Eight-Port VNA Aids Production Testing

Oct. 18, 2005
Eight completely independent test ports help to simplify and speed the evaluation of multiport and balanced devices from 300 kHz to 8 GHz.

Increasing numbers of multiport and balanced devices have increased the complexity of RF/microwave testing. Since balanced (differential) components and multiport devices require more test channels, conventional four-port vector network analyzers (VNAs) must literally work double-time to perform the necessary measurements on these components. One way to simplify and speed the measurements is to add more test channels, and this is the direct approach taken in the industry's first eight-port VNA, the 300-kHz-to-8-GHz model ZVT 8 from Rohde & Schwarz (Columbia, MD). Each of the analyzer's eight ports features its own reflectometer (VSWR bridge, measurement receiver, and reference receiver) and the reflectometers have independent generators. As a result, devices with multiple ports and even several multiport or balanced devices can be tested because all receivers can simultaneously capture, process, and transfer data from different measurements without reconnection.

A basic ZVT 8 (see figure) has two ports. Additional ports and accompanying generator/receivers can be added as options to the full complement of eight. The analyzer offers more than 120 dB of dynamic range over its full frequency range, and a power sweep range of –40 to +13 dBm. Trace noise is typically less than 0.01 dB. The analyzer takes no more than 8 ms to capture a trace of 201 points. General specifications for the ZVT 8 are shown in the table.

The ZVT 8 is designed for ease of use, even when measuring multiple parameters. The instrument is based on the Windows XP Embedded operating system and has both IEEE-488 and 10/100BaseT Ethernet interfaces for network connections. Local operation is achieved using an external monitor, keyboard, and mouse. Measurement results can be placed into various word processors or spreadsheets by saving screen contents as bmp, png, .jpg, or emf files, and trace data can be stored as Touchstone (*.snp), ASCII (*.csv), or Matlab (*.dat) files.

The ZVT 8 simultaneously measures all paths of a device in one direction, which considerably reduces the number of sweeps in multiport measurements and increases measurement speed. It also eliminates the attenuation between test ports and measurement receivers that is typical of multiport analyzers using multiport systems based on switching matrices. The instrument's receivers use the fundamental mixing technique that is now standard on all of the company's vector network analyzers, providing increased dynamic range and reduced trace noise.

The multigenerator technique, with its separate generator for each pair of reflectometers, allows test ports to be organized in groups that simultaneously perform measurements independently of the others. Consequently, several devices or several paths of one device can be measured in parallel. Different instrument test routines can be stored on hard disk and loaded simultaneously into random-access memory (RAM). In local operation, the mouse is used to select windows with different setups, and the user interface is simple and straightforward. Under remote control, test routines can be changes in typically less than 10 ms. Most network analyzers require at least 1 s to make this transition.

The analyzer supports as many as 100 channels, each with as many as 100 traces. By successively processing different channels, the ZVT 8 can characterize a device under test (DUT) under different conditions on each channel.

The ZVT 8 can be optionally configured for direct access to the generators and receivers of all reflectometers, sending the signal paths of the receivers and generators directly to the front panel via SMA connectors. This bypasses the VSWR bridges (and accompanying loss), so measurements can be performed at higher output levels and higher sensitivity, and high-blocking devices can be measured with a dynamic range of up to 140 dB at a 10-Hz IF bandwidth.

With each test port equipped with a dedicated reference receiver, a variety of calibration techniques can be employed in addition to standard through-open-short-match (TOSM) two-port calibration. Seven-term calibration methods such as through-open-match (TOM), through-reflect-match (TRM), and through-reflect-line (TRL) techniques can also be performed as well as multiport techniques. The ZVT8 also supports network/test fixture embedding and deembedding at all ports. Rohde & Schwarz, 8661A Robert Fulton Dr., Columbia, MD 21046; (410) 910-7800, FAX: (410) 910-7801, Internet: www.rohde-schwarz.com/usa.

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