Network Analyzers allow a multitude of measurements about the behaviour of a network or a device under Test. They allow for a in-depth characterzation of parts and assemblies.

They share many similarities with spectrum analyzers, though they are more complex.

A spectrum analyzer scans a unknown signal. As such it is scalar in nature.

A vector network analyzer always features a synthesizer and scans the response of the DUT to a known signal. That way it can retrieve amplitude and phase information.

When Keysight still were HP or Agilent they introduced some remarkable ´low frequency band´ VNAs  to the market. From those times devices like the HP3577A and HP8751A and 4195A come into mind up to the actual Keysight E5061B option 3L5. Machines that still can perform on very high levels of precision and accuracy if they are in good condition.