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Figure 5. Constellation Plot of 256-QAM

In Figure 5, a center frequency of 1 GHz, a symbol rate of 5.36 MS/s, and a

root raised cosine filter with 0.12 alpha was used. The RF power was set to -10 dBm

and analysis was performed with the NI PXIe-5663. In addition, the wide

bandwidth of an NI PXIe-5673/5673E combined with high-performance image

rejection enabled the generation of modulated signals at high symbol rates. For

example, Figure 5 shows a constellation plot of a 64-QAM signal at 40.99 MS/s

with an RMS EVM of 0.9 percent (1250 symbols, equalization disabled).

Figure 6. Constellation Plot of QPSK with 50 MHz of Bandwidth

In Figure 6, a center frequency of 825 MHz was used. The RF power was set

to -10 dBm and analysis was performed with the NI PXIe-5663. While an EVM

of 0.9 percent is a nominal value, the typical result is 1.1 percent.

In addition, the combination of high dynamic range and a linear front end

yields high-performance adjacent channel power measurements. In Figure 7,

observe the spectrum for a QPSK signal at 1 GHz center frequency.

Figure 7. Spectrum of QPSK Signal at 1 GHz

A symbol rate of 3.84 MS/s and a root raised cosine filter with alpha 0.22 is

used. As Figure 7 illustrates, an NI PXIe-5673/5673E yields an adjacent channel

power measurement of better than -69 dBc rejection when configured with the

settings described.

Flexible Software

With NI Modulation Toolkit for LabVIEW software, you can operate an

NI PXIe-5673/5673E as a general-purpose vector signal generator. Using

NI LabVIEW or LabWindows/CVI example programs, you can generate a variety

of modulated signals.

These modules are programmed with the NI-RFSG driver, which contains

several performance-enhancing characteristics. Using an optimized driver

stack combined with fast-settling VCO-based hardware, you can tune an

NI PXIe-5673/5673E to 0.1 ppm of its settling frequency with a typical tuning

time of less than 1.5 ms. With the NI PXIe-5673E, a wide-loop bandwidth

configuration results in tuning times to within 0.1 ppm of the final frequency

in under 300 µs.

You also can use the NI-RFSG driver to enhance the RF performance. With

an RF impairments API, you can manually or programmatically adjust I/Q

impairments such as gain imbalance, DC offset, and quadrature skew.

A LabVIEW property node that illustrates how you can adjust these parameters

on the fly is shown in Figure 8.

Figure 8. RFSG Impairments Property Node

Vector Signal Generator