Set or query the low phase noise enable state.   
When switching to LPN mode, perform the operations in the following order: 
1.  Set the sample clock frequency with FREQ:RAST:EXT 
2.  Set the LPN enable state with FREQ:RAST:LPN:ENAB 1 
3.  Supply the sample clock to both the external sample clock input and 
the LPN clock input of the front panel 
4.  Set the LPN active state with  FREQ:RAST:LPN:ACT 1 
When switching from LPN to standard mode,  perform the operations in the following 
order: 
1.  Clear the LPN active state with FREQ:RAST:LPN:ACT 0 
2.  Remove the sample clock from the LPN clock input of the front panel 
3.  Clear the LPN enable state with FREQ:RAST:LPN:ENAB 0