Optical Extenders Replace Coax in VNA Tests Over Distances Up to 1 Km
By Steve McCoy, Agilent Technologies
Most network analysis is performed on the test bench or in production environments where all ports of the device under test are readily available to the vector network analyzer (VNA).
However, applications such as test ranges, in-building, aircraft or shipboard RF distribution systems, and satellite communications terminals, require extremely long coaxial cable lengths.
As the loss of even the best-performing coax at high frequencies is enormous over long spans, the usual solution is to downconvert to lower frequencies, which introduces mixing products that must be dealt with to ensure measurement accuracy. The new Optical eXtenders for Instruments (OXI) Series of products (Figure 1) solve this problem by converting RF signals to optical wavelengths and vice versa, eliminating coaxial cables and replacing them with single-mode optical fiber that has negligible loss. The OXI solutions allow signals to be transmitted and received at up to 50 GHz with very high isolation over distances of at least 1 km. Models in the series are available to extend the test ports of Agilent’s PNA Series VNAs and as PXIe modules that exploit the modularity and scalability of the PXIe form factor.
To understand the problems associated with using coax over long distances, consider that a signal transmitted over only 100 ft. would incur loss of more than 25 dB even with high-performance, low-loss types designed for test and measurement. The cable alone would cost at least $10,000. In contrast, single-mode optical fiber typically suffers only about 0.2 dB of loss per kilometer. By using a preamplifier, loss incurred in RF-to-optical and optical-to-RF conversion can be dramatically reduced and noise figure improved as well. Consequently, replacing coax with fiber is an obvious choice, especially considering it offers an additional ability: to optically remote all of the functions, display, and USB peripherals connected to the VNA to the remote point using an optical USB 2.0 extender, another part of Agilent’s OXI family.
Optical solutions have been possible only relatively recently at frequencies up to 50 GHz thanks to the availability of drivers that can modulate an RF signal onto a single-mode fiber as an optical signal and detectors and drivers that convert the signal back to RF. For this, the microwave industry owes a debt of gratitude to the world of lightwave communications as it moves from 10 Gb/s to 40 Gb/s and soon to 100 Gb/s, driving optical component development.
Extending the PNA Measurement Plane
The optical port extenders in the OXI Series for the PNA Series VNAs include the local converter (U3020AY03) and remote microwave optical module (U3020AY04) that allow one or more ports of the instrument to be optically extended. The local converter changes the PNA’s REF, TEST, and SRC signals to optical signals and is connected to the remote optical module with three fiber optic cables, allowing measurement capability to be conducted at distances up to 1 km. The remote microwave optical module provides electrical-to-optical and optical-to-electrical signal conditioning for the local converter, and neither module requires external control. The local converter can support two-port extensions and additional converters can allow more ports to be accommodated. Models for 10 MHz to 26.5 GHz and 10 MHz to 50 GHz are available.
By using the optional USB 2.0 powered interface and USB-to-video converter, the complete PNA measurement environment including display and control can be operated remotely. If additional RF output power to the test port is required, an amplifier can be used and "ratioed" out of the measurement results. Attenuation can be applied to the return path to the PNA as well.
The Versatility of PXIe
PXIe modules benefit from the standard’s inherently compact, modular, scalable architecture, and Agilent’s OXI Series modules fully exploit its capability to significantly reduce cost and complexity. The new PXIe modules simply plug into a PXIe chassis and do not require a dedicated instrument controller. The user can mix and match modules at any time based on measurement needs.
The OXI Series includes:
2-slot 3U M9403A RF-to-optical converter and M9404A optical-to-RF converter
1-slot 3U M9405A preamplifier
2-slot 3U M9406A and M9407A USB 2.0 hub
2-slot 3U M9408A reflectometer
Complementary PXIe products include multimeters, waveform generators, local oscillators, digitizers, downconverters, and switch multiplexers.
The RF-to-optical converter modulates the RF signal onto a 1550-nm single mode optical signal and the optical-to-RF converter demodulates the optical signal and presents the resulting RF signal at its output. Both cards operate from 300 kHz to 26.5 GHz and optionally to 50 GHz. The preamplifier covers the same operating frequency range as the converters and delivers 30 dB of gain to improve noise figure and to overcome conversion loss. It has a 6 dB noise figure below 35 GHz and 8.5 dB to 26.5 GHz. The preamplifier is also available as an integrated part of each converter.
The preamplifier is a valuable addition to any remote system as there is a certain amount of noise associated with the link in typical electrical-to-optical and optical-to-electrical pairs. In Figure 2 there is about 34 dB of loss and about a 0.23 dB/GHz loss slope with an additional 5 dB above 22 GHz in a 1.5-km span. Noise figure is about 40 dB. By adding the preamplifier to the link (Figure 3), noise figure drops to 10 to 15 dB and gain/loss to 0 to -10 dB. At 18 GHz, for example, the result would be a 16.27 dB noise figure and 6.5 dB of loss (which remains flat across the 1.5 km span).
As with the solution for the PNA, the optical USB 2.0 ports extend full instrument capability, including a power sensor and E-Cal module. Calibration can be performed from the remote location as well as power measurements for verifying RF power levels at the remote device. A two-pair optical cable supports the transmit and receive paths from both the remote and modules.
Finally, when the VNA has a configurable test set, adding the reflectometer module allows full port extension to be achieved over the entire 1-km span. Even for short distances, the reflectometer extends the measurement plane and delivers more power to the device, allowing full two-port characterization that is not practical using coaxial cables. The reflectometer has a frequency range of 300 kHz to 50 GHz and can be used as an independent dual coupler as well.
Coaxial cable has always been an impediment to network analysis and calibration in applications that require long cable runs between both ends of the system. As there was no alternative, designers were required to resort to downconversion and its inherent mixing products, high-power amplifiers to compensate for cable losses and maintain signal levels, and other schemes. Agilent’s Optical eXtenders for Instruments products not only eliminate all of the problems associated with coaxial cable, but are easy to configure, provide associated features such as the ability to provide total control of the VNA, and dramatically reduce the number of elements required in the measurement system. More information is available at www.agilent.com/find/OXI.
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