100 MS/s, 16-Bit Arbitrary Waveform Generator with Onboard Signal Processing
–25
–30
• Pulse-shaping finite impulse response (FIR) filter – Shapes
and interpolates the waveform data. The FIR filter coefficients are
programmable and include flat, raised cosine, and root raised cosine.
Digital interpolation factors range from one to 16,384 times.
• Numerically controlled oscillator (NCO) – Produces sine and
cosine waveform data for quadrature digital upconversion and
features 355 nHz frequency resolution and 0.0055 deg phase
resolution for precise control of impairments such as frequency error
and quadrature skew. You can adjust NCO frequency and phase before
or during waveform generation.
–40
–50
–60
–70
–80
–90
• Quadrature digital upconversion with impairments – Upconverts
signals, models channel effects, and tests receiver robustness. In
quadrature upconversion, I and Q complex waveform data is stored in
waveform memory and is passed to the OSP block. OSP then shapes
and interpolates the baseband signal and upconverts it to a carrier
frequency of up to 43 MHz. You can choose to suppress the lower or
upper modulation sideband by adjusting the NCO in-phase and
quadrature output phase settings. For modeling channel effects and
testing the robustness of a receiver, you can use OSP to add several
impairments to the signal during waveform generation. Add IQ gain
imbalance and DC offset impairments by adjusting the per-filter gain
and offset settings, and introduce quadrature skew and frequency
error by adjusting the I or Q carrier phase and frequency.
–100
–105
10
15
20
25
30
35
40
Frequency (MHz)
Figure 2. The NI PXIe-5442 with onboard signal processing digitally upconverts and
generates the frequency spectrum of a W-CDMA physical layer signal. (external sample
clock = 92.16 MHz)
• Amplitude modulation (AM) – Generates AM radio signals. By
using only the in-phase (I) path of the OSP block, you can generate an
AM radio signal by directly downloading the message signal into
onboard memory. The message signal scales the amplitude of the
NCO programmable frequency output.
• Single tone and function generation – Generates standard,
user-defined waveforms. Using the OSP block NCO, the NI PXIe-5442
can generate sine, square, triangle, ramp, and other standard and
user-defined waveforms just as a function generator does. You may
adjust the frequency of the output waveform during generation with
355 nHz resolution for generating phase-continuous frequency sweeps
and hops. You also can adjust the phase relative to other synchronized
instruments, the PXI 10 MHz reference clock, or an externally supplied
reference clock.
• Baseband interpolation – Generates smooth baseband signals.
You can use the NI PXIe-5442 OSP block to interpolate low-sample-
rate waveforms to a much higher sample rate, thereby improving the
output frequency spectrum by relocating zero-order sample-and-hold
reconstruction images to higher frequencies. With the images at
higher frequencies, the NI PXIe-5442 seventh-order lowpass analog
filter can greatly suppress them without disturbing the signals’
amplitude response or phase information. For example, you can
interpolate a waveform created at a 10 kS/s sample rate to
10.24 MS/s by using 1024 times digital interpolation in the OSP.
The upsampled signal is then passed to the DAC, which can also
interpolate by two, four, or eight times, resulting in an effective
sampling rate of 81.92 MS/s (eight times DAC interpolation).
Because you sample the original waveform at only 10 kS/s, rather
than 81.92 MS/s, you achieve a 1:8,192 compression ratio,
resulting in dramatically faster waveform computation and download
times. Alternatively, you can use the resulting compression to
efficiently store data in the NI PXIe-5442 onboard memory to gain
significantly longer playback times. Extended generation is essential
for improving the statistical significance of many communications
measurements and displays such as bit error rate, trellis plots, and
constellation plots.
DAC Interpolation
The NI PXIe-5442 uses digital interpolation to improve the output signal
quality of smooth waveforms. Every DAC produces reconstruction images
in the frequency domain as a result of the conversion process. Appearing
at |fo nfs|, where fo is the frequency of the desired signal and fs is the
sampling rate, reconstruction images are undesirable for smooth signals,
such as sine waves.
Typically, arbitrary waveform generators suppress the reconstruction
images by using high-order lowpass filters with a cutoff frequency near
the generator’s Nyquist frequency (50 MHz for a 100 MS/s sample rate).
By using a high-order filter with such a low cutoff frequency, the filter’s
nonidealities, such as passband ripple and nonlinear phase, significantly
affect generator performance. The NI PXIe-5442 uses digital
BUY ONLINE at ni.com or CALL 800 813 3693 (U.S.)
2
