Designing a clean microwave signal source in the VXI format provides many interesting challenges from two very different perspectives. From a signal generator standpoint, good RF characteristics, spectral purity and RF power, fast frequency switching speed, and broadband and narrowband coverage are needed. From the VXI viewpoint, the requirements encompass programmability, flexibility and restricted physical dimensions.
Modern RF Architecture for Optimum Performance
A key performance criteria in microwave signal generators is spectral purity. Spectral purity includes signal characteristics such as harmonic, subharmonic, spurious, and single-sideband phase-noise content. Achieving good RF characteristics is always difficult. And accomplishing good RF characteristics in a VXI package, where the slots available for each instrument are always at a premium, provides additional challenges.
Using a clean RF source ensures that the test is measuring the device and not the generator. The best way to generate a clean signal is to start with a clean signal source. Also, if broadband measurements are required, a source that can be tuned over a wide frequency range is important.
The yttrium-iron-garnet (YIG) oscillator is the best candidate for achieving these goals. The YIG oscillator is a current-controlled frequency-tuned device providing phase noise in the -130-dBc range at 1-MHz offset from the carrier.
Figure 1 provides a typical example of a YIG-based microwave signal source using indirect frequency synthesis. Although the close-in phase noise of an open-loop YIG is less than desirable for some measurements, close-in phase noise is improved by the action of the phase-locked loop (PLL) circuit.
With the PLL, the output of the YIG takes on the frequency accuracy and phase- noise characteristics of the crystal oscillator. This improves the phase noise of the YIG to levels typically better than -97 dBc at 100-kHz offset.
Harmonics
The output of the YIG oscillator generates harmonics of the fundamental frequency in the range of -12 dBc to -20 dBc. Harmonics at this level can affect the accuracy of a measurement by as much as ±1 dB when the detector is measuring the fundamental and second harmonic simultaneously.
A typical method for attenuating harmonics to an acceptable level uses low-pass filters. Harmonics in the range of -50 dBc should be achievable when using proper filtering techniques, resulting in negligible measurement errors.
Frequency Accuracy
The output of the YIG oscillator is monitored through a sampling mixer that provides frequency information to the PLL. The PLL controls the frequency of the YIG oscillator so the output frequency has the same accuracy and stability as that of the fixed-reference oscillator.
The typical reference oscillator is a 10-MHz crystal with a frequency stability of one part-per-million per year. There is a direct relationship between the output stability of the crystal and the frequency output of the YIG. For instance, if the reference crystal has a stability specification of 3 × 10-8 per day, the possible frequency error of the signal generator at 1 GHz will be 900 Hz 30 days after the last calibration.
Singleband and Multiband Capabilities
The YIG oscillator can supply a multiband sweep using only one oscillator. Modern-day technologies can sweep from 2 GHz to 20 GHz in a single YIG oscillator. This degree of performance offers the possibility of a multiband signal generator in the same C-size VXI package as the singleband generator.
The broadband YIG also does not generate subharmonics. Subharmonics are generally far less desirable than harmonics since they are more difficult to filter after the source. Subharmonics are the residuals of multiplying a fundamental frequency to a higher band.
For instance, if the fundamental YIG operates from 2 GHz to 8 GHz and the desired frequency is 10 GHz, the 5-GHz fundamental must be doubled. The output of the multiplier is the 10-GHz signal and a 5-GHz subharmonic.
A bandpass filter filters the 5-GHz subharmonic and the 20-GHz second harmonic. As the frequency sweeps through multiple bands, the filter must track the fundamental. A typical method that sweeps the fundamental frequency uses a YIG-tuned bandpass tracking filter. The disadvantage of a YIG tracking filter in the VXI package is the additional room required for the filter and a substantial loss in RF power.
Along with the spectral purity of the RF signal, other characteristics are important. Consider the following performance features when you buy a VXI signal source. The importance of the feature will depend on the application of the source.
Signal Power—There is always a need for higher power, especially in an ATE environment where long RF cable runs may be unavoidable. Providing high RF power output in a VXI package is very challenging when considering the space and heat-transfer requirements.
To achieve power levels in the range required for most applications, pay attention to component layout, particularly to short RF paths to minimize losses. With the proper design approach, the typical output power of a microwave VXI signal generator can be as high as 8 dBm to 13 dBm in a standard configuration.
The flexibility of the VXI format provides a solution to system integrators requiring even more power than what is normally available. For instance, if the application requires power above +13 dBm, it is possible to provide an external amplifier as a separate module. This solution does not affect the design, cost and size of the standard VXI signal generator.
Switching Speed—Automated test systems perform large numbers of repetitive tasks. A good example is the need to perform measurements over many frequency points when characterizing the frequency response of a device. The capability to quickly switch from frequency to frequency reduces the total test-time requirements when performing a frequency characterization test.
