Enhance mmWave to sub-THz Test with PXI Vector Signal Transceivers and Third-Party UDCs

Overview

This white paper describes how using NI PXI Vector Signal Transceivers (VSTs) with third-party frequency upconverters and downconverters (UDCs) can address the growing need for versatile, high-frequency testing from mmWave to sub-THz frequencies. Highlighting NI PXI VST,  we’ll discuss the benefits of this approach such as scalability, precise calibration processes, and integration within the PXI platform for optimized performance. Practical applications in fields like 5G/6G prototyping, FMCW radar, and 60 GHz Wi-Fi are explored to demonstrate how this system setup supports advanced research and validation efforts, pushing the boundaries of wireless communication technology.

Contents

​​Spectrum Use From mmWave to Sub-THz

​In the last few decades, the way the available radio frequency (RF) spectrum is viewed has shifted drastically. More often, different applications and use cases are competing for the same span of spectrum that has been used since the advent of wireless communications. Mobile carriers bid huge amounts to acquire prime real estate in the wireless spectrum. Cellular bands are spaced increasingly closely together; adjacent bands can interfere with other frequency bands used for aeronautical and maritime navigation, television and radio broadcasting, satellite, and radar, among many more—and existing cellular bands are increasingly saturated with a high quantity of cellular (UE) traffic.

Figure 1: U.S. Frequency Allocations, Source: US Department of Commerce

​Now more than ever, available spectrum is at a premium, and the answer to solve the shortage of available spectrum is to go higher (in frequency). Wireless technologies such as cellular, Wi-Fi, and autonomous driving, to name a few, are now exploring the viability of micro, mmWave, and sub-THz frequencies for their wide expanse of open spectrum and numerous other benefits.

​Among the many benefits of using higher frequencies are the large spans of unused spectrum that are great for continuous, wide- channel bandwidths and as a result, high user capacity and data throughput. While this certainly solves the issue of crowded spectrum, it also comes with many technical challenges. Doppler shift, path loss, and the need for new antenna packaging and infrastructure are just a few.

​Figure 2: New Antenna Packaging for mmWave and Sub-THz Frequencies

​To overcome these technical challenges, engineers need new and better test equipment that can cover the higher frequencies and wider bandwidths required—quickly, reliably, and accurately.

​Introduction to PXI Vector Signal Transceivers

​At the core of NI’s portfolio of RF instruments are PXI Vector Signal Transceivers (VSTs). VSTs combine an RF and baseband vector signal analyzer and generator with a user-programmable FPGA and high-speed serial and parallel digital interfaces for real-time signal processing and control from baseband to mmWave.

​For test at mmWave frequencies, NI offers several fully developed options that combine PXI VSTs with a remote measurement module for extended frequency coverage up to 44 GHz or 54 GHz, depending on the VST chosen.

​Figure 3: PXIe-5842 with 54 GHz Frequency Extension

​There are two mmWave VSTs available, the PXIe-5842 with 54 GHz Frequency Extension and the PXIe-5831 mmWave VST. Both leverage the same PXI module and remote radio head architecture for test at mmWave frequencies.

SpecificationPXIe-5831PXIe-5842
mmWave I/O Frequency Range22.5 GHz – 44 GHz22.5 GHz – 54 GHz
IF I/O Frequency Range5 GHz – 21 GHz200 MHz – 23 GHz
Bandwidth1 GHz2 GHz
mmWave Heads1 – 21
Direct Ports2 unidirectional2 bidirectional
Switched PortsUp to 32N/A
PXI Slots66

 

Table 1: PXIe-5842 with 54 GHz Frequency Extension and PXIe-5831 mmWave VST Specification Comparison

Refer to Instrument Innovations for mmWave Test for more information on mmWave VSTs.

​The remote measurement module architecture has many advantages when operating at mmWave frequencies. To begin, it keeps the PXI test system compact and uses the same instrumentation that is required for low-frequency RF applications, which enables a greater amount of versatility and reuse of the test system. Additionally, having mmWave frequencies limited to the remote radio head means less path loss throughout the signal chain and lower power requirements from the system. Lastly, and what is perhaps most relevant for this white paper, is the fact that these advantages and architectures are also applicable when using PXI VSTs with third-party frequency converters—of which there are many to choose from, enabling a wide range of configuration options and frequency coverage.

​Using the PXIe-5842 with Third-Party Frequency Up/Downconverters

​System Setup

​In addition to options developed solely by NI, you can also use VSTs with third-party frequency upconverters and downconverters. Using this option, you can get the excellent RF performance of PXI VSTs in conjunction with high-performance frequency upconverters and downconverters developed by third-party organizations that specialize in RF hardware for mmWave and sub-THz applications.

​Many of these companies, such as Eravant or Virginia Diodes, have decades of expertise in high-frequency test. By combining this expertise with NI’s platform of RF test hardware and software, an almost endless amount of spectrum coverage is leveraged in conjunction with software and measurement IP that enables meaningful insights and excellent measurement performance in a single interactive and integrated test system.

​Third-party UDCs are integrated with PXI VSTs by connecting the upconverter and downconverter to the VST’s RF OUT and RF IN ports, respectively.

​The VST first generates an IF frequency that is sent to the upconverter. The frequency conversion is accomplished by connecting the upconverter to an additional LO, also included in the same PXI system as the VST, that mixes an LO output frequency that can be mixed with the IF frequency from the VST to obtain the desired RF frequency.

