise measurements. (The noise
measurement results provided when using this technique reflect the sum of all
contributing noise sources in the system.)
The best way to determine the noise characteristics of the reference source is
to measure them. If three comparable sources are available, the Three Source
Comparison technique can be used to determine the absolute noise level of
each of the three sources. If you are using as your reference source, a source
for which published specifications exist, compare your measurement results
against the noise and spur characteristics specified for that source.
If you have obtained an actual (measured) noise curve for the reference source
you are using, you can use it to determine if your measurement results have
been increased by the noise of the reference source. To do this, determine the
difference (in dB) between the level of the results graph and that of the
reference source. Then use the graph shown in Figure 261 to determine if the
measurement results need to be decreased to reflect the actual noise level of
the DUT.
For example, applying to the graph the 7 dB difference in noise levels at
10 kHz, reveals that the measured results should be decreased by about 1 dB at
10 kHz to reflect the actual noise of the DUT. See Figure 262 on page 329.
Figure 261 Compensation for added reference source noise
1510 11 12 13 145432109876
0.5
1.0
1.5
2.0
2.5
3.0
Amount measured level exceeds reference level (dB)
Increase due to
reference noise (dB)