e) From the External Source Conguration dialogue box  
  select the trigger mode.  
  Note: Hardware trigger is TTL and is faster than Software trigger.  
  To learn more, select the Help button.
  f) From the Select Sources dialogue box:
  i) Highlight source name. 
  ii) Select Add. 
  iii) Select OK.
If all of your sources have been setup properly then the external sources 
should start to sweep.
3. Verify operation:
  a) Go to Frequency Offset dialog box and you should see  
  the external source listed. 
81 | Keysight | Antenna Test – Selection Guide