HP 3048A Operating Manual page 106

When the time domain display on the HP 3561A is
showing less than one full cycle, decrement the
HP 3561A's span width. The [3561 Span key provided on
the HP 3048A's display decrements the HP 3561A's
span width each time it is pressed. (To increment the
span width, press the SHIFT key on your keyboard and
3561 Span at the same time.)
Reduce the beatnote frequency again until the time
domain display is showing less than one full cycle
across the new span width.
Continue decrementing the HP 3561A's span and
reducing the beatnote frequency until the beatnote is
within the Capture Range determined in step 1 of
this procedure. The following chart will help you select
the best span width for viewing the beatnote within the
Capture Range.
Note If you are not able to tune the beatnote to within the
Capture Range due to frequency drift, refer to Increasing a
Synthesizer's PTR in the Problem Solving section of this
chapter for information about measuring drifting signals.
3-56 Signal Source Applications