of substantially reduced stability and performance.
2.2.7.2 Extension Test Sets
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.
2.2.7.3 True‐Multiport VNAs
While the extension test set provides a directional‐coupler on each port of the test system, the reference coupler and the measurement receivers are shared, so the number of ports that can be measured simultaneously is limited to the number of receivers in the base instrument. Recently, improved integration has made it possible to include a full VNA test receiver on each port, so true‐multiport VNAs are now available. These come in a variety of form factors, but for the most part they are intended for manufacturing operations, where size and footprint are important.
One of the first offerings for a large‐port‐count true‐multiport VNA was the ZNBT from Rohde & Schwarz. It provides options from 8 to 24 ports, with a faceless instrument. In this configuration, it had six independent sources (one for each four ports) as well as receivers on each port.
A modular form of multiport VNAs has been introduced in a PXI format, which allows for configuring from 2 to more than 68 ports, potentially up to 100 ports, depending upon the number and model of VNA modules used. Figure 2.29 shows a modular system with eight 6‐port modules (Keysight model M9804‐006) and one 2‐port module (Keysight model M9804‐002), configured as a 50‐port VNA system. There is one source per module, but a full dual‐reflectometer and dual RF receiver for each port. Thus, the 2‐port modules have 1 source and 4 receivers; the 4‐port modules have 1 source and 8 receivers, and the 6‐port modules have 1 source and 12 receivers.
Figure 2.29 A 50‐port VNA system comprised of 6‐port and 2‐port modules.
Multiport VNAs in a modular format require the local oscillator to be shared across all modules to get the best trace noise performance. These systems provide a daisy chain approach to the connect the LO and the 10 MHz reference to each of the modules. The big advantage of a modular approach is the test system is easily reconfigured to support different test needs. For example, a 16‐port system, comprised of eight 2‐port modules, can be reconfigured into four sets of 4‐port VNAs.
While more expensive than a switched version of a VNA, the economics of a true‐multiport system readily become apparent when one considers the overall measurement time and number of sweeps needed to complete an N‐port calibrated measurement. Table 2.1 shows the number of sweeps needed to complete an N‐port calibrated measurement. From a strict sweep time point of view, a true multiport VNA greatly reduces the overall test time requirement.
Table 2.1 Sweeps needed for N‐port calibration
| Total Ports | Total Paths | Switched 2‐Port | Switched 4‐Port | True Multiport |
| 8 | 28 | 56 sweeps | 24 sweeps | 8 sweeps |
| 16 | 120 | 240 sweeps | 64 sweeps |
