A vector network analyzer (VNA) is piece of test equipment used in radio frequency (RF) engineering. A VNA can characterize the response of a device under test (DUT) by simultaneously measuring both forward and backward propagating waves. The figure below shows the simplified block diagram of a two-port VNA and its key signal processing components.

RF Source serves as the excitation source for the Device Under Test (DUT). The source can be programmed to generate a sinusoidal signal at a desired frequency and power level.

Signal separation devices like directional couplers isolate incident and reflected waves at each port. This enables the VNA to quantify the incident waves (a_1 and a_2) measured by the reference receiver (labelled REF RX 1 and REF RX 1) and reflected wave (b_1 and b_2) measured by the test receiver (labelled TEST RX 1 and TEST RX 2).

Receivers measure the incident, reflected, and transmitted signals. The signals that are separated out by directional couplers are down converted to IF. Reference receivers r_1 and r_2 measure the signal travelling from VNA towards the DUT (Incident signal). Test port receivers s_1 and s_2 measure the signal travelling away from the DUT towards VNA (reflected and transmitted signals). IF signals are digitized and digitally down-converted to baseband, followed by further DSP to return the complex traveling wave amplitudes. These steps are not shown in the simplified block diagram above.