PXI-Based VSA Scrutinizes 3.6 GHz

This highly accurate, low-noise signal/spectrum analyzer is based on the modular PXI instrument format and can also be used as a vector signal analyzer from 20 Hz to 3.6 GHz.

Rare is the signal analyzer that can study audio as well as microwave signals. But the NI PXIe-5665 signal/spectrum analyzer from National Instruments bridges an analysis range from 20 Hz through 3.6 GHz. The NI PXIe-5665 is based on the modular PXi format, allowing it to be housed with other instruments (such as a signal generator) within a compact PXI mainframe. It can be used as a spectrum or vector signal analyzer (VSA).

The NI PXIe-5665 (Fig. 1) joins the company's lower-frequency and higher-frequency PXI VSAs, the NI PXIe-5661 (9 kHz to 2.7 GHz) and the NI PXIe-5663 (10 MHz to 6.6 GHz), respectively. It is a low-cost solution for spectrum and signal analysis through 3.6 GHz, with a low-frequency range that permits studies of mechanical vibration on a device under test (DUT). Being modular, the NI PXIe-5665 analyzer actually consists of several different PXI modules: the NI PXIe-5603 RF downconverter module, the NI PXIe-5622 intermediate-frequency (IF) digitizer module, and the NI PXIe-5653 frequency synthesizer/local oscillator (LO) source. The NI PXIe 5653 internal 10-MHz frequency reference can also be used as part of the NI PXIe-5665 VSA for outstanding phase noise.

The modules serve to translate signals through 3.6 GHz to an IF range that can be captured and digitized by the 16-b analog-to-digital converter (ADC) in the IF digitizer module. The IF digitizer supports wide resolution bandwidths: 1 Hz to 25 MHz standard and 1 Hz to 50 MHz as an option.

The combination of modules comprising the NI PXIe-5665 VSA owes a great deal of its analysis capabilities to the performance of its individual components, such as the 16-b ADC and the low-noise LO source. The low phase noise of the LO source (with the NI PXIe 5653 internal reference), for example (Fig. 2), is equal to the performance of frequency synthesizers costing much more, at -129 dBc/Hz offset 10 kHz from an 800-MHz carrier. The analyzer boasts a wide dynamic range, with third-order intercept point (IP3) of better than +24 dBm from 700 MHz to 3.6 GHz. An onboard self-calibration tool makes it possible to achieve IF amplitude response of 0.15 dB and iF phase linearity of 0.1 deg. This enables error-vector-magnitude (EVM) measurement performance of better than 0.21 percent for a 256-state QAM signal (256QAM) signal. The NI PXIe-5665 delivers 0.35 dB typical amplitude flatness across a 20-MHz bandwidth.

By using measurement software tools such as NI's LabVIEW or LabWindows/CVI software, PXI instruments can be tied together for fast, automated measurements. The analyzer can also work in RF list mode to step through a user-defined RF test configurations using internal timing or an external trigger.

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