Portable VNAs Tune Filters In The Field

Aug. 19, 2010
The latest additions to a line of handheld portable vector network analyzers provide frequency coverage from 5 kHz to 20 GHz under battery power, with an option for time-domain capability.

Vector network analyzers (VNAs) were once found only in a facility's equipment racks. But compact instruments like the VNA Master line from Anritsu Co. (www.us.anritsu.com) pack two-port VNA measurement capability into an enclosure about the size of a notepad computer, with equal portability. These portable test tools allow engineers to tune filters and phase-match cables on site and in the field. And with the latest editions of VNA Masters offering high-frequency coverage and optional time-domain capabilities, the portable instruments are ideal for applications through 20 GHz, including using the time domain to simplify filtering tuning at base stations and cell sites.

The most recent generation of VNA Master portable VNAs (Fig. 1) includes models with extended frequency coverage of 5 kHz to 6 GHz and 5 kHz to 20 GHz. Compared to earlier VNA Master models, the low-frequency end has been extended (from 2 MHz) and the highfrequency end has more than tripled (from a former top frequency of 6 GHz). The measurement speed of the newer models has also increased, from to 0.5 ms/point, to facilitate real-time tuning of resonant circuits, such as filters. And operators of the VNA Master instruments have the flexibility of a variable intermediate-frequency (IF) bandwidth from 10 Hz to 100 kHz; in the earlier VNA Masters, the IF bandwidth was fixed at 1 kHz. Another enhancement in this new generation of portable VNAs is the availability of optional time-domain capability (Fig. 2).

Instruments in the VNA Master family show results on a bright, 8.4-in. fullcolor, thin-film-transistor (TFT) touchscreen display. The display can show one, two, three, or four traces at once. Combined with the one-connection, fully reversing switched-source capabilities of the portable VNAs, operators can make a single set of cable connections to a device under test (DUT) while measuring and displaying 12-term errorcorrected magnitude and phase for all four S-parameters (Fig. 3).

To facilitate such simple S-parameter measurements, the latest generation of VNA Master portable VNAs features a three-receiver architecture. One of the receivers is dedicated to signals incident on port 1 (for measuring S21 and S11), one is dedicated to signals incident on port 2 (for measuring S12 and S22), and one serves as a reference receiver. A switch routes test signals to either port 1 (to the port 1 receiver and the reference receiver) or port 2 (to the port 2 receiver and the reference receiver). The portable analyzers contain 2 GB internal memory to store more than 4000 traces and more than 4000 measurement setups. For ease of importing and exporting data, they provide an Ethernet port and a pair of USB 2 ports.

The measurement speeds of the portable VNA Masters (faster than 1 ms/ point) make them well suited for infield filter tuning and phase matching of cables in phased-array radar systems. These instruments also provide the accuracy associated with benchtop instruments, with S21 uncertainty of a mere 0.2 dB through 20 GHz. They can operate for more than two hours on a battery charge and include a memory card slot for data storage and data transfer. All of the VNA Master portable VNAs measure just 12.4 x 8.3 x 3.1 in. and weigh about 8 lbs., making them portable enough to bring the VNA to the DUT (Fig. 4), rather than bringing the DUT to the analyzer.

Tuning a multiple-resonator filter, such as a bandpass filter, can be difficult, even in a laboratory. Practical bandpass filters might have from five to eight poles.The pole count is usually increased due to the need for more stopband rejection, with the tradeoff of greater complexity in passband filter tuning. For a bandpass filter, for example, resonators are tuned such that the coupling between resonators satisifies filter requirements for input and output VSWR, passband insertion loss, passband amplitude ripple, and out-of-band rejection. In the field, such as in a broadcast tower or a cellular communications base station, a filter may even be connected to another component, such as a power combiner, as part of a multifunction assembly. Filter testing is typically performed by exciting a device under test (DUT) with a swept-frequency source and evaluating the filter's in-band and out-of-band responses. This can be done with a VNA by measuring the S-parameters at each port and path through the DUT.

