Learn about multi-port and multi-site test optimization techniques

(a) 2-port electronic calibration (b) 4-port electronic calibration

Figure 13. Electronic calibration simplifies multiport calibration while maintaining accuracy.

Finally, an ECal module can be embedded in the calibration switch matrix box and a user characterization can be performed at the end of each switch matrix port so that the user characterization provides calibration at each port. Figure 14 shows an example of an N-port custom calibration test set with an integrated ECal module. ECal characterization is performed at the test port. On this type of custom calibration test set, a multiport VNA can be calibrated with a single test port connection.

There are two caveats to this type of multiport solution. First, the number of physical connections required to use 2-port or 4-port Ecal is not actually reduced or eliminated; what is reduced is the number of times the calibration button is pressed on the VNA. Second, there is a tradeoff in calibration sensitivity or accuracy. As mentioned earlier, the effect of switch attenuation within the switch matrix test socket also applies to switch-based multiport Ecal solutions. Does this mean that compared to 2-port or 4-port Ecal,

Will calibration need to be performed more frequently? This depends on the parameters required of the device under test and how difficult its specifications are. This can be a bigger problem for frequencies above 10 GHz or for products that require high performance.

Multi-site technology improves throughput

Multisite or parallel measurement refers to the use of a single test station to test multiple devices simultaneously (Figure 15). Measuring multiple devices simultaneously on the same test station can reduce test costs by increasing throughput and reducing capital expenditures. The widespread use of wireless technology has driven the need for multisite testing in the high-volume production of passive devices, mobile phone SAW devices, and general components such as antennas, filters, cables, etc.

Figure 15. Multi-site solutions reduce test costs by increasing throughput and reducing capital expenditures.

Multisite testing may include multiple VNAs performing 2-port measurements, as well as any combination of ports and VNAs. While previously multisite may have meant stacking multiple ENA or PNA instruments in a rack, the new approach uses standalone PXI VNA modules to increase throughput and provide more flexibility than traditional solutions.

Unlike switch-based solutions that perform sequential measurements, the multi-site test capability provided by PXI VNA can simultaneously measure multiple paths of the device under test. For example, a two-port, four-port, or even more-port device under test can be tested simultaneously using PXI modules recognized by a single PXI controller. Different devices or different measurement paths of a single multi-port component can be measured simultaneously using optimized stimulus settings (such as frequency, power level, IFBW, or number of points) to increase the overall measurement throughput of production test. Figure 16 shows the multi-site measurement capability of the PXI VNA.

(a) Testing multiple devices simultaneously (b) Testing different devices simultaneously

图 16. PXI VNA 多站点测量能力可以提高测试吞吐量，从而显著降低每个器件的测试成本。

It is also important to note that this discussion is not about the number of ports a VNA has. The only effect of having more ports is that all the VNAs you need can be put into a single chassis.

Considerations when configuring a multi-site solution

First, let’s discuss some considerations when building a multi-site solution.

Controller/CPU

In a multi-site configuration, multiple instances of the VNA software are launched, each connected to a single or multi-port VNA. Each instance behaves as a separate instrument used simultaneously, and each VNA can be optimized for the specific measurement conditions of each device under test, thus achieving the best balance between measurement speed and accuracy.

Embedded Controllers vs. External Controllers

In a multi-site configuration there is one controller, which can be either an embedded controller or an external controller. Whether an embedded controller or an external PC controller is used has no impact on the multi-site solution, the speed and throughput measured are similar.

The bandwidth of the external PC is not an issue, the bottleneck is the CPU. As the number of independent VNA instances increases, the controller needs to do more computing work, and once it exceeds a certain limit, the processor performance begins to decline. In this case, the effect of embedded controller or external controller is the same.

Number of Cores and VNA

So, what affects measurement speed and throughput when multiple PXI network analyzer (VNA) instances are running simultaneously within a PXI chassis?

In recent testing of controller processor speed and performance, there was no significant reduction in processor speed for multi-site operation when the number of independent VNAs equaled the number of processor “cores.” Therefore, for the Keysight M9037A quad-core embedded controller, up to four independent VNA instances can be operated within a single PXI chassis with no significant drop in processing speed.

