Vector network analyzers (VNAs) are the workhorse test instruments of the microwave industry, every bit as home on a production line as in the lab. From the early days of the HP 8409 and HP 8510 series VNAs from Hewlett-Packard Co. (now Agilent Technologies), these instruments have gone through their share of changes. In any case, the right microwave VNA should meet the needs of an application's frequency range, while providing a broad dynamic range, accuracy, and measurement flexibility.
Modern microwave VNAs come in many package styles, from large rack-mount systems to portable, battery-powered units. The traditional rack-mount instrument, such as a four-channel ZVA Series VNA (Fig. 1) from Rohde & Schwarz, integrates precision test signal sources stabilized to a crystal oscillator.
Such a VNA provides four independent measurement channels for recording all four scattering (S) parameters on a device under test (DUT) over a frequency range of interest. Although it embeds a dedicated computer within its mainframe, it is typically connected to an external computer in many applications as part of an automatic-test-equipment (ATE) system with a variety of other measurement functions, including frequency, voltage, and power.
Versions of the ZVA microwave VNAs are available with four internal test sources with upper frequency limits to 67 GHz, and are capable of providing two-tone measurements of linearity for amplifiers and mixers. The analyzers can also be specified with intermediate-frequency (IF) bandwidths as wide as 30 MHz for performing pulse profiling measurements on amplifiers and mixers, with pulse resolution as fine as 12.5 ns. Models are available for frequency ranges of 300 kHz to 8 GHz as well as with coverage of 10 MHz to 24 GHz and at frequencies through 110 GHz, with typical power levels to +15 dBm or more and typical dynamic range of 140 dB or more. The analyzers offer differential measurement capability and measurement speed of 3.5 s per test point.
Because calibration is essential to achieving high accuracy with a VNA, a large number of calibration methods have been developed since the early days of these instruments. These techniques use well-understood passive circuit elements, such as transmission lines, attenuators, impedance matches, and reflections, to calibrate the reference levels of the VNA. As with many commercial microwave VNAs, the ZVA models support many different calibration methods, including through-reflect-line (TRL), line-reflect-line (LRL), through-reflect-match (TRM), through-open-match (TOM), and through-attenuation-symmetric network (TNA) techniques.
The ZVA analyzers, which run under a Windows XP Embedded operating system, allow over 100 measurement traces to be combined in any manner or assigned to 100 different channels. In fact, like many modern microwave VNAs, the ZVA analyzers are as well suited for production as for the laboratory, since test ports can be used in parallel.
Agilent Technologies has built a 45-year legacy of excellence in microwave VNAs, starting with the HP 8409A and HP 8510A models, and now offering a wide range of models in its PNA series (Fig. 2). The latter includes the economy PNA-L series instruments and the high-end PNA-X series analyzers. Low-cost PNA-L series analyzers can be specified with standard top frequencies from 6 GHz through 50 GHz as needed, while standard PNA analyzers are available with top frequencies of 13.5, 26.5, 43.5, 50.0, and 67.0 GHz for extremely broadband coverage.
Another major supplier in benchtop microwave VNAs is Anritsu Co., with its premium VNA instrument line known as the VectorStar family of analyzers. The firm's MS4640A Series VNAs (Fig. 3) provide outstanding performance from 70 kHz to 70 GHz, while the company's ME7838A Series analyzers include systems spanning a range from 70 kHz to 110 GHz in a single 1-mm coaxial test port. The MS4640A VNA boasts 100-dB dynamic range at 70 GHz with 80-dB low-level signal sensitivity.
Offering a variety of measurement solutions in modular PXI format, National Instruments has developed a PXI microwave VNA, its model NI-PXIe-5630 PXI VNA. With a frequency range of 10 MHz to 6 GHz and better than 100-dB dynamic range, the modular analyzer can execute sweep speeds of 400 s per point. The company offers a preconfigured NI Vector Network Analyzer Startup Kit that includes the NI PXIe-5630 VNA and all necessary hardware components for performing vector network analysis through 6 GHz.
The modular VNA works with an external computer running the flexible NI LabVIEW graphical development environment and LabWindows/CVI ANSI C programming interface. Using these, one can create automated tests for incorporating network analysis into design validation and production lines. The software-defined instrument approach makes it possible, for example, to add an NI PXI-2598 transfer switch for reverse S- parameter measurements.
Probing Portability
The Agilent N9923A FieldFox is part of a new movement in high-frequency VNAs, toward portability and in-field testing. The N9923A (Fig. 4) is available from 2 MHz to either 4 or 6 GHz, with 1-Hz frequency resolution to 3.2 GHz and 1.2-Hz frequency resolution beyond 3.2 GHz. It can measure all four S-parameters using from 101 through 10,001 data points in a measurement. It offers IF bandwidths of 300 Hz, 1 kHz, 3 kHz, 10 kHz, and 30 kHz. It can achieve a minimum 90-dB dynamic range from 2 MHz to 6 GHz with a 300-Hz IF bandwidth, along with typical dynamic range of 100 dB under those conditions. Test signal levels can be set from -40 to +5 dBm in 1-dB steps, with receiver compression (0.1 dB) occurring with a +10-dBm input signal level. The analyzer can be prepared for measurements using a number of different calibration methods, including a simple one-port calibration to correct for directivity, frequency response, and source-match errors as well as an (optional) two-port 12-term error-correction calibration.
Many of these new portable VNAs offer performance levels that compare favorably to benchtop or rack-mount instruments. The VNA Master model MS2028C from Anritsu Co., for example, has a frequency range of 5 kHz to 20 GHz (Fig. 5). The two-port instrument can measure all four S-parameters with a single connection using a 12-term error-correction algorithm. It provides fast measurement speed, with 350 s per data point, and provides results on a user-configurable four-quadrant display that can show all four S-parameters at once.
For those on tight budgets, the Planar 304/1 VNA from Copper Mountain Technologies is designed to run with an external personal computer. It measures all four S-parameters from 300 kHz to 3.2 GHz with measurement bandwidths from 1 Hz to 30 kHz (Fig. 6). It achieves 1-Hz frequency resolution with an output-power range of -55 to +10 dBm and 1 dB accuracy at the test port. The measurement speed is 125 s per point, and the compact, integrated instrument can deliver as many as 16 independent measurement channels, with as many as 16 data traces displayed in each channel window. The company offers potential users an opportunity to demo the test software.
For those on even tighter budgets, Electro Rent Corp. offers a wide selection of microwave VNAs as used equipment and for lease arrangements. During a recent check on the Electro Rent site, the firm was offering several Agilent VNAs from 45 MHz to 50 GHz, a number of two-port PNA-L analyzers with frequency range from 10 MHz to 20 GHz from Anritsu, and several four-port PNA-L analyzers from Anritsu with frequency coverage of 300 kHz to 20 GHz. Additional locations to search for good used microwave VNAs include Global Test Equipment, Test Equipment Connection, and ValueTronics International.