Portable vector network analyzers (VNAs) have brought new mobility to one of the most trusted instruments in the RF/microwave industry. The new model LA19-13-02 portable analyzer from LA Techniques (www.latechniques.com), for example, features a compact footprint but full S-parameter measurement capability from 3 MHz to 3 GHz. Of course, a major consideration before acquiring a portable VNA is its accuracy and reliability.

Recently, a collaboration between LA Techniques and the United Kingdom's national measurement standards laboratory, the National Physical Laboratory (NPL), focused on establishing reliable accuracy metrics for this low-cost portable VNA (Fig. 1). This work built on other recent work where new calibration methods were developed to enable this type of VNA to achieve unprecedented levels of accuracy using budget priced calibration kits.^1,2

In order to understand this assessment of portable VNA accuracy and reliability, it may be useful to examine the portable VNA's system architecture, and then review the methods used to characterize the calibration standards, including the concept of load "breeding." In load breeding, a parent load can be used to produce offspring loads with characterized performance so that the offspring loads can also be used as reliable VNA calibration standards. Following an evaluation of these loads, this article will review the achievable accuracy for this type of VNA and shows some measurement comparisons with the UK's national standard facilities to demonstrate the accuracy statements.

Figure 2 shows a schematic diagram of the LA19-13-02 VNA system. The unit operates under the control of an external personal computer (PC). The VNA's internal controller is tasked with setting the hardware operation and carrying out partial averaging of the measured data in order to minimize the traffic through the serial interface.

The VNA employs a tuned receiver with mixer-based frequency down-converters and synchronous detectors. A noise-cancellation technique allows very low trace noise to be achieved with a simple hardware implementation. The unit includes two frequency synthesizers, one to generate the test signal (at levels from –20 to 0 dBm) and another to generate the local-oscillator (LO) signals for the receiver. The synthesizers are based on direct-digital-synthesis (DDS) technology and achieve a worst-case settling time of 290 µs to within 1 kHz of a new frequency with a worst-case phase noise of –71 dBc/Hz offset 22 kHz from the carrier.

Because of space constraints, directional bridges (C1 and C2 in Fig. 2) are used to extract reflection and transmission signals. Bridges are typically more compact than other types of directional couplers (e.g., stripline edge couplers) for very wideband applications albeit with a higher insertion loss. Included in the VNA are wideband bias Ts to allow as much as 250 mA DC bias injection when testing active devices such as transistors and amplifiers.

The user-interface (UI) software runs on the external PC and carries out virtually all of the data processing. It includes numerous features such as real time de-embedding, automatic and manual reference plane extension, importing and displaying of data files for live comparisons with measurement data, vector mathematics on memory traces, and numerous marker functions such as peak and minimum search and 3-dB-bandwidth calculations.

Control of the instrument from other programs such as LabView® from National Instruments (www.ni.com) and VEE® from Agilent Technologies (www.agilent.com) is possible using the dynamic-link-library (DLL) code supplied with the instrument. The library supports high-level commands to simplify the control of the VNA in automatic-test-equipment (ATE) systems, making control of the VNA in automated test systems easy.

The LA19-13-02 VNA is calibrated using the short-open-load-through (SOLT) technique. The calibration kits (Fig. 3) supplied with the VNA contain budget-priced components so that the overall cost of the VNA package is kept to a minimum. However, the performance of these budget-priced components is greatly enhanced by applying accurate characterization routines using the VNA's firmware. This results in state-of-the-art VNA performance, i.e., as good as can be achieved using high-precision calibration standards and/or more sophisticated calibration routines.

In conventional calibration schemes, the VNA standards are often assumed to have ideal characteristics. For example, short- and open-circuit standards are assumed to have a magnitude voltage reflection coefficient (VRC) equal to unity. Similarly, the load standard is assumed to have a VRC of zero, i.e., it is assumed to be a perfect match. For short-circuit/open-circuit standards, such assumptions are reasonable for most applications. However, the assumption that the load standard is a perfect match can never truly be met. This is particularly relevant when the load is not a high-precision component (as is the case here) and this causes significant residual errors to remain in the VNA after a conventional calibration.

The approach to calibration used by the LA19-13-02 VNA acknowledges that the load standard does not provide a perfect match and uses measurement data to 'characterize' the load so that its nonzero VRC can be taken into account and used during the calibration process. This method continues to work well even when the calibration load produces significant amounts of reflection—a condition that is usually associated with poor calibration performance. The technique, therefore, is particularly useful when low-cost components are used as the standards. Under these conditions, the only requirement is that the measured value of the standard is repeatable, and this is achieved using a precision connector at the device interface.

The characterization of the load standard involves obtaining reliable measurements of the load at DC and RF. Specifically, the resistance is measured at DC and the complex VRC is measured at RF. Only modest accuracy is required for the DC measurement, e.g., to within a few milliohms, and this can be achieved using an off-the-shelf calibrated digital ohmmeter. Greater accuracy is required for the complex VRC measurements, and these are made over a range of frequencies that corresponds to the VNA's bandwidth, i.e., 3 MHz to 3 GHz. These RF measurements were originally supplied to the VNA manufacturer (LA Techniques) by NPL using the UK's primary national standard measurement system.