Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 116

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	16 sweeps
24	276	552 sweeps	144 sweeps	24 sweeps