The LA19-14-06 instrument is a compact vector network analyzer (VNA) module designed to be embedded into original equipment manufacturer (OEM) applications. It is a turnkey solution that enables fast and flexible microwave vector measurements within an embedded system. The module is provided with extensive cross-platform software and programming examples are available in multiple languages. The instrument features a novel calibration store feature with a patent application that aids temperature measurement stability over time.
VECTOR NETWORK ANALYZERS
VNAs are increasingly employed in medical and industrial processes, requiring microwave reflection and transmission measurements. Recent examples of novel VNA applications include detecting total dissolved solids in steam boilers, detecting corrosion under insulation in steel pipes, detecting positive margins in breast cancer surgery and detecting biomarkers in assessing traumatic brain injury. These novel, dedicated applications are challenging to serve using traditional desktop or handheld VNAs mainly because of cost, size and frequency and practicality of calibration.
LA Techniques Ltd has developed an OEM VNA module to address these challenges. With this development, the company plans to enable the potential of VNAs to be fully realized in embedded applications. Using the module can significantly reduce the time-to-market, development and production costs of any new embedded application involving RF vector measurements.
The OEM VNA module operates from 300 kHz to 6.4 GHz. The compact module measures only 117 × 69 × 19 mm. It provides a measurement speed down to 100 μs per complex reflection coefficient measurement point, a low distortion test signal with 10 Hz setting resolution and up to 10,001 measurement points. It features a unique calibration store that significantly reduces temperature drift of reflection measurements and reduces the number of calibrations required over time. Optionally, the VNA module can also measure complex transmission coefficients.
FAST AND FLEXIBLE SOFTWARE SUPPORT
Figure 1 OEM VNA module can be paired with a Raspberry Pi.
Extensive cross-platform software support for the new OEM VNA is provided. A user interface and SCPI control are available to prototype and develop new applications rapidly. Programming examples are available in the C++, C and Python programming languages to facilitate the transition of new applications into production. Out-of-the-box, the software can be run on Windows, Linux, macOS and Raspberry Pi. It can be made available for other embedded platforms on request. Figure 1 illustrates a possible remote access arrangement.
CALIBRATION TEMPERATURE DRIFT COMPENSATION
VNA calibration is used to remove inherent errors introduced by imperfections in the VNA as well as test fixtures and cables between the VNA and the DUT. In addition, there will be a temperature dependence, mainly due to variations in the characteristics of circuit components within the VNA. In the case of reflection measurements, these imperfections can be conveniently modeled by three error terms: e11 (directivity), e22 (source match) and e121 (frequency tracking). Conventionally, these errors can be corrected by calibrating immediately before use.
Figure 2 OEM VNA module circuit with built-in electronic calibrator.
The OEM VNA module will likely be used over a wide range of ambient temperatures. In many applications, it will not be practical to perform a calibration before use to reduce the effect of temperature drift. The LA19-14-06 OEM VNA module employs an internal electronic calibrator, a new technique to reduce drift and improve the quality of reflection measurements by refreshing the external calibration. The arrangement used is shown in Figure 2.
STORING AND REFRESHING A CALIBRATION
Once a conventional calibration using an external calibration kit is complete, the user can make the calibration resilient to temperature drift by immediately measuring the calibrator’s internal short, open and load (SOL) standards and storing these measurements in non-volatile memory. New algorithms developed by LA Techniques Ltd can then use these measurements to remove the effect of any temperature drift later in the measurements by refreshing the calibration, which computes a new set of error terms e11, e22 and e121. The ability to instantly compute a new set of error terms makes the feature unique and potentially transparent to the use of the module.
The measurements of the calibrator’s embedded SOL standards are not meaningful by themselves. They cannot be used directly for a conventional reflection measurement calibration. The advantage of the novel calibration store/refresh technique is that, with the new mathematical processing developed, the internal standards do not have to be known or fabricated accurately to be later used to remove the effects of temperature drift from a conventional reflection measurement calibration.
The advantage of using the embedded electronic calibrator can be seen by measuring a device with a suitable reflection coefficient and observing how the measurement varies as the VNA temperature varies. A mismatched device with a nominal linear reflection coefficient of 0.25 (12 dB return loss) was chosen for this test. The VNA was calibrated with an external calibration kit when the VNA case temperature was 32°C. The DUT was then measured at 32°C, 10°C and 47°C. Figure 3 shows the results with and without the temperature stabilization algorithm at a temperature change of -22°C and Figure 4 shows the results at a temperature change of +15°C.
Figure 3 Measured S11 magnitude and phase deviation (ΔT = -22°C)
Figure 4 Measured S11 magnitude and phase deviation (ΔT = +15°C).
Conclusion
A new, compact OEM VNA module has been described with a module that includes an embedded electronic calibrator. This module, together with a novel algorithm, can minimize the effect of temperature drift. This provides clear benefits in reducing measurement errors caused by large changes in the operating temperature of the VNA, without the need for an operator to perform a new calibration. The low-cost module is supplied with fast and flexible software options, making it ideally suited for many dedicated OEM and embedded applications.
LA Techniques Ltd
Surbiton, U.K.
www.latechniques.net
AAI Robotics Ltd
Cambridge, U.K.
www.aairobotics.com