Tuning a filter with a VNA can be done in a number of ways. Filters can be tuned according to a template response that may be based on a "golden" filter, a kind of "reference" component that provides performance closest to the ideal response. This measurement is then stored in the memory of the VNA for comparison to other filter measurements. The response for an ideal filter can also be created by modeling the desired filter with a computer-aidedengineering tool, such as Microwave Office from AWR or the Advanced Design System (ADS) suite of design programs from Agilent Technologies. These simulated responses are then saved in the memory of the VNA for comparison with measured data on actual filters.

Yet another means of tuning filters, with a properly equipped VNA, is to adjust resonators and coupling while studying the effects of the tuning in the time domain. Unlike a frequencydomain display, which shows magnitude or phase across a frequency range, timedomain displays can show the individual responses of a filter's resonators separated by time or distance through the DUT. In a VNA, time-domain analysis was once associated with fixed-site test systems, but the VNA Master instruments bring this capability to the field.

Efficient filter tuning requires the capability of measuring and displaying at least two S-parameters at once. For example, in tuning a bandpass filter for a flat passband response (by measuring and viewing its S21 response), the effects of reflected signals should also be studied by observing the filter's S11 return-loss response (Fig. 5). Of course, the effects of adjustments made while tuning should appear in real time on an analyzer's screen, dictating fast tuning and display refresh rates for the VNA. The latest generation of VNA Masters achieves real-time displays of performance changes from tuning by means of 0.5 ms/point data refresh rates.

In applying the time domain to filter tuning, it is important to consider the path through the filter as it interacts with the test signal from the VNA's built-in source. Since the newest versions of the VNA Masters offer automatic source switching between ports and multiplegraph display capabilities, it is possible to look at both the input and output ports of a filter under test at the same time. For example, two measurement channels can be used to measure the return loss (S11) for the resonators starting from the input port, and the reverse return loss (S22) starting from the output port. For the S11 measurements, the first resonator in the time-domain plot will be the one closest to the filter's input port. For the S22 measurements, the first resonator represented in a time-domain plot will be the one closest to the filter's output port.

Since the time-domain data is based on an inverse FFT of the measured frequency domain data, the frequency span should be selected to include all of the passband and some of the skirts from the response of a filter under test. As a practical tip, the VNA's measured frequency span should be from two to five times the bandwidth of the filter to provide the optimal resolution for tuning the filter's resonators.

In support of field tuning of filters, the VNA Masters allow users to employ an arbitrary number of data points to optimize the instrument's display resolution when evaluating both broadband and passband characteristics. The portable VNAs also offer an integral Global Positioning System (GPS) receiver as an option, for time/location stamping of measurement data for simple identification of different test sites. They can also be equipped with an option that allows power monitoring through 20 GHz when using an additional power detector. The option enables a power measurement range of -50 to +20 dBm and a total power display range of -80 to +80 dBm. The analyzers also feature flexible marker readouts, and can provide output data in a variety of formats, including .txt, .csv, and .s2p file formats, for ease of export to external computers for further analysis and archiving. The new versions of the Vector Master VNAs also provide waveguide support for testing filters and other waveguide components in satellite-communications (satcom) and point-to-point-radio systems.

In addition to tuning such components as filters and cables in the field, this most recent family of VNA Masters incorporates new capabilities, such as lowpass processing and gating, which equip them for more esoteric applications, such as measuring the moisture content of materials samples or the stealth characteristics of stealth structures in military systems. The latest series of VNA Masters offers a number of additional options, including power-monitoring capability (with an external power sensor), a built-in bias tee for active device testing, a vector voltmeter for precision phase measurements, differential (balanced) S-parameter measurement capability, and a secure data option where test data can only be stored on a removable storage medium, such as a Flash memory drive. Anritsu Co., 490 Jarvis Dr., Morgan Hill, CA 95037-2809; (408) 778-2000, FAX: (408) 776-1744, Internet: www.us.anritsu.com.

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