Frequency stability: A general phrase that
covers both short- and long-term LO instability.
The sweep ramp that tunes the LO also
determines where a signal should appear
on the display. Any long term variation in LO
frequency (drift) with respect to the sweep
ramp causes a signal to slowly shift its horizontal
position on the display. Shorter-term
LO instability can appear as random FM or
phase noise on an otherwise stable signal.