page 1 of 6 Fa m i l y o f PXI Do w n c o n v e r t e r Mo d u l e s Br i n g s 26.5 GHz RF/MW Measurement Technology to the PXI Platform by Michael N. Granieri, Ph.D. Background: The PXI platform is known for providing world-class, economical, small form-factor solutions in support of analog, digital, and RF measurement applications up to 6.6 GHz. However, the platform has not been a contender for mainstream RF/MW measurement applications in the X- to K-band until now. Spurred on by the need for small formfactor/multipurpose/high-speed test equipment, a new PXI RF/MW capability was developed by Phase Matrix, Inc. (http://www.phasematrix.com) and was introduced at this year s Autotestcon conference. The genesis of this innovative dual-use (i.e., commercial and military) PXI product was a Small Business Innovative Research (SBIR) Program sponsored by the Navy (NAVAIR PMA 260 D). Since the advent of commercialgrade instrumentation during the post WWII era, the automatic test systems (ATSs) industry, both military and commercial, has grown into a multibillion dollar market, resulting in a large array of instrument types in factory and field (see Figure 1). Some challenges that have been identified Figure 1: Collage of traditional ATSs with traditional ATSs are, one, inflexibility of functionality because traditional ATSs employ hardware-centric instruments that are married to fixed firmware; two, an increase in obsolescence of aging ATSs as technology gallops forward; and, three, the growing limitation of reusing traditional instruments to address new and emerging test requirements. Thus, the primary motivation behind the Navy-sponsored SBIR contract award was to overcome these challenges, and the solution is recognized as a major paradigm shift. In addition to these challenges, as Phase Matrix discovered from its customers, are increasing demands for smaller form-factor test equipment, more portability, broadband range, faster processing speed, modular architecture, more adaptability and multifunctional capabilities, and, of course, lower cost. A paradigm that embodies the solution to these needs is synthetic instrumentation (SI). Synthetic Instrumentation: SI, as defined by the Synthetic Instrument Working Group, is a reconfigurable system that links a series of elemental hardware and software components with standardized interfaces to generate signals or make measurements using numeric processing techniques. 1 The common attributes of a SI system are, one, a virtual (software) interface or graphical user interface (GUI); two, integration of a common core of hardware and software components such as downconverters, upconverters, analog-to-digital and digital-to-analog converters, and digital-signal processing (DSP) software; and, three, implementation of a numeric processing technique to generate signals and make measurements. When employing SI both the stimulus and measurement capability are implemented primarily by software; hardware reconfiguration is not required. Figure 2 is a notational view of a synthetic instrument. A synthetic instrument primarily consists of three functional blocks, a stimulus block (signal path at bottom in Figure 2), a measurement block (signal path at top in Figure 2), and a signal-processing block, which is commonly hosted in an ATSs main CPU. Other variations of system architecture are possible; however, fundamentally they will adhere to the framework depicted in figure 2. There are three distinct signal paths inherent in the system architecture: a RF/MW signal path, which typically is in the GHz range; a baseband or intermediate frequency (IF) signal path, which is in the
page 2 of 6 Figure 2: Notational view of SI system from a high-level perspective MHz range; and a digital signal path. Once the RF/MW signal has been downconverted to a lower frequency signal where it can be digitized with a minimum amount of distortion, it is sent to the DSP software that is hosted in the ATS s CPU for conversion into a measurement. A similar process is effected in generating a stimulus signal: a digital stimulus command is sent to the synthetic instrument s digital-to-analog converter or arbitrary waveform generator, converted into a baseband signal, upconverted into a RF/MW signal, conditioned, then sent to the device under test (DUT). Size, Cost, Speed Size, cost, and speed are the most cited attributes by customers. With respect to the first two, it should be noted that in a large number of applications, SI is often called upon to emulate or replace signal-analysis or signal-generation functionality that is resident in a number of legacy RF/ MW instruments such as spectrum analyzers, digital storage oscilloscopes, and vector signal generators. Thus, an exact side-by-side comparison is problematic in these cases Comparative Analysis: Functionality, Stimulus/Measurement Hardware, User interface Manufacturers of traditional