HP 3048A Operating Manual page 71

Checking the Beatnote
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.
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 the VCO Tuning Constant by the 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.)
Note 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 3561 A. The HP 3561A can be configured
to display the beatnote signal in both the frequency domain
and the time domain. If the beatnote does not appear on the
HP 35 61 A, then the beatnote is either greater than 100 kHz
or it does not exist.
rev.l2JUL88 Signal Source Applications 3-21