N5511A Phase Noise Test System User’s Guide 131
Absolute Measurement Examples
Dual Channel (EFC)
New Measurement
Close Define Window. From the File Menu:
Measure
,
New Measurement
.
If the beat note is not within the frequency range required, the software will
prompt to tune the reference.
Checking the Beat Note
Use an oscilloscope (connected to the Monitor port on the test set) or a
counter to check the beat note being created between the reference source
and your DUT.
Figure 6-18
Oscilloscope displaying monitor output of phase detector
The objective of checking the beat note is to ensure that the center frequencies
of the two sources are close enough in frequency to create a beat note that is
within the capture range of the system. The phase lock loop (PLL) capture
range is 5% of the peak tuning range of the VCO source you are using. (The
peak tuning range for your VCO can be estimated by multiplying the VCO
tuning constant by the tune range of VCO. Refer to
if you are not familiar with the relationship
between the PLL capture range and the peak tuning range of the VCO. The
beat note frequency is set by the relative frequency difference between the two
sources. If you have two very accurate sources set at the same frequency, the
resulting beat note will be very close to 0 Hz. Searching for the beat note will
require that you adjust the center frequency of one of the sources above and
below the frequency of the other source until the beat note appears on the
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