Configuring Hybrid PXI/Benchtop Vector Signal Generators

This tutorial is part of the National Instruments Signal Generator Baseband Developer Kit. Each tutorial in this series will teach basic concepts about the architecture, features, or strategies for baseband IQ signal generation.

Introduction

In the following document, learn how to use PXI arbitrary waveform generators (AWGs) as a baseband source for traditional benchtop vector signal generators (VSGs).  This tutorial is part of the National Instruments Signal Generator Fundamentals. Each tutorial in this series will teach basic concepts about the architecture, features, or strategies for signal generation. 

As communication systems continue to evolve in complexity, the requirements for generating modulated RF signals have become increasingly challenging. For example, testing next-generation MIMO systems requires test architectures with multiple channels of synchronized RF generation and acquisition. In addition, large test waveforms in automated test (ATE) systems require a platform with high bus speeds and tight integration with analysis software. As a result, while PXI-only test solutions can solve the RF test needs of many applications, it is also important to evaluate hybrid PXI/benchtop solutions to meet niche application needs such as MIMO test systems and wide-bandwidth signal generation. In this white paper, we will discuss the technologies required for hybrid systems to solve niche application needs. In addition, we will discus the architecture for a typical hybrid system and the benefits that it provides.

Basics of Baseband Upconversion

Modern architectures for RF signal generation typically implement a combination of digital-to-analog converters and signal mixers. Typically, a transceiver generates a modulated signal either as a pair of in-phase (I) and quadrature-phase (Q) baseband signals or as a signal that has been digitally upconvertered to an intermediate frequency (IF). The first approach is commonly referred to as the homodyne or direct approach; the second approach is known as heterodyne or IF upconversion. Traditional benchtop VSGs typically use the direct upconversion approach, which is illustrated below:

Figure 1. Frequency Domain of Direct Upconversion

As the figure above illustrates, direct upconversion translated two baseband I and Q signals into a signal at the desired RF frequency. When upconversion is done directly, the center frequency of the RF signal is the same as local oscillator (LO) frequency of the upconversion circuitry.

With hybrid PXI/benchtop systems, PXI AWGs are used to generate baseband I and Q signals. Using external I and Q inputs, the vector signal generator mixes the baseband signals with its internal LO to produce the final RF signal. On a hardware level, this process is rather simplistic and can be represented by a simple block diagram, shown below:

Figure 2. Block Diagram of a Direct Upconverter (VSG)

As the circuit illustrates, baseband I and Q signals are independently mixed with two local oscillators, each of which is 90 deg out of phase. Translated I and Q signals are then summed to produce an RF output. As the figure illustrates, the local oscillator produces the carrier of the RF signal internally in the VSG.

Basic Architecture of Hybrid PXI/Benchtop Systems

In hybrid PXI/benchtop RF generation systems, two AWGs are synchronized to provide the baseband inputs to a benchtop VSG. The benchtop VSG then performs direct upconversion to mix each of the baseband I and Q signals with the appropriate LO or frequency synthesizer. In addition, a GPIB controller on the PXI system is able to fully automate the RF generator. A block diagram of a typical system configuration is shown below:

Figure 3. Architecture of a Hybrid PXI/Benchtop VSG

As the figure illustrates, hybrid PXI/benchtop RF generation systems can be configured easily by using the external I and Q inputs available on most benchtop VSGs. In addition, with hybrid systems, users can take advantage of the synchronization, throughput, and analog performance of PXI AWGs

Configuring a Hybrid System

The process of configuring a hybrid PXI/Benchtop RF generator requires three basic steps. Each is highlighted in Figure 4.

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Figure 4. Hybrid PXI/Benchtop Instrumentation Configuration

Step 1. Configuring the Baseband Generation

As the image illustrates, a PXI system with two AWGs and a GPIB interface must be connected to the VSG. When configuring the AWGs, each must be synchronized as tightly as possible to ensure minimal baseband error. In addition, the waveforms can be created from software such as the LabVIEW Modulation Toolkit. With these VIs, a baseband signal is created as a complex waveform with real (in-phase) and imaginary (quadrature-phase) components. Thus, this waveform must be split into its real and imaginary components, with one AWG generating I (real), and the other generating Q (imaginary). This is illustrated in the LabVIEW block diagram below

Figure 5. AWG Synchronization with NI-TClk

As the block diagram illustrates, each AWG can be configured independently in a for loop. Also, an array of session handles is passed to the three NI-TClk VIs, which are responsible for synchronizing the instruments. As a result of the simple diagram above, the two signal generators will generate baseband I and Q signals.

