September 29, 2017
Over the past decade, millimeter wave (mmWave) frequencies have moved beyond the battlefield into commercial applications, ranging from automotive radar to 5G. One reason mmWave frequencies are used in commercial designs is technology improvements in semiconductors, components, cables, and connectors. Advances in test equipment, such as vector network analyzers (VNAs), are another contributor to the expansion, particularly given the issues of connector mismatch error and cable loss when making measurements at mmWave frequencies.
The increased need for accurate, cost-effective mmWave measurements during R&D and manufacturing are evident by the growing number of commercial applications operating at the higher frequencies. Some of the more prevalent areas are:
- 5G - In July 2016, the US Federal Communications Commission (FCC) opened nearly 11 GHz of spectrum in the mmWave frequency range, specifically 27.5 GHz to 28.35 GHz, 37 GHz to 38.6 GHz, 38.6 GHz to 40 GHz, and 64 GHz to 71 GHz. 5G is still evolving but it appears initial use will be for last mile residential service. Mobile devices and base stations will have beam forming antennas to compensate for the higher path losses at mmWave frequencies so wireless networks can accommodate 5G applications, as well.
- Automotive radar – The 24 GHz, 77 GHz, and 79 GHz frequency bands are used in connected car systems such as collision avoidance.
- 60 GHz Wi-Fi – IEEE 802.11ad, which was recently expanded to 71 GHz, is being used in high-speed wireless multimedia services, including uncompressed HDTV, and instantaneous music and image data transmissions. Point-to-point communications links - mmWave backhaul, particularly E-band, is becoming widespread in picocells, microcells, and metro cells.
- Security – If you’ve done any air travel over the past few years, you probably have passed through a mmWave screen. These security systems can detect metallic and nonmetallic threats, including weapons and explosives, without any physical contact.
High Frequency Testing
The growing applications in which mmWave technology is used presents an opportunity for many companies. It also creates numerous challenges for engineers. Among the more common are higher propagation loss and connection repeatability.
- Propagation loss – As most RF engineers know, the loss of signals propagating at RF and microwave frequencies is proportional to the frequency and distance:
Loss (d) = (4*π*d*f /c)2.
An added consideration at mmWave frequencies is increased attenuation from elements of the earth’s atmosphere, especially oxygen absorption at 60 GHz. This presents a challenge to verify designs and analyze product performance at these elevated frequencies because test solutions need higher power signals or improved sensitivity to make accurate measurements.
- Connection repeatability – At 70 GHz, the diameter of the coax center pin is just ½ mm. Scratches and dust particles on the connector interface are more damaging to the impedance match at mmWave frequencies, so connections at these bands require significantly more care. Connector interfaces should at a minimum be visually inspected before each use and inspected often with a microscope and cleaned, as needed. A torque wrench should be used to tighten the connectors to the proper specification (8 in-lbs. max).
Overcoming mmWave Challenges
To characterize impedances at microwave and mmWave frequencies, ρ (or rho), the voltage reflection coefficient is commonly used. For each value ρ, the + and – uncertainty in dB can be as large as:
ρ can be measured with a VNA, such as the Anritsu VectorStar® and ShockLine™ Series. Models with coaxial connections are available to 145 GHz. The results of these equations are shown in Figure 1.
Figure 1: Profile of mismatch uncertainty (dB) values resulting from two reflection coefficients.
Precision, low-loss cables can improve system performance. A 2-foot-long precision test cable will typically cost more than $1,000 and will still add uncertainty from mismatch and insertion loss. Cable loss becomes more complicated if multiple cables are used in a system and an engineer tries to use the same set of values to correct all measurements for loss. The only way to simply and completely remove the cable impact is to eliminate cables altogether and take measurements directly at the device under test (DUT).
The VectorStar-based ME7838A broadband system (figure 2) addresses this issue with unique, highly integrated, very small mmWave frequency extension modules. Their compact size is achieved via Anritsu’s exclusive non-linear transmission line (NLTL) technology that allows the probe tips to be mounted directly to the modules, eliminating cable interconnects. This greatly enhances measurement and calibration stability.
Figure 2: The ME7838A broadband VNA system has very small mmWave frequency extension modules that eliminate the need for cable interconnects.
The ShockLine VNAs (figure 3) use the same NLTL technology in an E-Band VNA configuration and have small remote mmWave extension modules to provide a banded 55 GHz - 92 GHz high-performance VNA system. It has a “headless” design that can be controlled via external display, mouse and keyboard or an external touch screen display to lower cost while maintaining high performance.
Figure 3: ShockLine has small remote mmWave extension modules to provide a banded 55 GHz – 92 GHz VNA system.
To learn more about advances that have made VNAs well suited for mmWave applications, download a white paper entitled Scaling the Test Equipment Size to Match Millimeter Wave Test Needs.