Agilent Technologies E4438C - Manual (Page 85)

Performance Tests
Phase Noise and Residual FM (Manual Test - Option UNJ or Option 506 Only)
3- 37
Using Electronic Frequency Locking (EFC)
Figure 3-33 shows the result of using the electronic frequency control (EFC) method of phase-locking the DUT
to the Agilent E5500 Series phase noise measurement system (see its manual for set up procedures). This
method produces the best result, but is not available on all sources (such as the Agilent 8644B
high-performance RF signal generator). Note that the close-in phase noise is now much improved and the
60 Hz related spikes are eliminated. Although the two large spikes still exist, they are the result of the
Agilent 8662A/8663A high-performance RF signal generator and are not characteristics of the DUT.
Figure 3-33
To get an accurate idea of the actual phase noise of the DUT, it may be necessary to combine the two plots,
always using the lowest result of both (see
Figure 3-34 ). Keep in mind that the actual phase noise results are
probably better than what is displayed.
The results shown on the Agilent E5500 Series phase noise measurement system may be worse than the
actual DUT. If a tested device has the same phase noise characteristics as the system’s down-converting
source, the displayed result will be 3 dB worse than either device. To eliminate this error, measure three
different sources. This results in three equations and three unknowns, and the Service Support Software can
then sort out the actual results for each device. This 3-source substitution method generates a correction table
that can be used for future measurements, effectively eliminating the combined error (of up to 3 dB) for
subsequent measurements. For best results, use this method. Refer to the Agilent E5500 Series phase noise
measurement system’s documentation for instructions on how to implement a 3-source substitution. When
the plots in this section were taken, this method was not used. If it had been used, portions of the plots could
have been up to 3 dB better.
Another way of stating the above: for any phase noise measurement where a 3-source substitution is not
performed, the actual phase noise of one of the two sources involved (DUT or down-converting source) is at
least 3 dB better than shown.