The Keysight Agilent HP 3570A Tracking Receiver has two identical channels for fast, high accuracy "B-A" measurements of gain or insertion loss of two-port devices and to measure the phase shift between input and output ports.
The Keysight Agilent HP 3570A can also function as a limit comparator to determine how closely the gain and phase response of a device matches that of a reference. The frequency range covers from 50 Hz to 13 MHz.