Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 113

For a class of RF and microwave devices, the normal port count of two to four ports is not sufficient, and multiport measurements are required. There are two distinct classes of multiport test requirements that generate the need for two distinct RF architectures to support the measurements. In the past, RF switching test sets provide the basis for these multiport extensions, but recently true‐multiport VNAs have been introduced.

The first class of devices requires multiple sets of 2‐, 3‐, and 4‐port measurements. As such, the native mode measurements of the VNA are sufficient, and all that is required is RF switching to route the VNA ports to the various port pairs of the DUT. One example of such a DUT is a satellite multichannel‐diplexer (or multiplexer), which filters and separates signals from a common antenna path to each of several output channels, as shown in Figure 2.24. This unit has waveguide filters and interconnections to provide for the lowest possible loss.

Figure 2.24 A satellite multiplexer with many outputs.

Source: Courtesy ComDev Ltd., with permission.

This device requires two‐port measurements for each path from the common port so a 2‐port VNA with one common port and one switch port can make all the required measurements. These are sometimes known as switching test sets or simple switch trees.

The second class of devices requires a measurement from each port to every other port, and in general the response of any path depends upon the loading or match applied to every other port. A “Butler matrix” is a kind of signal dividing network used in phased‐array radar systems, which has this attribute. An eight‐port Butler has four inputs and four outputs, and the proper description is an 8 × 8 S‐parameter matrix. To measure such a device, a switch matrix must be able to allow measuring every path of the device. Informally, these types of switch matrixes are called full cross‐bar switches, which implies that from the two ports of the VNA, any path of the DUT can be measured.

There is a further requirement on the Butler matrix; a full N‐by‐N port calibration measurement must be able to be performed to correct for the imperfect match of at each port. This requires not only a full cross‐bar matrix but one that supports N‐by‐N calibration as well. A third style of test set allows such N‐by‐N S‐parameters called an extension test set, which extends or adds to the number of test ports from a VNA, but these have largely been replaced with true‐multiport VNAs.

More recently, several vendors have developed high‐port‐count VNAs with up to 24 internal ports (R&S ZNBT) or configurable module multiport VNAs (Keysight M9875), in which PXI‐based VNA modules can be flexibly configured to large numbers of ports (more than 66 ports).

The various forms of multiport configurations are described next.