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How does a Vector Network Analyzer (VNA) measure S parameters actually?

Namely we have equations:

\$b_1=S_{11}a_1+S_{12}a_2\$

\$b_2=S_{12}a_1+S_{22}a_2 \$

In theory, if i wanted to calculate scattering parameters I would have to be able to make perfect matching on the source (\$a_1=0\$) and on the load side (\$a_2=0\$).

Unfortunately I do not understand how is this done inside the VNA. If I want to measure scattering parameters of a random network with input and output impedance different than 50ohm, how is in this case achieved \$a_1=0\$ and \$a_2=0\$?

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Here's a good basic explanation about how a VNA works.

An excerpt follows:

Simplified block diagram of a VNA with two test ports

To measure S11 or S21, the switch would route the signal toward Port 1. The signal first enters a directional device, shown as Directional Device R1 in Figure. This directional device could be a directional coupler or bridge. A power divider could also be used here. The purpose of this device is to direct a portion of the input signal to a reference receiver, shown as Reference Receiver 1. Once the signal enters this reference receiver, it’s downconverted and ultimately processed.

The signal that passes through Directional Device R1 then passes through another directional device, shown as Directional Device M1 in Figure, before arriving at the DUT. When the signal does arrive at the DUT, a portion is reflected back to Port 1 of the VNA and reenters Directional Device M1. This device now directs a portion of the reflected signal to a measurement receiver, shown as Measurement Receiver 1. Next, this signal is downconverted and processed. The VNA can then compare the data obtained from Measurement Receiver 1 with the data obtained from Reference Receiver 1. This process summarizes how S11 is measured.

The signal applied to the DUT also results in a transmitted signal, which exits the DUT and enters Port 2 of the VNA. This transmitted signal enters another directional device, shown as Directional Device M2 in Figure. This device directs a portion of the signal to another measurement receiver, shown as Measurement Receiver 2. This signal is subsequently downconverted and processed. Now, the VNA can compare the data obtained from Measurement Receiver 2 with the data obtained from Reference Receiver 1. This process summarizes how S21 is measured.

To measure S22 or S12, the switch would route the signal to Port 2. The same process occurs in the opposite direction: The signal first enters Directional Device R2 in Figure, which directs a portion of the signal to Reference Receiver 2. After entering this reference receiver, the signal is downconverted and processed.

The signal that passes through Directional Device R2 then passes through Directional Device M2 on its way toward the DUT. After arriving at the DUT, a portion of the signal is reflected back to Port 2 of the VNA and reenters Directional Device M2. Now, a portion of this reflected signal is directed to Measurement Receiver 2. Afterward, this signal is downconverted and processed. The data obtained from Measurement Receiver 2 can then be compared with the data obtained from Reference Receiver 2, thus summarizing the S22 measurement process.

The signal that’s transmitted through the DUT enters Port 1 of the VNA. This transmitted signal enters Directional Device M1 in Figure, which directs a portion of the signal to Measurement Receiver 1. This signal is likewise downconverted and processed. The data obtained from Measurement Receiver 1 can now be compared with the data obtained from Reference Receiver 2, thus summarizing the S12 measurement process.

If you want to obtain S-parameters of a network with input/output impedance other than 50-ohm, these must be recalculated afterwards from the already measured, 50-ohm referenced S-parameters.

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The HP8505 used directional couplers inside a special module located below the display/sweep/storage/compute unit.

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  • \$\begingroup\$ For what purpose did thy use them? \$\endgroup\$ – Nexy_sm Sep 1 '17 at 9:05

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