Labs Worldwide Address High-frequency OTA and Propagation Measurement Challenges With ShockLine™ VNA

July 20, 2021 Anritsu Company

July 20, 2021

Most traditional vector network analyzers (VNAs) are designed with ports mounted in a single chassis. The reason is that it keeps the source and measurement circuitry close together, which simplifies designs and enables the very strict synchronization needed for high frequency vector S-parameter measurements. For most benchtop environments, this approach is not a major issue. With so many commercial and military/aerospace systems now utilizing microwave and millimeter wave (mmWave) spectrum, however, the effect of cabling on S-parameter measurements is more significant.

Standard software techniques de-embed the characteristics of the cabling from the measurement so that the test results reflect the device under test (DUT) performance. For the most part, this method suffices in a typical bench application. The setup is reasonably close to the VNA and the RF characteristics of the cables and fixtures stay very stable, so that their effect on the measurement from device-to-device is effectively removed.

Long Cable Issues

Long cables connecting the VNA ports to the DUT can have several negative effects on S-parameter measurements. Two prominent issues are:

Insertion Loss (IL) – At mmWave frequencies, a few meters of cable can add significant IL between the DUT and the VNA. For example, a coax cable at 40 GHz can add approximately 4 dB per meter loss to the measurement path. A typical midsize to large Over-the-air (OTA) chamber with the VNA outside the chamber can easily require up to 5 meters of expensive microwave coax cable to connect the VNA ports to the source antenna and antenna under test (AUT) in the chamber. If 5 meters of cable is required on each port, up to 40 dB of extra IL can be added to the measurement path at 40 GHz. The effective dynamic range of the overall system is significantly reduced due to this cable loss.

Instability – Long cables also introduce instability into the measurement setup. With minor changes in ambient temperature or with moderate movement, coax cables can add several degrees of phase shift to a measurement, causing significant deviations. Because these phase deviations are due to environmental conditions, it is very difficult to de-embed the cable affects from the measurement in a reliable fashion.

New Improved Approach

The ShockLine ME7868A VNA (figure 1) addresses these issues. It is a unique, distributed modular 2-port VNA system that delivers the capabilities of a typical single chassis VNA but with the flexibility of portable remote ports. It makes IL testing over distance more convenient, less expensive, and more accurate and stable then existing mmWave VNA solutions.

ShockLine ME7868A VNA system
Figure 1: ShockLine ME7868A VNA system.

A groundbreaking technology - PhaseLync™ - is a key reason how this system changes long-distance S-parameter applications. With PhaseLync technology, 1-port compact, lightweight VNAs are phase synchronized to enable full 2-port vector S-parameter measurements. This architecture eliminates the need for long cables by bringing the VNA port to the DUT. This simplifies and improves the S-parameter measurements over distance in OTA, large vehicle, and similar shielding and propagation testing applications.

Engineers designing high-frequency products throughout the world are realizing the benefits of the unique ShockLine ME7868A. Here are just two examples.  

OTA Chamber Measurements

In Europe, a research lab sought to upgrade equipment used with a 20-meter tapered range for antenna testing. The legacy OTA chamber solution consisted of a 26.5 GHz spectrum analyzer and a lower frequency 3 GHz signal generator.

Frequency coverage of the OTA chamber needed to be enhanced. Different options were considered, including upgrading the signal generator to 26.5 GHz or replacing both instruments to support up to 40 GHz.  

A demo of the ShockLine ME7868A changed their mindset completely. By eliminating long test port cables with distributed ports, the innovative system (figure 2) improves the effective dynamic range and phase stability of their antenna IL measurements. Plus, with frequency coverage to 43.5 GHz, the customer is satisfying all its requirements with a single instrument.

OTA test system featuring ShockLine ME7868A
Figure 2: OTA test system featuring ShockLine ME7868A.

Propagation Measurements Over Long Distances

In Japan, a laboratory conducting electromagnetic compatibility (EMC) research needs to conduct propagation measurements over 10 meters at 38 GHz. The application involves making an IL measurement between one fixed antenna and a second antenna that moved slightly. The test configuration had an assortment of VNAs from multiple instrument manufacturers.

There were measurement issues when more than 10 meters of cable were used between antennas. It was discovered that moving the long cables in the setup introduced too much phase instability to enable a repeatable measurement.  

The ShockLine ME7868A 2-port VNA solved their issues. With two, small ShockLine MS46131A 1-port VNAs acting as distributed VNA ports, integration into their setup was easy. The phase compensation technology in PhaseLync enables the ME7868A to conduct a more stable phase measurement over the 10 meters of distance than would have been possible with the conventional VNAs and cables. With the ShockLine-based system, engineers can perform the repeatable measurements necessary to verify the designs more efficiently and accurately, giving them greater confidence in their products’ performance.

To learn more about S-parameter measurements in high-frequency designs, download this Improving High Frequency S-Parameter Measurements by Moving the VNA Port to the DUT application note.  

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