instruments primarily define instrument functionality by way of firmware whereas with SI the user defines functionality of a synthetic instrument by virtue of PC-based field programmable gate arrays (FPGAs) or user-defined application software (see Figure 3). Fixed stimulus/measurement hardware is inherent in traditional instruments. With SI, on the other hand, stimulus/measurement is generic; the user employs a core set of generic stimulus/measurement hardware. The user interface of a traditional instrument is an instrument s physical front panel, which includes knobs, buttons, dials, etc. A virtual software interface or GUI is how the user controls testing with a synthetic instrument. Figure 3: Baseline instrument architecture vs. SI paradigm unless one addresses specifics. With that being said, PXI SI systems typically reside in PXI 3U chassis, which range from eight-slot units to 18-slot units. Synthetic instruments often replace multiple half-rack and full-rack instruments at a fraction of their cost when the user employs a software package such as LabVIEW as well as associated signal
November 2009 page 3 of 6 processing and modulation toolkits that are used to develop measurement and stimulus software modules. In terms of speed, PXI (125 MB/second) and PXI Express (250 MB/ second to 1 GB/second) provide world-class bus speed with a significant decrease in latency time when compared against traditional RF/MW instruments that employ classical rackand-stack bus system architectures. Phase Matrix s Family of PXI RF/MW Downconverter Modules: Phase Matrix s family of PXI RF/MW downconverter modules (see Figure 4) provide the needed front end downconversion function to implement the measurement path of a SI system. The family consists of five modules: the local oscillator (PXI-1450), the preselector/ attenuator module (PXI-1410), and the RF, IF, and MW downconverter modules (PXI-1430, PXI-1440, and PXI-1420 respectively). Data sheets for each of these modules are available at the Phase Matrix website (http://www.phasematrix.com). The wide range of applications that this family of downconverters serves includes synthetic/virtual instrumentation applications such as spectral analysis, time-domain analysis, and modulation analysis; portable and high-mobility test systems; signal Figure 4: Phase Matrix s family of PXI downconverter modules intelligence; hybrid test systems (PXI, VXI, GBIB, and LXI); and embedded test systems (e.g., health monitoring and diagnostics). Features, Advantages, and Benefits Features, advantages, and benefits of these downconverter modules are summarized in Table 1. This family of PXI downconverter modules is the PXI industry s first broadband downconverter and has the capability of being configured into any one of six user configurations, resulting in flexibility in providing tailored solutions for users needs. System Architecture The family of PXI downconverter modules have been system engineered to ensure that their functionality and associated system I/Os function in various system configurations. The complete family of modules are interconnected to synthesize a full up 100 khz to 26.5 GHz downconverter solution. Other solution variants include 100 khz to 2.9 GHz and 2.7 to 26.5 GHz. As an example of its operation, use Figure 4 to trace the 100 khz to 26.5 GHz signal path as follows. Input signals less than 2.8 GHz (i.e., lowband signals) are routed by Table 1: Features, advantages, and benefits of Phase Matrix s family of PXI downconverter modules
page 4 of 6 Figure 5: System architecture of Phase Matrix s family of PXI downconverter modules the system software through the preselector module and a programmable attenuator to the RF downconverter module where the signal undergoes dual downconversion by the local oscillator (LO) module and an internal LO source and undergoes signal conditioning. The resulting 250 MHz IF signal with a bandwidth of 40 MHz is sent to the IF downconverter module where the signal is either passed through and applied to the system s wideband digitizer or is downconverted internally to the IF module and converted to a narrowband 21.4 MHz signal. The narrowband IF signal is then processed by a 30 khz or 8 MHz optional filter and applied to the system s narrowband digitizer. For signals greater than 2.8 GHz, the process is similar. The signal is either passed through a YIG-tuned filter or bypasses the filter and is applied to the MW downconverter module. The signal is then downconverted by the local oscillator, conditioned, and passed through a 350 MHz band-pass filter. It then is processed by the IF downconverter module in a similar fashion as low-band signals. The primary inputs required of the user are, one, the input power level and, two, the frequency span of interest for the DUT s input signal. The operation of the set of modules is entirely automatic after the user provides the two controllable inputs. Use Configurations As mentioned earlier, the family of five downconverter modules can be configured six different ways. Table 2 contains a summary of the six primary user configurations. The table shows the total number of PXI slots required to effect a SI solution including downconverters slots, embedded control- Table 2: Primary user configurations
