The OSP delivers four main benefits
• Reduced computational and download times of waveforms to onboard waveform memory
• Longer waveform playtimes
• On-the-fly signal adjustments such as signal impairments and frequency changes
• Linking and looping of baseband waveforms
The OSP effectively transfers the creation of waveforms from the host PC to the hardware, thus reducing the computational and download times significantly. The following scenario depicts a typical use case. The computation and download of a waveform representing a 65,000 sample 3.84 MS/s QPSK signal generated at the maximum sample rate of 100 MS/s takes more than 85 s using the host PC processor (PXI-8187 controller with 2.5 GHz Pentium 4 running Windows XP). The computation and download time of the same waveform using OSP takes under 10 s; an >8.5X improvement. Additionally the waveform memory requirements are reduced significantly as well. The host PC method takes up to 3.15 MB of onboard memory whilst the OSP method takes up to 0.5 MB; a 6.3X improvement. This is because the host PC method creates the root raised cosine pulse shaped waveforms from I/Q chip data and resamples it to a 100 MS/s sampling rate, thus taking up considerable waveform memory. The OSP creates the waveforms on the fly from I/Q chip data stored on the onboard memory. The pulse shaping, interpolation, and upconversion of the data at baseband to an IF frequency is done in line - the data is fed to the OSP at the symbol/chip rate and the OSP feeds the waveforms at 100 MS/s to the DAC.
The following table illustrates some of the common communications applications the waveform playtimes possible on the PXI-5441 and PXI-5671, allowing for large PN sequence signal generation for enhanced bit-error-rate test (BERT) of receivers.
And the OSP delivers on-the-fly signal impairments, which allows things such as I/Q gain, offset, and quadrature skew discussed in detail later in this paper.
Quadrature Digital Upconversion with Impairments
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 data using the FIR filters, interpolates it up to a high sample rate using the CIC filters, and then up-converts the data to a programmable carrier frequency 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, the OSP can add several impairments to the signal on the fly (during waveform generation). IQ gain imbalance and DC offset impairments are implemented by adjusting the one-filter gain and offset settings, while quadrature skew and frequency error can be introduced by adjusting the I or Q carrier phase and frequency.
Baseband Interpolation for I/Q Signal Generation
Useful for generating baseband signals, such as I and Q signals, you can use the PXI-5441 OSP block to interpolate low-data-rate waveforms to a much higher sampling 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 seventh-order lowpass analog filter in the PXI-5441 can greatly suppress them without disturbing the amplitude or phase of the signal. For example, a waveform created at 3 MS/s sampling rate can be interpolated to 96 MS/s by using 4X FIR interpolation and 8X CIC interpolation. The upsampled signal is then passed to the DAC which can also interpolate by 2, 4, or 8X resulting in an effective sampling rate of 384 MS/s (4X DAC interpolation). The resulting sampling rate of 384 MS/s places the sampling images well above the lowpass analog filter cutoff frequency, thus effectively attenuating the images. Because the original waveform was sampled at only 3 MS/s, rather than 96 MS/s, a 1:32 compression ratio is achieved, resulting in dramatically faster waveform computation and download times. The resulting compression can be used to efficiently store data in the PXI-5441 onboard memory so that you can achieve much longer playback time without streaming from arrays of high-speed disk drives. Long playback times are essential for improving the statistical significance of many communications measurements and displays such as bit error rate, CCDF, trellis plots, and constellation plots.
Using T-Clock synchronization technology, multiple PXI-5441 modules can be synchronized for applications requiring a greater number of channels, such as I/Q signal generator or multiple IF generation for MIMO systems. Because it is built into the SMC, T-Clock can synchronize the PXI-5441 with SMC-based high-speed digitizers and digital waveform generator/analyzers for tight correlation of analog and digital stimulus and response. Using onboard calibration measurements and compensation, T-Clock can automatically synchronize any combination of SMC-based modules with less than 500 ps module-to-module skew. Greatly improved from traditional synchronization methods, the skew between modules does not increase as the number of modules increases. To achieve even better performance, a high-bandwidth oscilloscope such as the Tektronix CSA8000 Communications Signal Analyzer can be used to precisely measure the module-to-module skew. Using the oscilloscope measurement for calibration information, T-Clock can achieve <20 ps module-to-module skew (Figure 6).
Channel-to-channel jitter is also an important consideration for I/Q signal generation. Significant jitter leads to unwanted quadrature skew and gain imbalance. The total system jitter between two PXI-5441 outputs is less then <20 psrms (Figure 7) thus allowing for wideband I/Q signal generation.
Creating I/Q Data for Use with OSP
A number of additional tools exist to create I and Q waveform sample data. Commonly, data resulting from simulations using a math package such as MATRIXx X-Math are stored to disk. National Instruments LabVIEW and LabWindows/CVI can read a variety of data file types and convert them to either I16s or complex double-precision floating-point numbers - two formats that the NI-FGEN driver directly accepts. By first normalizing the waveform data to ±1 V and extracting the gain multiplier, the AWG can use the full 16-bits of the DAC and amplify or attenuate the output signal using the front-end analog electronics to ensure the best output signal quality.
LabVIEW can also directly generate I/Q data using the NI Modulation Toolkit. The Modulation Toolkit provides LabVIEW VIs for modulating and demodulating both analog and digital schemes such as AM, FM, PM, QPSK, and QAM. Figure 8 shows how to use the toolkit to create I and Q data for a frequency-modulated (FM) signal. With the first VI, you generate the FM message signal by selecting from standard waveforms such as sine, square, or triangle, and specifying the carrier frequency and frequency deviation. The next VI performs the modulation and returns the complex envelope of the FM signal. Lastly, two VIs extract I and Q data from the complex envelope signal and download it to two T-Clk synchronized arbs. The toolkit can also modulate a user-defined message signal and extract the modulated signal magnitude and phase (polar form) components to test a polar-based digital modulator. The programming for creating waveforms with different modulation schemes (such as QAM and QPSK) follows a similar structure.
To model channel effects, the Modulation Toolkit offers Rayleigh and Rician fading profiles; or you can create your own custom-defined fading profile based on the simulation tool output.
Double-Sideband Amplitude Modulation (AM)
By using only the in-phase (I) path of the OSP block, one can generate an AM signal by directly downloading the message signal into onboard memory. The message signal scales the amplitude of the programmable frequency output of the NCO.
Single-Tone and Function Generation
Using the OSP NCO, the PXI-5441 can generate sine, square, triangle, ramp and other standard and user-defined waveforms similar to function generators (The PXI-5671employs the NCO to generate RF tones). The benefit of using the OSP for function generation is elimination of waveform creation and downloads to the onboard memory for function generation and phase-continuous frequency changes. For example a tone with <50 Hz resolution may require many cycles of a sine waveform to be downloaded for generation at 100 MS/s, consuming valuable host PC processor and onboard memory resources. With the NCO, no waveforms are downloaded to arbitrary waveform memory. (For further information on NCO functionality please refer to .)
The frequency of the output waveform may be adjusted during generation with 355 nHz resolution for generating phase continuous frequency sweeps and hops. The phase is also adjustable relative to other synchronized instruments, the PXI 10 MHz reference clock or an externally supplied reference clock.