Providing fast-frequency switching capabilities is largely the responsibility of the PLL section. The PLL monitors the frequency output of the YIG oscillator and compares it with the reference input which is phase-locked to the 10-MHz time base.
If there is a difference between the reference and the YIG frequency, the PLL generates a DC correction voltage proportional to the phase error. When the YIG is tuned to the same frequency as the reference, the source is locked and assumes the same accuracy and stability as the reference input.
An important component within the PLL circuit that provides fast-frequency switching is the direct digital synthesizer (DDS) chip. A DDS chip provides the interface between the CPU and the reference arm of the PLL circuit. The DDS quickly supplies the correct reference input to the PLL.
An additional advantage of the DDS chip is its size. The DDS performs many operations that previously required discrete components. Using the DDS chip in the PLL circuit reduces components and board size, always a necessity in VXI design considerations. With proper PLL circuit design, switching speeds of 200 ms or less can be achieved even when switching over a range as wide as 20 GHz.
Broadband and Narrowband Coverage—The availability of the VXI signal generator module in broadband or narrowband configurations gives the system integrator flexibility when designing a test system. Broadband coverage provides the capability to test a device over a wide frequency range without switching from one source to another.
A broadband source also offers the flexibility to adapt to different test requirements as the need arises. A narrowband source provides the capability to configure a system that meets the application requirements without having to pay for unnecessary range.
Along with the concern for signal-generator performance, specific VXI characteristics must be considered when selecting a VXI microwave signal generator. Some of the issues related to the VXI format include programmability, flexibility and size.
Programmability
A key characteristic of the VXI format is the reliance on programmability as the primary interface to the instrument. A typical VXI signal generator is available in a three-slot-wide, C-size package using message-based protocol.
A drawback to the programming-only design is the potential for increased interface problems when integrating instruments from multiple vendors into a single system. VXIplug&play has standardized the high-level instrument drivers, relieving the programmer from the low-level I/O communications interface and providing ease of use in a multivendor open-architecture environment.
Along with the instrument driver, VXIplug&play provides a path for an instrument soft panel. The soft panel is an executable file that supplies a visual interface to the instrument. This capability helps verify the instrument performance before incorporating it into the system. The soft panel also provides a quick troubleshooting tool for isolating faulty instruments within the system.
Flexibility and Size
The modularity of the VXI platform is a major reason for its popularity. The capability to quickly replace a faulty module and minimize system downtime is a major benefit. The size and flexibility of the modules also minimize backup inventory requirements.
During the design phase, the objective of fitting the salient characteristics of a signal generator into as few slots as possible is always a prime driver. The final version of the architectural layout must reflect the concern for space.
One example is the modulation capability typically found in a VXI signal source. Although it is possible to include a modulation generator internally within the VXI signal source, the space requirements are a serious drawback. This is especially true if you do not need modulation or already have a modulation generator located in the VXI mainframe. Consequently, a VXI signal source typically has modulation capabilities but does not have an internal modulation generator located in the signal generator.
The functional layout of the signal generator in Figure 1 identifies two areas: the reference section and the microwave section. The reference section includes the 10-MHz crystal that is the frequency standard for the unit. The reference section encompasses the controlling signals for the PLL circuits.
If an application requires more than one signal source, it is redundant to include a separate reference section for each additional microwave section. One way to reduce the number of slots required is to multiplex the reference signals from a single module.
Figure 2 shows how this may be accomplished. The reference module in this example is only one slot wide while the microwave section is two slots wide.
If an application requires three RF sources, using a single reference (controller) module requires only a seven-slot configuration rather than a nine-slot configuration if the reference is repeated in each unit. This design approach in the VXI format is a good example of how the VXI architecture achieves a compact, flexible test system in an open-architecture platform.
Summary
Providing microwave signals in the VXI format presents unique challenges. Performance specifications such as single-sideband phase noise and high output power must be carefully considered during the layout of the RF components. Proper isolation and signal-path considerations are critical in achieving a compact, lightweight instrument that meets the performance objectives of microwave tests.
VXIplug&play is a major step in reducing the design time required to integrate multiple instruments in VXI. A VXIplug&play instrument driver is an important ingredient in the total package solution.
The flexibility of the VXI architecture provides the basis by which an instrument can easily adapt to the requirements of specific applications. Separate modules for specific tasks can be added as needed, providing a path for expanding the performance capabilities of the most basic unit without using unnecessary slots within the VXI mainframe.
About the Author
Steve Reyes is the product marketing manager for power meters and RF and microwave synthesizers at Giga-tronics. He is a graduate of the University of California, and has been in the RF and microwave test and measurement industry for more than 17 years. Giga-tronics, 4650 Norris Canyon Rd., San Ramon, CA 94583, (510) 328-4650.
Copyright 1997 Nelson Publishing Inc.
February 1997
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