​The upconverter and downconverters used often have frequency multipliers that must be accounted for in the calculation of the final center frequency. The formula for obtaining the desired frequency follows, where [Equation] is the frequency multiplier in the upconverter, is the frequency of the LO, and is the frequency output of the VST. 

​Integrated software tools work with information from the UDCs to pick the best LO and IF numbers for the desired RF frequency. This process ensures the best spectral quality and measurement results.  

​Figure 4: Diagram Showing Different Ways to Obtain a Specific RF Frequency

​The same process occurs in the reverse for the frequency downconversion. The LO signal used in the frequency upconversion process is split and used by the downconverter as well. The result is an IF frequency to the RF IN port of the VST that is at the same frequency as the IF output signal originally generated from the RF OUT port of the VST.

​Figure 5: Block Diagram of Using the PXIe-5842 VST with Third-Party Frequency Up/Downconverters

​The exact range of output frequencies depends on the specifications of the frequency up and down converters that are used. There are many options available, from many different vendors, ranging from 50 GHz up to sub-THz frequencies. You can even use custom hardware, if desired.

​In addition to the NI hardware and UDCs in the test system, a fully functioning system also requires various attenuators, cables, waveguide connected components, and other accessories. These components can vary based on frequency range, power, test application, and other factors. Contact an NI technical expert for assistance selecting and configuring the proper accessory components.

​Calibration

​With so many moving pieces and components in the signal chain, calibration can be tricky if not correctly accounted for. This challenge can be especially true since PXI VSTs are calibrated and optimized for performance at their RF IN and RF OUT ports, not the input and output ports of the UDCs which is the signal of interest in these applications.

​To overcome this issue, NI has created software IP that accounts for all external components in the measurement path. The calibration software accounts for the inclusion of these components and calibration occurs at the output of the upconverter and at the input of the downconverter.  

The Benefits of NI’s Approach

​There are many advantages to scaling a mmWave test system with this methodology in a PXI form factor. To begin, the footprint of the test system itself does not change. The required LO is integrated into the same PXI chassis alongside the VST. Additionally, being in the same test system means that they are both controlled with the same PXI embedded controller, simplifying timing and synchronization.

​Figure 6: PXI-Based Test Systems Can Accommodate a Wide Range of Modules

​The advantages of hosting instruments in the same PXI test system are not limited to the LO. In fact, many other PXI instruments are available to be integrated alongside RF instruments at mmWave frequencies for a comprehensive set of measurement capabilities for a given application. This can include DC instruments for DT biasing and digital instruments for DUT control, among others.

​Also important is the versatility of the core instrument, PXI VSTs, which can be used with or without third-party frequency extenders. By removing the connections to the UDCs and additional components, and recalibrating the instrument, users now have the capabilities of a fully functioning signal analyzer and signal generator that is great for test and validation of Wi-Fi, Bluetooth, UWB, or cellular components, power amplifiers, and RF front ends.

​Applications

In 5G, cellular communications expanded into mmWave frequency bands, and in 6G there is discussion around whether the sub-THz spectrum could be used for specific applications like joint communications and sensing and gesture recognition. The wide channel bandwidths and available spectrum make moving up into higher frequencies appealing. The 802.11ad Wi-Fi standard made use of unlicensed 60 GHz spectrum, although with limited success; however, there is a renewed interest with Wi-Fi 8 (802.11bn). Automotive radar has leveraged the wide bandwidths available around 79 GHz to provide high-accuracy radar measurements

​FMCW Radar

​As the automotive industry incorporates more advanced driver assistance systems (ADAS) into modern automobiles, frequency modulated continuous waveform (FMCW) chipsets are increasingly being used due to their simpler circuit design and increased distance and velocity accuracy.

Radar sensors operate at frequencies from 50 GHz to 75 GHz with instantaneous bandwidths up to 5 GHz. PXI VSTs, such as the PXIe-5842, are ideally suited to address the test requirements of these chipsets in a scalable, versatile, and high-performance test system.

​60 GHz Wi-Fi

​Still early in definition and development, there is ongoing exploration into expanding Wi-Fi 8 (802.11bn) into the 60 GHz range. Wi-Fi could turn out to be ideally suited for mmWave frequencies, where high data throughput is required and range not as crucial a limitation as with cellular communication.

​Using PXI VSTs with third-party UDCs provides the RF capabilities necessary for design, validation, and verification of new, 60 GHz Wi-Fi components and devices.

​5G and 6G Wireless Research and Prototyping

​In recent years, 3GPP specifications have outlined mmWave and even sub-THz frequencies as potential use cases for cellular applications. These extreme high frequencies present the possibility of very high data throughput and a large increase in network capacity. However, research into their feasibility is still ongoing. NI offers a high-level starting point for sub-THz research intended to provide users with a platform to design custom sub-THz test applications, and include real-time, sustained data streaming.

​Figure 7: Sub-THz Test System Diagram

Learn more about configuring custom sub-THz test applications

​Conclusion

​​Setting up mmWave and sub-THz test systems can introduce many technical challenges, but these frequencies could prove very important to the future of wireless technologies. By combining PXI VSTs with third-party UDCs, NI provides an integrated, scalable, and easy-to-use test system for test and validation at mmWave and sub-THz frequencies.

Contact NI to learn more about using PXI VSTs with third-party UDCs for FMCW radar sensor validation, 60 GHz Wi-Fi, sub-THz test, and other high frequency test applications.