November 2009 page 5 of 6 ler slots, and digitizer slots as well as the total number of slots dedicated to the downconverter function. It also indicates whether the preselector module is integral to the subject configuration and lists the number of IF signals employed. Figure 6 depicts a front panel view of the Phase Matrix family of PXI downconverter modules required to support configuration #6, the full up configuration, which covers the frequency band of 100 khz to 26.5 GHz. Low-loss, flexible coax cables are used to effect the system interconnects between modules. This configuration is popular in the aerospace and defense communities in which testing of X-, Ku-, and K-band radar systems is common. A frequency extension to 40 GHz can be integrated into this configuration by incorporating a harmonic mixer in order to test Ka-band Figure 6: Phase Matrix PXI downconverter modules in configuration #6 radar systems in the 27 to 40 GHz frequency band. Applications and Implementation Configuration #6 consumes 11 slots of an 18-slot Figure 7 shows a high-level system view of the Phase PXI chassis. The remaining seven slots may be used to Matrix family of PXI downconverter modules implemented incorporate user-defined stimulus and switching. The flexin a modular PXI SI system context. The RF/MW ibility to employ a generic signal-analysis capability as well downconverter front end of the SI system performs as the ability to implement customization tuned to unique the requisite RF/MW attenuation, YIG-tuned filtering, customer requirements over a broadband frequency range are and frequency translation down to an IF frequency for good reasons to consider utilization of configuration #6. Figure 7: System view of SI-based measurement modularity
subsequent digitization and processing by SI DSP software. Data-conversion hardware (i.e., digitizers, DACS, arbitrary waveform generators) is available from manufactures such as National Instruments (NI). DSP software may be provided from companies such as BAE Systems, and PXI chassis may be provided by vendors such as Geotest and NI. SI DSP toolkits are provided by software infrastructure companies such as NI. The advantage to this PXI SI system is its inherent modularity whereby multiple sources of supply can be used, resulting in mitigation of obsolescence risk, which was mentioned earlier as one of the primary challenges to be overcome by the Navy-sponsored SBIR program. Another advantage of employing this type of modular PXI SI system is the various implementations that may be realized. A customer could choose to self-integrate a customized solution by using COTS PXI modules along with his or her own home-brewed DSP software. Another approach would be to work with a PXI manufacturer such as Phase Matrix to integrate a core system of COTS hardware and associated DSP software tools to provide a SI subsystem capability, or a customer could seek a turnkey system provider who could supply both an integrated test Figure 8: Screen shot of SI-based test equipment page 6 of 6 system and associated test program sets per the customer s unique requirements. Figure 8 shows an insightful screen shot that exemplifies the power of SI used in conjunction with the PXI platform. Observe that with one data capture instance, multiple views of the measurement (i.e., spectrum analysis, oscilloscope, and spectrum 2D waterfall) can be displayed by selecting the applicable tab toward the top of the GUI. No hardware reconfiguration or switching is required to reset the test data within any one of the measurment tabs. Summary: Phase Matrix s family of PXI downconverter modules that cover an operating range of 100 khz to 26.5 GHz has been released to the marketplace. The set of modules serve in SI ATSs, overcoming traditional instrumentation challenges such as inflexibility, increased obsolescence, and limited ability to address new test requirements. Features, advantages, and benefits of the family of PXI downconverter modules include broadband coverage for dual-use applications (i.e., military and commercial), six primary user configurations for user-tailored solutions, modularity for incremental upgrade of technology, and increased testing speed. REFERENCES: 1. Synthetic Instrument Working Group Joint participation between DoD, Defense Prime Contractors, and Suppliers. Dr. Granieri is Vice President of Advanced Programs and Business Development at Phase Matrix, Inc. and is a recognized expert in the T&M industry, contributing to both the technical and technology management domains.