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7
Absolute Measurement Examples
Free-Running RF Oscillator
Checking the
Beatnote
7-28
E5500 Phase Noise Measurement System Version A.02.00
While the connect diagram is still displayed, recommend that you use
an oscilloscope (connected to the Monitor port on the Agilent
70420A) or a counter to check the beatnote being created between
the reference source and your device-under-test. 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
Measurement Results" if you are not familiar with the relationship
between the PLL capture range and the peak tuning range of the
VCO.)
NOTE: If the center frequencies of the sources are not close enough
to create a beatnote within 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.
Chapter
15, "Evaluating Your
Document Part No. E5500-90024 Ed. 1.0
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