The Agilent 35689A is a test set that is available for the Agilent 3589A Spectrum/Network Analyzer. The 35689A is designed for 50-ohm devices. The Agilent 35689A has a separate spectrum input to let you make spectrum measurements without disconnecting the test set.
The Agilent 35689A contains directional couplers that are optimized for operation above 100 kHz. Although you can make network measurements with start frequencies lower than 100 kHz, the analyzer's performance is not specified below 100 kHz when using the Agilent 35689A Test Set.