Each AWG should be connected to the external I and Q inputs of the VSG. For this configuration, it is important that two cables of the same length are used, to prevent quadrature skew. Because signals propagate at approximately 2 nanoseconds per meter, unmatched cable lengths can result in significant baseband skew.  In fact, this is particularly problematic at high baseband symbol rates.

Figure 6. Hybrid PXI/Benchtop Instrumentation Configuration

As the Figure 6 illustrates, VSGs such as the E443x series from Agilent typically provide options for baseband I and Q inputs. In addition, the RF bandwidth can be increased in most case by supplying baseband signals through the external inputs. Once the external inputs have been connected, the VSG must be configured to use the appropriate generation mode. This step is illustrated below.

Step 2. Configuring the Benchtop VSG

The next step requires configuration of the benchtop VSG. As shown above, each of the baseband outputs of the PXI system is configured to the external I and Q inputs of the VSG. The VSG must then be configured to use these inputs as the source of the baseband signal. The configuration step required to do this varies from generator to generator. In Figure 7, we show the appropriate setting to select external IQ mode on an Agilent E443x VSG.

Figure 7. Selecting External IQ Inputs on Agilent E443x Series VSG

Step 3. Characterizing the RF Output

The final step involves characterizing the performance of the hybrid RF generator with a variety of tests making both modulation and spectral measurements. In this section, we will discuss two different types of measurements that can be made to verify the accuracy of the hybrid generator.

Modulation Measurements

Typical modulation measurements that are used to characterize an RF signal include error vector magnitude (EVM), modulation error ratio (MER), quadrature skew, and IQ gain imbalance.  These measurements provide a more detailed analysis of the overall system performance and can be made with a vector signal analyzer (VSA).  A typical hardware configuration is shown in Figure 8:

Figure 8. System Architecture for Modulation Measurements

As the figure illustrates, the external baseband inputs of the VSG are supplied with modulated I and Q signals. Also, a PXI vector signal analyzer can be used to characterize the signal quality of the RF output. Thus, we can analyze the gain, phase, and offset attributes of the hybrid VSG system with the RF analyzer. In addition, we can make modulation measurements on the modulated signal, such as error vector magnitude, IQ gain imbalance, and quadrature skew. Visually, these measurements are also observed with a constellation plot, shown below.

Figure 9. Constellation Graph Showing Quadrature Skew

As Figure 9 illustrates, quadrature skew in the benchtop VSG translated to significant error in the modulated signal. This can be represented graphically in the constellation plot above or characterized with measurement functions of the LabVIEW Modulation Toolkit.

However, modulation measurements alone cannot be used to fully characterize system performance. Because the accuracy of these measurements is also dependent on the accuracy VSA, it is also important to characterize the system with spectral measurements such as sideband and carrier suppression, described below.

Spectral Measurements

Spectral measurements such as carrier suppression (also known as image suppression) and sideband suppression can be used to characterize effects of errors in phase, gain, and DC error of the baseband inputs. This measurement is performed by generating single tones (sine and cosine) on I and Q inputs at the same frequency. A typical measurement configuration is shown below.

Figure 10. Single-Tone Characterization

As Figure 10 illustrates, each AWGs generates single tones that are 90 deg out of phase (sine and cosine), and are at the same frequency. In an ideal system, these tones will be mixed with an LO so that the LO - tone signals are exactly 180 deg out of phase with one another. This tone should cancel out when the translated I and Q signals are summed. In the ideal case, the resulting output should have a single tone at the sum of the local oscillator and the input frequency. This is illustrated in Figure 11:

Figure 11. Single-Tone Measurements

Realistically, LO - tone signal components will not entirely cancel and we are left with some signal power at that frequency. In addition, any DC offset present at either the I or Q inputs will translate into an additional tone at the LO frequency. These two signals are commonly referred to as the unsuppressed sideband and unsuppressed carrier. In addition, we can we can evaluate system performance by measuring the amplitude of these signal components with respect to the amplitude of the local oscillator.

Figure 12. Sideband and Carrier Suppression

As Figure 12 illustrates, a nonideal system will result in two additional tones at frequencies of the local oscillator (LO) and at the frequency difference of the LO and the stimulus tone.  The peak at LO - Tone, referred to as sideband, is a product of: (1) gain error either at the baseband inputs or the upconverter analog mixers, or (2) phase error between the in-phase and quadrature-phase LO. If the the carrier is well suppressed (less than -70 dBc), we can be certain of tight baseband synchronization. The unsuppressed carrier appears at the center frequency of the LO (center frequency) of the VSG. This peak results from DC offset between I and Q. As Figure 13 illustrates, we can achieve a carrier suppression of better than -65 dBc when using external baseband inputs to an Agilent E443x series VSG.

