I want to use an external source to go below the 0.1 Hz frequency limit of AM or Phase Modulation, but I need to know the input power range so that I should set that external source to.
Ext1 & Ext2 an externally applied signal is used as the modulation input.
Connect the signal to the EXT 1 or EXT 2 connector on the rear panel of the instrument.
The N5181B is a signal generator. This is the LCR meters and impedance analyzers forum. In other words, you have posted the question in the wrong place. Here
Signal Generators and Sources
would be a better place to ask.
Your question also looks rather badly phrased - I can't work out exactly what you want to do. Your best bet is to ask in the correct place, and put as much information as possible - model number of equipment, options, and firmware version.
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