VNA Considerations when Making E-Band Measurements
The rollout of 70 GHz–80 GHz 5G systems and the subsequent need for more E-band devices has cast greater importance on vector network analyzers (VNAs), which have numerous advantages when testing high-frequency components. Many of these active and passive devices will be frequency selective and can be high-rejection parts, such as duplexers and bandpass filters. VNAs will need excellent Spurious Free Dynamic Range (SFDR) measurement capability to characterize and test the out-of-band performance of these high frequency components during production.
SFDR performance in testing high-rejection devices is an indicator of the level of sampler (or mixer) bounce associated with a VNA. Sampler bounce is caused by LO and LO harmonic leakage from a VNA test port and IF frequency modulation sidebands with that LO leakage. Figure 1 shows the effect of sampler bounce and LO leakage on the out-of-band noise floor of a filter. The result is that the filter response will appear worse than it is in reality.
When measuring a reflective, filter there is a large RF signal being transmitted into port 1 during the stopband portion of the S21 measurement. Some harmonics of the port 1 LO leakage land in the filter passband and are passed through to port 2. The IF sidebands on those LO harmonics are down converted by the port 2 receiver and displayed as part of the stopband of the filter, causing an incorrect measurement result.
Improved SFDR Performance
Anritsu’s E-band frequency option for the ShockLineTM 2-port MS46522B VNA (Figure 2) addresses this issue by delivering excellent SFDR performance for applications from 55 GHz to 92 GHz. The performance is achieved through a design with high isolation between the LO and RF and reduced sampler bounce in the measure modules. This enables the E-band MS46522B VNA to more accurately test passive millimeter wave (mm-wave) components, including waveguide duplexers and filters used in 5G small cell networks. The ShockLine VNA, with the 55 GHz to 92 GHz frequency option, is also a cost-efficient test solution for measuring E-band components — including mm-wave radar connectors and antennas — used in higher frequency 77 GHz driver assist radar systems that are gaining traction in the marketplace.
The benefits of good SFDR can be seen in Figure 3, which shows the results of measuring the same filter used in Figure 1. High isolation between RF and LO paths and the capability to turn off the unused receiver (spur reduction) improves the filter stopband measurement by about 20 dB.
Economics are another consideration in the development of E-band components. Balancing the cost-of-test edicts from management with having a high degree of confidence that a component meets specification is a challenge for engineers. Utilizing Anritsu’s proprietary Nonlinear Transmission Line (NLTL) technology, the ShockLine MS46522B VNAs can deliver the performance at the proper price point to satisfy the strict market requirements.
Integration of the NLTL technology in an E-Band VNA configuration provides many benefits for engineers developing systems utilizing mm-wave frequencies. One advantage is that the MS46522B E-band VNA has permanently attached tethered source/receiver modules that make the solution more convenient to use and deliver SFDR performance equivalent to much more expensive E-band solutions.
The modules are attached to the base 3U-high chassis of the VNA through one-meter cables that are permanently attached to the unit, creating a compact, ready-to-use E-band VNA. Because the modules are small and lightweight enough to connect directly to the DUT, all the critical measurement circuitry is as close to the DUT as possible. Eliminating cables results in minimized RF loss and increased stability.
The remote modules have native WR12 waveguide interfaces for convenient interfacing to typical waveguide devices. The configuration also makes the ShockLine MS46522B VNA less prone to setup errors compared to broadband VNA solutions that rely on loose cables to interface between frequency multiplier modules and the instrument.
Anritsu also designed the ShockLine VNA on a small and flexible platform that has a “headless” design. Engineers can control the VNA via external display, mouse and keyboard or an external touch screen. In addition to producing a smaller footprint in manufacturing environments, this design removes some of the costs associated with front panels and monitors while retaining high performance and reliability. The MS46522B series uses the same GUI, software, command syntax, drivers, and programming environments as the other VNAs in the ShockLine family to streamline operation.
E-band designs are becoming commonplace in more commercial applications, ranging from automotive radar to emerging 5G systems. To verify the components used in these applications, VNAs will need to provide high performance, including excellent SFDR measurement capability. Anritsu’s E-band module design for the ShockLine MS46522B VNA series enables out-of-band measurements on the order of -80 dB, which is comparable to more expensive broadband solutions, while maintaining the cost efficiencies necessary for production.