PXI Express: Enabling a High-Performance Mixed-Signal Test Platform

PXI Express instruments offer new PXI platform benefits through the use of the PCI Express bus. This bus provides up to 2 GB/s of data throughput to multiple instrument systems and enables applications that were previously possible only with custom hardware.

Benefits of PXI Express

Now that the PXI standard includes PCI Express technology, PXI automated test systems offer higher performance than ever before. In some cases, PXI instruments now can perform measurements that were not previously possible. 

PXI Express is an extension of PXI. New PXI Express chassis provide hybrid-compatible slots that enable PXI and PXI Express modules to work in the same system. As a result, both PXI and PXI Express systems feature the following three key benefits for automated test applications:

• Flexible, software-defined instrumentation
• Integration of modular instruments
• High data throughput

With the flexibility of a software-defined approach to instrumentation, you can reconfigure test systems for various measurements. This is particularly pertinent in RF/communications manufacturing test, for which standard-specific measurements from multiple protocols are required to test the same device.

Second, through the integration of modular instruments into the same system, you can choose from more than 1,500 existing PXI instruments. These instruments include the highest-performing instruments in the test and measurement industry:

• High-accuracy multifunction data acquisition with 18-bit resolution
• High-resolution digitizer with up to 24 bits at 500 kS/s
• High-accuracy arbitrary waveform generator at 16 bits and 200 kS/s
• Fast 7½-digit digital multimeter with picoamp accuracy and 1000 V capability
• High-density switching with up to 512 crosspoints in a single 3U slot
• RF signal analysis and generation up to 6.6 GHz
• High-density channel count and synchronization (up to 5,000 dynamic channels)

With the ability to use multiple PXI instruments in the same system, you can test a variety of mixed-signal devices with a single test system. One common application is the use of PXI systems for mixed-signal semiconductor ASIC characterization.

Finally, both PXI and PXI Express instruments feature a high-performance data bus to transfer information from the instrument to the host PC. PXI instruments are capable of up to 132 MB/s of shared bandwidth for all devices on the bus. PXI Express instruments are capable of even greater throughput with the PCI Express bus. The PCI Express bus is a point-to-point high-speed serial bus that can scale from 250 MB/s to 2 GB/s per slot. As an example, a x4 ("by four") PXI Express slot offers up to 1 GB/s of dedicated bandwidth to the device in that slot. In addition, total system throughput increases as you add more instruments to the system. 

 

 

 

Figure 1. PCI Express Throughput Scales with the Number of Instruments Used

 

You can use the improved throughput of PXI Express instruments to create several new applications. Because of bus throughput, you can use PXI Express instruments with PXI Express RAID hard drives in high-speed stream-to-disk or stream-from-disk configurations. Two specific applications, signal intelligence and digital video test, benefit greatly from this capability.

Software-Defined Measurements: RF and Communications Test

With the virtual instrumentation approach, the PCI or PCI Express bus acts as a data bus from the instrument to the PC. You can analyze data with custom algorithms or common measurements such as rise time or THD (total harmonic distortion). With software-defined measurements, you can reconfigure instruments to perform a variety of tasks. 

One application area where software-defined measurements are necessary is RF and communications manufacturing test. Because today’s wireless devices use multiple communication protocols, such as 802.11g, GSM, GPS, and Bluetooth, the test challenges and test costs of wireless devices are increasing. In the past, you needed multiple instruments to characterize device performance for various communications standards. Unfortunately, the use of multiple stand-alone instruments was expensive. Today, the ability to perform software-defined RF measurements enables the reuse of RF instruments for compliance testing of multiple protocols. As a result, you can use the same instrument with multiple software personalities. One common example of instrument reuse for wireless standards compliance test is cell phone manufacturing, illustrated in Figure 2.

 

 

 

Figure 2. Cellular Manufacturing Test Requiring Multiple Standards

 

As the figure illustrates, a single PXI vector signal analyzer captures RF signals at various frequencies for different communications standards. Because the communications protocol stack is implemented in software, you can reuse the same instrument for each communications standard. Thus, the software-defined measurement approach reduces test cost and footprint.

Multiple Instrument Integration: Mixed-Signal ASIC Characterization

Another advantage of PXI for mixed-signal test is that it provides the ability to tightly integrate multiple instruments in the same system. This delivers accurate synchronization between instruments as well as correlation of analog and digital data, and it lends to a smaller instrument footprint. A variety of test applications greatly benefits from the mixed-signal approach to instrumentation. One example is a characterization of multichannel mixed-signal application-specific integrated circuits (ASICs), such as a digital-to-analog converter.

Modern ASICs require mixed-signal inputs and outputs with various signal requirements. Traditionally, automated test of these devices required multiple benchtop instruments that were expensive and needed a large amount physical space. Today, PXI instrumentation provides a single-platform solution with which you can integrate multiple instruments into a single test. 

