Characterizing multiport devices with a vector network analyzer (VNA) in the 75-to-110-GHz range has always been performed using an external frequency converter that increases the usable frequency range of the VNA. Such converters are invariably offered as options by VNA manufacturers and, while they generally offer adequate frequency coverage through millimeter-wave frequencies, are often limited in terms of test signal power and dynamic range. Rohde & Schwarz (www.rohde-schwarz.com) has taken a different approach to VNA frequency extension by creating its own frequency converter, the model R&S ZVA-Z110, which boasts as much as 110 dB dynamic range. The converter is designed for simple integration within the VNA's firmware and the ability to characterize devices with as many as six ports.
The R&S ZVA-Z110 converter (Fig. 1) works in conjunction with the company's flagship models R&S ZVA24, R&S ZVA40, R&S ZVA50, and R&S ZVT20 VNAs to make measurements from 75 to 110 GHz without an additional test source generator. The converter's dynamic range is greater than 95 dB and typically 110 dB, or as much as 15 dB more than competing solutions. An integrated mechanical attenuator allows RF output power to be varied by as much as 25 dB, a significant advantage when characterizing low-noise amplifiers (LNAs). Directivity with error correction is greater than 35 dB, and trace stability is +/-2 dB. The converter delivers RF output power of +2 dBm that can be reduced to -25 dBm by means of the attenuator (see table).
For each test setup, one signal generator is required to act as the common source for all local-oscillator (LO) signals for all extenders. Using an R&S ZVx VNA with four test ports (i.e., two internal sources) allows test setups with two frequency extenders for two-port measurements without the need for an external signal generator, cutting measurement time and cost to a minimum while extending the frequency range.
A test setup can also be configured with as many as six converters in conjunction with the R&S ZVT20 six-port VNA and an external signal generator. The six ports of the VNA provide the RF signal and the external signal generator supports the LO distribution network. The six-port R&S ZVT20 and converters can also be used to make measurements on frequency-converting waveguide devices. The RF stimulus signals for waveguide test ports 1 and 3 are delivered by different oscillators of the VNA, which makes it possible to apply stimulus signals to the waveguide test ports with a frequency offset.
Since any external components employed in a network analysis test setup add complexity, the designers of the R&S ZVA-Z110 frequency converter took pains to make it as small and light as possible, while also configuring the enclosure to allow easier access to the screw-connected flange joints. The fans traditionally used to cool millimeter- wave converters were eliminated by the design, which employs a passive cooling scheme. This provides silent operation and makes the unit usable in particle-sensitive environments such as clean rooms.
Optional firmware (R&S ZVA-K8) for the VNAs allows the converter to be fully configured and controlled by the instrument, with diagrams on the display that guide the user through the set-up process. The firmware allows the user to select the measurement set-up (two internal generators or the internal and one external generator), automatically configures the internal generators to provide the desired RF and LO power and all frequency conversion ratios, selects the test port type (e.g., WR10 waveguide) and calibration kit, and determines x-axis scaling. RF power delivered to the converter is automatically limited to keep it from being overdriven. Integration with the instrument also increases measurement speed, since no external controllers are required to operate the converter.
The R&S ZVA-Z110 can be operated below its specified minimum frequency (from 65 to 75 GHz), with reduced performance. When used with a waveguide power sensor, calibration of the analyzer's reference and measurement receiver power can be performed using the VNA's internal receivers, which when compared with direct measurement using a power meter provides wider dynamic range.
The converter comes with two exchangeable test port adapters that accommodate many waveguide sizes, a DC power supply, and two intermediate-frequency (IF) cables. Several options, including calibration kits, are also available. Rohde & Schwarz, 8661A Robert Fulton Drive, Columbia, MD 21046; (410) 910-7800, Internet: www.rohde-schwarz.
