HP 70427A User Manual page 259

Softkeys and Displays
100 MHz no tune
100 MHz no tune
This softkey disables the external tuning of the reference chain. The 100
MHz oscillator is free-running and the 600 MHz oscillator is locked to it.
Select this sensitivity for applications that require the best phase noise at
>1 kHz offsets and when external tuning is not needed.
Path
Ref Chain Tune Sens 100 MHz no tune
HP-IB command
REF:TS 4
Figure 10-15 Reference Chain
10-16 HP 70427A/HP 70428A User's Guide