Speedy VNA Spans 70 kHz To 70 GHz

Jan. 26, 2009
The widest-bandwidth member of this new line of high-performance vector network analyzers offers outstanding accuracy, dynamic range, and measurement speed to 70 GHz.

Bandwidth is crucial to a vector network analyzer (VNA), which is often called upon to make critical measurements on a passive or active device in order to develop a software model that is then used to optimize the design. In terms of VNA bandwidth, it doesn't get much better than the new VectorStar VNAs from Anritsu Co., which includes a model (MS4647A) with standard bandwidth of 10 MHz to 70 GHz (and optional extension to 70 kHz). The new family of premium, two-port VNAs aims not only at wide bandwidth but at outstanding dynamic range and accuracy, with fast measurement speed that makes them as well suited for the production line as for the laboratory.

The VectorStar VNA line includes the 10-MHz-to-20-GHz model MS4642A, the 10-MHz-to-40 GHz model MS4644A, and the 10-MHz-to-70-GHz model MS4647A (Fig. 1). All of the VNAs offer optional extension to 70 kHz and optional time-domain analysis capability. The VNAs support coaxial measurements to 110 GHz and measurements through 500 GHz in waveguide bands using frequency-extension units.

A VNA extending as low as 70 kHz provides seven octaves of test data well beyond test results available from instruments with a lower-frequency cutoff of 10 MHz. Such lower-frequency information offers greater insight into the lower-frequency and even DC-coupled behavior of a device under test (DUT). The measured lower-frequency scattering parameters (S-parameters) below 10 MHz can also be applied to creating more accurate device and component models in a computer-aided-engineering (CAE) software simulator, rather than simply using higher-frequency measurements to extrapolate the values of S-parameters at these close-to-DC frequencies. In support of their lower-frequency range, the VectorStar VNAs have been designed to provide increased stability below 1 GHz, eliminating coupler rolloff effects compared to existing commercial microwave VNAs.

Broad bandwidth in a VNA would provide small value without high accuracy, and the VectorStar VNAs provide the wide-dynamic-range performance needed to achieve high accuracy. The specified dynamic range at 70 GHz is 100 dB. That dynamic range is a measure of the difference between the sensitivity of the analyzer's receiver, and the amplitude of the largest signal that the receiver can measure before going into 0.1-dB signal compression. The receiver used in the VectorStar VNAs features sensitivity of -103 dBm at 70 GHz. At the other end of the dynamic range, the VNA receiver exhibits only 0.1-dB compression at 70 GHz with a +10-dBm input signal. The high accuracy of the VectorStar VNAs is also supported by a new Precision AutoCal and calibration kits based on K and V connectors for 70 kHz to 40 GHz and 70 kHz to 70 GHz, respectively. Calibrations with the Precision AutoCal routine help achieve residual directivity of 42 dB at 70 GHz and 50 dB at 20 GHz.

Measurement speed is as important in a VNA for research as for one making measurements in a production application. Faster measurement speed, which is also essential for tuning purposes, means more data captured during a given test time. VNAs have traditionally employed a number of methods to achieve faster tuning speeds, including using a test signal source in unlocked (unstabilized) operation. The VectorStar VNAs achieve measurement speed of 20 s/point without restrictions or resorting to unlocked frequency sweeps.

In order to achieve the broad bandwidth, high accuracy, and fast measurement speed, the VectorStar VNAs employ an innovative analog architecture, where a low-frequency VNA and high-frequency VNA are multiplexed for seamless broadband operation. Below 2.5 GHz, a superheterodyne mixer-based receiver with directional bridges is used for coverage down to 70 kHz. Above 2.5 GHz, a harmonic-sampling receiver with directional couplers provides high-frequency coverage. The receiver is based on nonlinear-transmissionline (NLTL) technology, also known as Shockline technology. Unlike traditional VNA architectures, these couplers are not stretched below their effective frequency range (1 GHz and below); thus, the usual sacrifice in raw directivity and coupling rolloff from using directional couplers down to 10 MHz is avoided. The result is high stability at lower frequencies.

The VectorStar VNAs incorporate software based on the Microsoft Windows XP Pro operating system (OS). Results are shown on a 26-cm touch-screen liquid-crystal-display (LCD) screen, with a variety of connectivity choices, including Ethernet, GPIB, and Universal Serial Bus (USB) 2.0 interfaces. The analyzers include enough memory and graphics capability for 16 independent measurement channels with 16 traces each and 13 markers per trace and as many as 25,000 data points per channel. As many as 100,000 points are available in single-channel mode for applications that require as many points as possible for analysis, such as when performing time-domain studies. For complex measurements, such as on frequency-conversion components and systems, as many as four external synthesizers can be connected to and controlled by a VectorStar VNA.

