modulation. The default source is internal. 
If the modulation source is set to external, the 
phase deviation is controlled by the ±5V MOD 
INPUT terminal on the rear panel. For example, if 
the modulation voltage is +5V, then the phase 
deviation is equal to the phase deviation setting. If 
the modulation voltage is less than +5V, then the 
phase deviation will be less than the phase 
deviation setting.