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