The system architecture of the VectorStar VNAs is based on two VNAs in parallel, with one for the lower frequencies and one for the higher frequencies. Both of the VNA sections share a common intermediate-frequency (IF) subsystem and common core frequency synthesizer (as LO) to minimize cost and complexity and maximize reliability. The approach provides relatively seamless frequency conversion over the operating bandwidth. The architecture is fully frequency synthesized for maximum application flexibility, allowing arbitrary programming while maintaining frequency accuracy and phase synchronization.

For example, the VNAs combine both FET and PIN diode switch technologies to achieve the best combination of high isolation and low insertion loss at a given frequency. Signal switching capability can be critical to overall VNA performance, with isolation requirements often greater than 120 dB. Insertion loss must also be low and power-handling capability must often exceed +20 dBm. GaAs FET switches have been known to provide excellent electrical performance at low frequencies, but tend to be limited in performance above 40 GHz. GaAs PIN switches are well suited to higher frequencies, but suffer degraded performance below the 1-to-100-MHz range. As a result, in the VectorStar VNAs GaAs FET switches are used in the low-band subsystem while GaAs PIN switches are used in the high-band subsystem. This hybrid switching function results in high isolation and less than 2.5 dB insertion loss at 2.5 GHz for the low-frequency section (Fig. 2). At the same time, the high-frequency section has more than 120 dB isolation with less than 8 dB insertion loss at 70 GHz and less than 5 dB insertion loss at 40 GHz.

A VNA's receiver establishes the instrument's noise floor as well as its compression limits. The front end can be based on a number of different approaches, including fundamental mixing, harmonic mixing, and harmonic sampling. Fundamental mixing can provide optimum noise performance, but is usually not practical at the high frequencies (70 GHz) of the VectorStar VNAs. A choice between harmonic mixing and sampling depends upon the harmonic number (Fig. 3). The sampling approach will lead to better noise performance above a harmonic mixing number of about 3. By matching the technology to the frequency range, fundamental mixing is used for the lowband section and harmonic sampling for the high-band section.

Along with the receiver architecture, the choice of device technology plays a large role in VNA performance. High isolation in the frequency-conversion signal paths can minimize crosstalk and improve accuracy. A leading cause of signal leakage in the RF-to-LO signal path in many frequency converters relates to the physical symmetry of the mixer baluns or sampler distribution network. Using a fully MMIC structure can ensure the symmetry required to achieve 50 dB RF-to-LO isolation and crosstalk of better than 120 dB in the VectorStar VNAs. This inherent symmetry also improves linearity at high frequencies. The port-referred third-order intercept (IP3) performance in the VectorStars generally exceeds +30 dBm.

For fast measurement speed in a VNA, the source and LO are critical. These signal sources must provide fast settling time and be spectrally clean with low noise floors to maintain low trace noise in the VNA. A tiered loop system was developed to achieve both goals without phase-locking the sources through the IF circuitry. Such a phase-lock approach can complicate cer-tain measurements, such as mixer, multiplier, and intermodulation-distortion (IMD) measurements, where the RF source and LO may be at different frequencies.

A VNA receiver's IF system also plays a role in noise floor and trace noise as well as contributing to measurement speed. The IF section should contribute only minimally to the overall receiver noise figure, while handling wide signal dynamic ranges (140 dB), minimize spurious content, minimize distortion, and not hamper measurement speed. The VectorStar VNAs achieve this through a judicious combination of analog and digital filtering, a flexible variable gain system, and a high-speed analog-todigital- conversion (ADC) system.

For improved automatic calibration in the VectorStar VNAs, a large number of impedance states are used, with lowloss transmission structures serving as calibration standards. A novel low-loss switching structure allows a large number of impedance states to be covered even at 70 GHz. Electronic switching was vital to achieving repeatability, stability, and longevity in the automatic calibration system. VectorStar VNAs are calibrated at the factory with highquality line-reflect-line (LRL) airlines, which contributes to automatic calibrations that can deliver high residual directivity and source match in the 40-dB range through 70 GHz.

In the VectorStar VNAs, time-domain selections for all VectorStar traces are independent. This allows frequency- and time-domain data to be shown simultaneously in any format for any parameter. The VectorStar VNAs feature a wide range of time-domain window and gate shapes, including Kaiser and Dolph- Chebyshev windows and gates.

Many improvements have been made to the embedding/de-embedding system. In addition to embedding and de-embedding .s2p files (representing different networks) for a DUT, a number of circuit elements, such as resistive elements, transmission lines (with loss) and LC networks, can be cascaded and edited for each port. The VNAs include a network extraction program to generate files for de-embedding. Anritsu Co., 490 Jarvis Dr., Morgan Hill, CA 95037; (408) 778-2000, FAX: (408) 201-1093, Internet: www.us.anritsu.com.

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