• Resolution Bandwidth = 100 Hz
• Carrier LO = 1 GHz, 0 dBM
• IQ Tones = 5 MHz, 100 mV_pp

Figure 13. Single-Tone Spectral Measurements

As Figure 13 illustrates, system performance can be evaluated by measuring the amplitude of the unsuppressed sideband and unsuppressed carrier.

Considerations for Baseband Generation

Using PXI AWGs to generate baseband I and Q signals for hybrid systems provides several basic benefits. These include tight multi-module synchronization, wide modulation bandwidth, and increased system throughput with onboard processing.

Tight Baseband I and Q Synchronization

Because of the built-in synchronization features in the PXI architecture, PXI AWGs can implement the NI-TClk driver to generate baseband signals with minimal quadrature skew. Using this driver, AWGs use a common reference clock to synchronize generation of baseband I and Q signals. Typical channel-to-channel skew ranges from 100 ps to 1 ns. In addition, AWGs can be calibrated to reduce the typical channel-to-channel skew to 20 ps.

When generating baseband I and Q signals, minimal skew between I and Q is essential to generating accurate RF signals. In fact, even minimal baseband skew can result in significant error. The error is frequently characterized by an error vector magnitude measurement (EVM), and exponentially increases as the channel skew increases. This is illustrated in Figure 14.

Figure 14. EVM versus Baseband Skew for 40 MHz 802.11n Signal

(Compliments of Lyocom, Inc.)

As the graph illustrates, tight baseband I and Q synchronization is essential to reduce the EVM of the signal. Thus, for standards such as 802.11n, which require a channel bandwidth of 40 MHz, even small amounts of baseband skew can cause significant system error.

Modulation Bandwidth

Secondly, use of PXI AWGs as external I and Q baseband sources enables traditional benchtop VSGs to generate RF signals with a wider channel bandwidth. Many VSGs offer a modulation bandwidth of 40 MHz or less when using the standard internal baseband source. However, many benchtop VSGs can generate wider-bandwidth RF signals simply by using an external baseband source. In fact, the bandwidth of the RF signal will be twice the bandwidth of each individual I or Q input. For example, when synchronizing two PXI-5422 AWGs to generate baseband I and Q signals, you can use a hybrid solution and achieve up to 160 MHz of total modulation bandwidth. Thus, when bandwidth signal generation is required, you can use PXI AWGs to meet the system performance requirements while maintaining the advantages of a PXI-based architecture.

Measurement Speed

Finally, use of PXI AWGs as a baseband source for hybrid PXI/benchtop RF generators increases system throughput by taking advantage of two factors - availability of onboard signal processing (OSP) and bus speed of the PXI architecture. First, OSP on the PXI-5441 AWG offers two major functions that increase system throughput by decreasing waveform download and times. These are illustrated in the figure below:

Figure 15. Onboard Signal Processing (OSP) Functional Blocks

As the figure illustrates, OSP enables both digital interpolation and pulse-shaped filtering. By implementing both features in hardware on a field-programmable gate array (FPGA), we are able to significantly reduce download times of the baseband waveform. Using OSP, you can download to the AWG waveforms that are sampled at the symbol rate of the baseband signal. Thus, because interpolation is able to increase the effective sampling rate in hardware, you can gain the advantages of a high effective sampling rate while observing high system throughput.

Conclusions

By integrating PXI and benchtop instruments, you can configure automated test systems that take advantage of both test platforms. Using PXI as a platform for baseband generation, you can easily configure systems with multiple channels of synchronized RF generation. In addition, you can use the wide modulation bandwidths offered by PXI AWGs. Finally, you can configure systems with high system throughput by taking advantage of OSP to reduce data rates, and by relying on the fast bus speeds of the PCI bus. Thus, when configuring RF test systems, consider using a hybrid PXI/benchtop system architecture for applications requiring accurate RF signals, wide bandwidth, and fast measurement speed.

Related Products

NI modular instruments can be used with the LabVIEW Modulation Toolkit to implement a complete communications system - from baseband waveforms to RF signals.  As an example, common NI products include the following:

• NI PXI-5441 - 100 MS/s arbitrary waveform generator with onboard signal processing
• NI PXI-5671 - 2.7 GHz vector signal generator with digital upconversion
• NI PXI-5661 - 2.7 GHz vector signal analyzer with digital downconversion
• NI PXI-5142 - 100 MS/s digitizer with onboard signal processing

Related Links

• ni.com/rf
• ni.com/signalgenerators
• ni.com/digitizers
• ni.com/communications

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