HP 3048A Operating Manual page 133

Checking the Beatnote
While the hardware Connect Diagram is being displayed on
the HP 3048A, we recommend that you 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.
For this measurement setup, the phase lock loop (PLL)
Capture Range is 10% of the Peak Tuning Range of the
VCO source you are using. (The Peak Tuning Range for your
VCO can be estimated by multiplying its Tuning Constant
by its Voltage Tuning Range. Refer to Chapter 2 if you are
not familiar with the relationship between the PLL Capture
Range and the Peak Tuning Range of the VCO.)
NotG If the center frequencies of the sources are not close enough
to create a beatnote within the Capture Range, the
HP 3048A will not be able to complete its measurement.
If the beatnote frequency is below 100 kHz it should now
appear on the HP 3561A. The HP 3561A is configured to
display the beatnote signal in both the frequency domain
and the time domain. If the beatnote does not appear on the
HP 3561A, then the beatnote is either greater than 100 kHz
or it does not exist.
Signal Source Applications 3-83