This instrument is used to measure the impedance across its terminals, for instance in an RF circuit or a resonance circuit such as an antenna. The circuit being measured on is called a network. Since impedance is frequency dependent, the instrument sweeps across a range of frequencies. The instrument inputs a signal of the given frequency into the network, and measures the response. The same operation is repeated for many different (a spectrum of) frequencies. This is called a frequency sweep.
It measures S-parameters.
HP8753A
300kHz - 3.0GHz two port network analyzer.