A network analyser for today’s technology
01 September 2019
Component characterisation nowadays can be a challenging task, due to the complexity of DUTs, meaning network analysers must meet various diverse & stringent requirements.
This article was originally featured in the September 2019 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.
In the aerospace and defence sector, the emphasis is on stable data and reproducible measurements. Research institutes and universities demand top performance and versatility from a measuring instrument. Andreas Henkel, Product Manager, Network Analysis at T&M experts, Rohde & Schwarz explains how its new R&S ZNA high-end vector network analyser can meet the needs of both user groups – as well as other demanding users...
To fulfill the growing requirements on high-frequency components in practical applications, test and measurement equipment must be able to characterise components quickly and precisely. This requires excellent RF properties and versatile measurement functions. However, extensive instrument settings are usually necessary to use these functions, and it takes a lot of experience and familiarity to set them up properly. Automatic test configuration and operating aids that support users can significantly boost productivity, especially when working with network analysers. Development of the R&S ZNA vector network analyser (VNA) was driven by these two main objectives: excellent data and efficient operation.
The R&S ZNA offers the flexibility necessary to master current and future test requirements for characterising active and passive components. Its modular, high-performance hardware architecture, with four signal sources and eight measurement receivers, supports complex tasks such as intermodulation measurements on mixers or beamforming and MIMO tests on 5G antenna arrays. The extremely low noise floor of less than 0.001 dB at 1 kHz IF bandwidth and an outstanding dynamic range of up to 170 dB provide the basis for stable and reproducible measurements.
A market innovation is its DUT-centric operating concept, with configuration wizards that significantly accelerate and simplify instrument setup. The R&S ZNA is the first VNA to feature touch-only operation (Figure 2). This enables application-based configuration of the user interface to simplify and accelerate test setups as much as possible.
The R&S ZNA is initially available with frequency ranges up to 26.5 GHz or 43.5 GHz and with two or four test ports each. The 43.5 GHz version is available with a choice of 2.92 mm or 2.4 mm connectors. The 2.4 mm version is specified over the entire frequency range. The 2.92 mm version is specified up to 40 GHz, with typical values provided for the range from 40 GHz to 43.5 GHz.
Mixer and amplifier characterisation easier than ever
Configuring measurement traces and various test parameters to determine the mixing loss in terms of amplitude and phase, as well as for matching and isolation measurements, is usually a time-consuming task. The DUT-centric operating concept of the R&S ZNA make this task unnecessary. After selecting the DUT type and defining some basic parameters, such as the frequency range and power level, the user directly selects the test parameters and quantities, and the instrument configures the test setup. This means measurements can be configured quickly, without specific knowledge of the optimal analyser settings, which saves time and ensures high reproducibility.
The phase-coherent signal sources allow phase delay and group delay measurements without a reference mixer. The second internal local oscillator (LO) enables simultaneous measurement of RF and IF signals, resulting in higher measurement accuracy and doubling the measurement speed compared to previous test approaches.
Intermodulation measurements on active DUTs can be performed as a frequency sweep with fixed or variable carrier spacing, or as a level sweep. Digital automatic level control (ALC) delivers the precise amplitude of both carriers over the entire frequency range, regardless of the DUT’s input reflection coefficient.
The electronic power sweep range up to 100 dB (Figure 3) significantly simplifies compression point measurements on amplifiers and eliminates the influence of switching operations that occur with mechanical attenuators. The FFT-based spectrum analysis enables deeper analysis of the behavior of a DUT when S-parameter measurements are not sufficient. Interference signals in converters and TX/RX modules are detected quickly, while the measurement setup does not affect the measurement result thanks to the scalar system error correction.
The four internal pulse modulators, which can be driven either by internal or external pulse generators, enable point-in-pulse measurements with up to 30 MHz measurement bandwidth. They deliver pulse profile measurements up to 50 times faster than conventional measurements, with a resolution as fine as 8 ns. This capability is often required in the A&D and wireless sectors, where many amplifiers can only be tested in pulse mode.
Quick and reliable measurements on frequency converters without LO access
Frequency converters without LO access, such as are used in the satellite industry, are difficult to measure due to their frequency drift. To solve this problem, Rohde & Schwarz has developed a patented test solution for determining phase delay and group delay. It uses a two-tone method to simultaneously measure both signal tones, making it immune to the effects of frequency converter drift. Since both tones are affected by this drift, they cancel each other out in the measurement result. The R&S ZNA also features an LO tracking function to compensate for drift effects that exceed the measurement bandwidth. For such an application, the second internal LO doubles the measurement speed.
Highest dynamic range for filter measurements
The high-rejection filters used in A&D applications and base stations impose stringent requirements on the dynamic measurement range of network analysers.
Not only is a wide dynamic range essential for measuring very low signal levels, it also enables the use of a larger IF bandwidth for the desired dynamic range and leads to faster and simpler filter tuning.
In reverse coupler mode, the R&S ZNA has a typical dynamic range of up to 170 dB (Figure 4). A segmented sweep with different measurement settings for the passband and cutoff band prevents saturation of the measurement receiver in the filter passband.
Fast and accurate antenna characterisation
The high sensitivity and wide dynamic range of the R&S ZNA are also required for fast antenna measurements (near-field and far-field measurements in CATRs, radar cross-section). The direct IF inputs on the rear of the instrument for frequencies up to 1 GHz cover a wide spectrum of IF frequencies used by external mixers in antenna systems. The parallel measurement receivers enable amplitude and phase recording of up to eight signals, which means the horizontal and vertical polarisation of measurement and reference antennas can be determined simultaneously.
Frequency converters for measurements up into the terahertz range
In addition to basic research, imaging, antenna measurements and material measurements, commercial applications such as automotive radar (77/79 GHz) and mobile communications (5G NR) in the millimetre wave and terahertz range are becoming more common. The R&S ZNA can be extended up to these frequencies with frequency converters (Figure 6). Rohde & Schwarz frequency converters feature high output levels and a wide dynamic range – both important for on-wafer measurements and antenna characterisation. By using the direct IF inputs, effectively bypassing the input mixers, the dynamic range can be improved by an additional 7 dB. Up to four converters can be connected to the R&S ZNA to characterise frequency-converting multiport and differential DUTs. Even with a two-port device, two converters can be used without an external signal generator for the LO signal.
Calibration methods for every measurement task
A broad spectrum of calibration methods, including traditional TOSM, TRM, TSM, TOM, TRL/LRL, UOSM and adapter removal, ensure accurate measurement results for every measurement task (Figure 5). Digital ALC compensates for thermal drift after calibration. Manual economy and high-end calibration kits with individual S-parameter characterisation, automatic calibration modules with two or four ports, inline calibration modules and power sensors for level calibration can be used for calibration according to the measurement task.
R&S ZNA users can easily keep pace with the rising requirements of RF technology. With its high-performance and function-rich hardware, it can handle even the most challenging measurement tasks. The analyser has four internal phase-coherent signal sources that enable independent control of the signal frequency at each port, as well as phase measurements on mixers. It has two internal local oscillators, a true multichannel receiver architecture, pulse modulators and extensive trigger and synchronisation options. These hardware features make the R&S ZNA a universal, compact test system for characterising passive and active DUTs. Intermodulation measurements on mixers and receivers can be performed without external signal generators, minimising test times and simplifying test configuration. Finally, the DUT-centric approach boosts productivity with a configuration wizard and leads to valid measurement results without detours. The touch-only operation with two independent screens is unique.
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