System Reference Source (Frequency Reference)

Use the System Reference Source radio buttons to specify whether the measurement hardware should lock to its internal clock or to the signal at the Ext Ref In connector of the Reference Instrument.

If the External is selected, the hardware uses the signal at the Ext Ref In connector as the frequency reference.

If the Internal selected, that hardware uses its internal clock as the frequency reference.

If the hardware does not have an Ext Ref In connection, the System Reference Source is automatically set to Internal, the radio buttons are greyed and External cannot be specified.