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6
Absolute Measurement Examples
Checking the beatnote
While the connect diagram is still displayed, use an oscilloscope (connected to
the Monitor port on the test set) or a counter to check the beatnote being
created between the reference source and your DUT. The objective of checking
the beatnote is to ensure that the center frequencies of the two sources are
close enough in frequency to create a beatnote 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
Chapter
14, "Evaluating Your Measurement Results" if you are not
familiar with the relationship between the PLL capture range and the peak
tuning range of the VCO.
If the center frequencies of the sources are not close enough to create a beatnote within
NO T E
the capture range, the system will not be able to complete its measurement.
The beatnote 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 beatnote will be very close to 0 Hz.
Searching for the beatnote will require that you adjust the center frequency of
one of the sources above and below the frequency of the other source until the
beatnote appears on the oscilloscope's display.
If incrementing the frequency of one of the sources does not produce a
beatnote, you will need to verify the presence of an output signal from each
source before proceeding.
Agilent E5505A User's Guide
139
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