网络分析仪(VNA) 的中频带宽（IFBW）设置对测量的总体吞吐量有显著影响。对于较低的分辨率带宽（如 1 kHz），VNA 实例的数量可以远远超出内核数量。但是，对于 100 kHz 或更高的 IFBW，每个内核只能处理一个 VNA 实例。

Let's look at an example of multi-site testing by adding 16 2-port VNA modules to see how the system scales as modules are added. In this example, we test dual-core and quad-core controllers at 1 kHz and 100 kHz IFBW. Add up to 16 modules in each of the four cases and see how many DUTs can be measured per second.

The multi-site performance of a PXI VNA depends heavily on how CPU-intensive the task is. As expected, the results show that the quad-core performs better than the dual-core.

The actual measurement parameters needed in production test and their impact on the CPU are the most critical factors, in this case the IFBW setting determines the overall throughput. The 1 kHz setting scales with the number of modules. At an IFBW of 100 kHz, performance scales with the number of CPU cores.

At the higher 100 kHz IFBW, the results do not scale quite as expected. For example, one quad-core PXI VNA can measure 28 DUTs per second, an additional PXI VNA can measure 52 DUTs, and an additional PXI VNA can measure 73 DUTs. This is not as efficient as it could be. In theory, three PXI VNAs should be able to measure 84 DUTs per second. This is likely due to the need to allocate some processor overhead to coordinate multiple cores. This example highlights how measurement results can vary depending on the actual VNA setup required for a specific application.

将来的调查能够确定，当内核数量增加到超出本测试中使用的四个时会出现什么结果。如果 IFBW 值更高，那么将现在使用的 CPU 更换为性能更高的 CPU，可以显著提高可扩展性。

Backplane Speed ​​and Memory

For multi-port, multi-site PXI applications, PXI backplane speed limitations are least likely to affect throughput. Consider a 16-port VNA with 100 kHz IF bandwidth using eight PXI VNA modules.

A 100 kHz IF bandwidth means 100,000 points per second or 10 microseconds per point, multiply that by the number of receivers – 32 in this case for eight PXI VNA modules. Now there are 3.2 million cells per second. Each cell is 32 bits or 64 bits, let’s assume 64 bits or 8 bytes, this becomes 25.6 million bits per second (25.6 MB/s). The bandwidth of the external controller is 4 GB/s, so there is obviously a lot of headroom.

RAM storage is also unlikely to be a limiting factor. Each instance uses very little "dedicated" memory. On a generous basis, each instance might use 200MB of dedicated memory, and 8 instances would use less than 2GB.

PXI Chassis

Since a PXI network analyzer (VNA) requires only one PXI slot, up to 16 PXI VNA modules can fit into one chassis. Be sure to use a PXIe or hybrid slot to get the full performance of the PXI VNA.

Figure 21. A single  M9018B PXIe 18-slot chassis can accommodate up to 16 PXI VNA modules.

Multi-display/Multi-user

Multi-site testing is often used in highly automated environments, and there are also some applications that require multi-site testing for multiple users. For example, low-cost filter tuning manufacturing. Figure 22 shows a 4-port VNA multi-site configuration that provides workstations for four testers. The limit on the number of test stations is determined by the hardware and ports available on the PXI chassis and computer peripherals.

The points that require attention are explained below.

Figure 22. Multi-site capability enables multi-user testing solutions.

Controller Hardware

Figure 22 shows a multi-user configuration using a Keysight  M9037A embedded controller . The controller has two display output ports to connect two monitors directly. The USB3 ports allow an external USB3 hub to be connected, and the two paths on the hub are routed to a USB3 to DVI video converter and then extended to two more monitors. At this point, additional monitors can be added in addition to the four monitors shown in the example above.

Another approach is to use an external controller, perhaps a desktop computer with a more powerful video game board, which allows for more standard video connections without the need for a USB3/video converter. When choosing between an embedded or external controller, it should be noted that this configuration is best suited for VNA display information. Transferring video-intensive information between displays, such as high-definition training videos, can degrade performance. For VNA testing, this is not a problem.

In the above example, the USB3 hub is also used to transmit the control signals for the touch screen display or the test station mouse.

Microsoft Windows limitations

The computer operating system must be configured to support multiple monitors and be able to distinguish between the test stations. This is a standard feature of Microsoft Windows.

First, the computer needs to be set up to recognize each monitor (Figure 23). Then, each monitor needs to be configured so that it can be recognized as a specific test station (Figure 24). This should also include calibration of the positioning station - touch screen or mouse.