Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 115

To satisfy the requirement for making full N‐by‐N calibrated measurements, often referred to as full N‐port cal measurements, a test set design has been developed that includes both directional‐couplers and switches. The original implementation of this style of extension test sets was configured to supply two additional ports to a two‐port VNA to create a 4‐port VNA for making the first balanced and differential measurements. The general idea of an extension test set is to essentially extend the source switch matrix of the VNA to more outputs through a source switch and also extend the internal receivers to more ports through a receiver switch. This requires that an additional test port coupler be provided for each additional port. Because the switching occurs behind the VNA directional‐couplers, they are still available as test ports: the ports on the test set extend the total number of ports available, which is why it's called an extension test set. Figure 2.27 shows block diagrams for a simple two‐port extension test set.

Figure 2.27 Extension test set block diagram.

One key point of the block diagram is that the test set breaks into the source and receiver loops behind the test port coupler. Since any number of switch paths can be supplied behind the test couplers, there is in theory no limit to the number of ports that can be used. Further, this block diagram allows additional test sets to be added so that any number of test ports can be created by stacking extension test sets. Common configurations are 4‐port extension test sets for a 4‐port VNA to extend to a total of 8 ports, 10‐port extension test sets for a 2‐port VNA to achieve a total 12 ports, and 12‐port extension test sets for a 4‐port VNA to achieve a total of 16 ports. Figure 2.28 shows a 4‐port VNA with two 4‐port extension test sets to create a 12‐port system.

Figure 2.28 12‐port system using a 4‐port VNA and two extension test sets.

Source: Photo courtesy of Keysight Technologies.

The switches may be either mechanical switches or solid‐state switches. Because all the switching occurs behind the test port couplers, the stability and performance of the measurements are much better than that of switching test sets, and loss in the switch, while it reduces the dynamic range, has no effect on stability of the measurements.

In some cases, an option may be provided to add a low‐noise amplifier (LNA) between the coupled port of the test coupler and the switch input. This improves performance as the gain of the LNA improves the dynamic range. Adding amplifiers in between the coupled arm and the switch also removes another source of error. In some cases, the source‐match of a port changes when the source and test port share the same VNA receiver, for example ports 1 and 3 in Figure 2.27. This error is typically small as the difference between the match of the VNA receiver and the match of the switch is small (on the order of −10 dB) and is further reduced by twice the coupling loss (32 dB) resulting in a typical source‐match error smaller than −40 dB. In most cases it has a negligible effect, but in some measurements, particularly circulators or couplers, it can become significant and is not removed in calibration, so adding an amplifier ensures that the match presented to the coupled arm is constant. The test ports also change load characteristics depending upon if they are terminated in a switch or the VNA internal load; however, the N‐port calibration methods characterize both these states and fully correct for the difference.