Modular test instrument formats such as PXI and PXIe offer tremendous flexibility in a fraction of the space of traditional benchtop instruments. The M9381A PXIe vector signal generator (VSG) from Agilent Technologies, for example, produces stable vector [in-phase (I) and quadrature (Q)] signals from either 1 MHz to 3 GHz or 1 MHz to 6 GHz depending upon the option while occupying only five slots in a PXIe chassis. The PXIe format uses a control module or separate computer and software to take the place of the screen and controls typically found on a larger benchtop instrument.
The M9381A (Fig. 1) comprises several modules that together fill five slots in a PXIe chassis: the M9300A frequency reference module (one slot), the M9301A PXIe synthesizer module (one slot), the M9310A PXIe source output module (one slot), and the M9311A PXIe digital vector modulator (two slots). These modules can be used with additional test function modules and a controller module in the PXIe chassis or with the chassis and an external computer running the appropriate drivers and test software, such as Agilent’s Signal Studio signal creation software (Fig. 2). Signal Studio provides validated and performance optimized reference signals for cellular and wireless connectivity standards with an easy-to-use, application-specific graphical user interface (GUI).
Also, the M9381A has a base configuration (model M9381A-F03) that provides a “starting point” for performance that can be readily extended through a large number of options. The base configuration covers a frequency range of 1 MHz to 3 GHz with 32-Msample memory and 40-MHz modulation bandwidth. By selecting a higher-frequency option (model M9381A-F06), the frequency range is extended from 1 MHz to 6 GHz. The frequency tuning resolution is 0.01 Hz in both cases. Additional options boost the frequency/amplitude switching speed, the output-power level, the sample memory to either 512 or 1024 Msamples, and the modulation bandwidth to 100 or 160 MHz.
The M9381A can generate a variety of modulation formats over those modulation bandwidths, including amplitude modulation (AM), frequency modulation (FM), pulse modulation, and multitone modulation. Across the 40-MHz modulation bandwidth, the typical amplitude flatness is ±0.2 dB, with ±0.3-dB typical flatness across the 100-MHz modulation bandwidth and ±0.5-dB typical flatness across the 160-MHz modulation bandwidth.
The PXIe backplane allows for simple interconnection of different function modules. That backplane supports almost instantaneous interaction among the modules in the M9381A VSG, allowing it to execute rapid changes in frequency and/or amplitude. In the standard model M9381A, the frequency and amplitude switching speeds are typically better than 5 ms. However, with the high-switching-speed option, option UNZ, the 5-ms time drops to 2 ms or less for frequency and/or amplitude changes. When using the M9381A’s list-mode switching capability, with 3201 list mode points for storing test point information, the signal generator changes in frequency and amplitude in only 220 μs with option UNZ and changes in amplitude alone in only 125 μs.
The M9381A’s baseband tuning capability can also perform extremely fast frequency and amplitude changes within ±80 MHz of a carrier frequency and within 0 to –20 dB from an RF power level, driven by a custom ASIC. With option UNZ, changes in amplitude and frequency are typically 250 μs when responding to commands from a controller and typically only 10 μs when using the list-mode function. The custom ASIC enables real-time corrections to phase and amplitude during tuning. In addition, the ASIC ensures outstanding error-vector-magnitude (EVM) performance, as required for testing many modern communications standards, including WCDMA.
In the base model M9381A, the output power can be set from –130 to +10.7 dBm with 0.02-dB nominal resolution with and without automatic level control (ALC) in I/Q mode. The ALC consists of precision step attenuators that help maintain precise desired output levels. The maximum output power in the standard model M9381A is +10 dBm through either 3 or 6 GHz. By selecting option 1EA, the output power can be set from –130 to +20 dBm, with as much as +19-dBm output power available to 2.5 GHz and as much as +18-dBm power available to 6 GHz.
This fast frequency and amplitude switching-speed capability can dramatically reduce test time for any device under test (DUT) that must be evaluated with wideband digital I/Q modulation. As many as 80 channel parameters can be set in list mode, including frequency, power, and modulation, so a DUT can be effectively exercised with test signals that represent real-world signals. The M9381A is suitable for testing a variety of different communications standards, including IEEE 802.11a/b/g/n/ac wireless local area networks (WLANs), Mobile WiMAX, Long-Term Evolution (LTE) cellular, GSM/EDGE/Evo cellular, and Bluetooth equipment.
Ignoring the versatility of the PXIe format, the M9381A delivers outstanding spectral purity when considered simply as a test signal source. It draws its stability from the M9300A frequency reference module, which is built around a low-noise 10-MHz oven-controlled crystal oscillator and 100-MHz phase-locked loop (PLL). It provides access to either reference frequency: 10 MHz at a nominal output level of +9.5 dBm and 100 MHz at +10 dBm. The reference module exhibits a daily aging rate of better than ±0.5 ppb/day after a 72-hour warmup period and a yearly aging rate of better than ±0.10 ppm/year after a 72-hour warmup period. It can also accept an external frequency reference sinewave input signal from 10 to 110 MHz over a nominal amplitude range of 0 to +10 dBm.
The high quality of the M9300A reference module contributes to the M9381A’s low phase noise of –122 dBc/Hz offset 20 kHz from a 1-GHz carrier and –108 dBc/Hz offset 20 kHz from a 6-GHz carrier. Harmonic levels are typically –35 dBc for carriers at power levels to +10 dBm and carrier frequencies to 1 GHz, and typically –30 dBc at power levels to +10 dBm and carrier frequencies to 2.5 GHz. Nonharmonic-related spurious signals are nominally –70 dBc or lower, with no subharmonic content present for carriers from 1 MHz to 6 GHz. For wideband-code-division-multiple-access (WCDMA) test model 1, with 64 dedicated physical channels (64 DPCH), the M9381A achieves excellent adjacent channel power ratio (ACPR) performance at test signal output levels to +15 dBm, typically better than –70 dBc.
The M9381A VSG supports the open PXI/PXIe modular instrument format and will readily work with other PXIe modules in the same chassis or system. It is supplied with a CD with product information, example programs, drivers, soft front panel, and Agilent I/O Libraries Suite software. The M9381A will also support interfaces for VisualStudio from Microsoft, MATLAB from the MathWorks, and LabVIEW from National Instruments.
The base model M9381A VSG (1 MHz to 3 GHz) starts at $25,663 but performance can readily be modified through license-key upgrades. The modules are factory calibrated as shipped. The VSG is supported by Agilent’s N7800A Test Management Environment, which helps users perform a calibration. In addition, the firm offers an 18-slot PXI chassis for the modules, as model M9018A, and PXIe embedded controller, model M9036A, along with a CW signal source (without vector modulation) version of the M9381A, as model M9380A, which costs about $19,000 for the 3-GHz version and available with license-key upgrades.