Vector Network Analyzer Architectures

Network analyzers are available as both scalar (magnitude only) and vector (magnitude and phase) instruments. Scalar instruments were once widely used because of their simplicity and lower cost. Vector instruments offer better error correction and more complex measurement capability. With advances in technology, integration, computing power, and cost reduction, VNAs are increasingly common.

Modern VNAs have the same basic functional process. A signal source produces the incident signal and feeds into the DUT input. The signal source enters the DUT through the signal separation section, also known as a test set. At this stage, the reflected and transmitted signals are separated. For each frequency point, the device measures the individual voltages and computes the parameter value (for example S₁₁ or VSWR).

Test sets are designed as either transmission/reflection (T/R) or full S-parameter.

T/R Test Set

The T/R test set is shown in the following figure.

1  Source 2  Directional Coupler 3  Reference Receiver 4  Receiver B1	5  Directional Coupler 6  VNA Port 1 7  VNA Port 2 8  Receiver B2

The T/R architecture includes a stable source that supplies a sinusoidal wave signal at a given frequency and power. A reference receiver, A, connected with a power divider or directional coupler, measures the incident signal. The incident signal exits the VNA through port one and enters the DUT. The B₁ receiver measures signals reflected back to port one. The signal transmitted through the DUT enters port two, where the B₂ receiver measures the signal. A coherent LO shared among all receivers enables accurate, ratioed measurements.

T/R architectures are cost-effective, simple, and offer good performance. They measure only forward S-parameters. To measure reverse S-parameters, you need to disconnect and reverse the DUT or rely on external switching.

Full S-Parameter Test Set

In a full S-parameter architecture, shown in the following figure, a switch is embedded in the signal path after the reference receiver.

1  Source 2  Directional Coupler 3  Reference Receiver 4  Receiver B1 5  Directional Coupler	6  VNA Port 1 7  VNA Port 2 8  Receiver B2 9  Switch

With the switch in the port-one position, the VNA measures forward S-parameters. In the port-two position, the VNA measures the reverse S-parameters.

Full S-Parameter Test Set with Dual Sources

The following figure illustrates a full S-parameter test set architecture with dual sources.

1  Source 1 2  Directional Coupler 3  Reference Receiver A1 4  Receiver B1 5  Directional Coupler	6  VNA Port 1 7  VNA Port 2 8  Receiver B2 9  Reference Receiver A2 10  Source 2

This architecture includes four receivers and two sources rather than a single source and transfer switch. A dual source architecture eliminates the need for a transfer switch, which improves port-to-port isolation. The absence of a transfer switch also removes any errors during switching and allows for delivery of higher port power to the DUT.