As an example, consider the test instrumentation required to characterize a four-channel, 12-bit, 100 MHz digital-to-analog converter. This ASIC requires more than 48 channels of synchronized digital I/O, four channels of accurate analog input, and a programmable DC power source. Using PXI instrumentation, you can meet this test challenge by integrating multiple PXI instruments into the same system. As Figure 3 illustrates, you can synchronize multiple digital I/O modules to provide 48 channels operating within 1 ns of channel-to-channel skew. In addition, a PXI high-speed digitizer provides 14-bit precision at 100 MS/s. For the test system, you need only one digitizer combined with a low-insertion loss RF switch. Finally, you can use a PXI programmable power supply to sweep voltage from 0 to 6 V in 120 µV steps. The reference architecture for a multichannel DAC is shown in Figure 3.

 

 

Figure 3. Reference Architecture for 4-Channel DAC Characterization

 

Using PXI modular instruments, you can integrate a mixed-signal test platform into a single test system. The system required to test a four-channel DAC is shown in Figure 4.

 

 

Figure 4. Mixed-Signal PXI Instrumentation System

With a modular approach to instrumentation, you can reconfigure the system or expand it to meet future test requirements. In addition, by connecting to the National Instruments LabVIEW programming environment, you can take measurements such as THD, SFDR, and SINAD. In this system, you can comprehensively characterize the device under test by observing performance according to a variety of factors such as power and current.

High Data Throughput: Stream-to-Disk Applications

The greatest benefit of PXI Express instruments is the high data throughput of the PCI Express bus. It not only improves test times on common automated test applications but also enables new applications that were not possible with commercial off-the-shelf hardware until now. One example of this is stream-to-disk scenarios for applications such as signal intelligence and digital video test.

Traditional benchtop instrumentation such as arbitrary waveform generators, logic analyzers, and oscilloscopes use limited onboard memory as a temporary buffer to store waveform data. Onboard memory is expensive and comes in only limited sizes. These instruments could then transfer waveforms to or from a PC through a GPIB, LAN, or USB interface. Unfortunately, the data throughput is only several megabytes per second. For stream-to-disk or stream-to-memory applications, much higher throughput is required. PXI Express offers a compelling solution because of its high throughput and low bus latency. 

Fortunately, you can use the NI LabVIEW multithreaded programming model to easily optimize stream-to-disk applications. Because LabVIEW dynamically assigns programming tasks to multiple threads, you can achieve greater throughput by dividing instrument I/O and file I/O into two independent while loops. The recommended programming approach is the producer-consumer loop structure, which is shown in Figure 5.

 

 

 

Figure 5. Producer-Consumer Loop Architecture with Queue Structure

In the example above, the top loop (producer) acquires data from a high-speed digitizer and passes it to a queue structure (a LabVIEW FIFO). You can use the queue structure to pass data among multiple while loops in LabVIEW. The bottom loop (consumer) reads data from the queue structure and writes to disk. The producer-consumer loop structure delivers the best performance for stream-to-disk applications because the producer loop can continue to acquire data while the consumer loop is writing data to disk. 

 

Benchmarking Stream-to-Disk Applications

With the improved throughput of PXI Express instruments, you can achieve higher sample rates and channel count in stream-to-disk scenarios. To benchmark exact throughput for stream-to-disk applications, use the following equation:

 

As an example, consider a stream-to-disk scenario on two channels of the NI PXIe-5122 high-speed digitizer at a maximum sample rate of 100 MS/s. Note that the NI PXIe-5122 is a 14-bit digitizer. Thus, each sample requires 2 B of memory or disk space. The maximum throughput for the NI PXIe-5122 is shown below:

To precisely characterize the performance of an actual system, use a PXI Express dual-core embedded controller with a x4 PXI Express RAID-0 hard drive rated at 650 MB/s. For the test, use an acquisition size of 40 GB. The test results below reflect the use of multiple NI PXIe-5122 digitizers with 256 MB of onboard memory. Table 1 illustrates the maximum sample rate for stream-to-disk applications according to the number of channels required. 

 

 

Table 1. Benchmarked Stream-to-Disk Rates for the NI PXIe-5122 High-Speed Digitizer

As a variation on a stream-to-disk application, you can also stream data from a high-speed digitizer into the onboard memory of your PXI controller. This method does not require a RAID hard disk configuration, and throughput is not limited by the disk write speeds of a hard disk. Instead, throughput is limited by the bandwidth of the PCI Express bus and acquisition size is limited by the amount of available PC memory. In a typical stream-to-memory application, PC memory is only a temporary buffer for data. Because a typical embedded controller is capable of disk write speeds of 40 MB/s, you can store data in memory until you can write it to disk.

The following benchmark for a stream-to-memory scenario uses a PXI Express dual-core controller with 2GB of onboard memory. With an acquisition size of 100 million samples per channel, the test requires up to 1.2 GB of PC memory for six channels. Again, use multiple NI PXIe-5122 digitizers with 256 MB of onboard memory for the best outcome. The results are shown in Table 2.

 

 

 

Table 2. Maximum Stream-to-Memory Rates for the NI PXIe-5122 High-Speed Digitizer

 

One reason why stream-to-disk and stream-to-memory applications are able to achieve such high throughput in PXI is the use of a high-bandwidth and low-latency data bus – PCI Express. If you compare this bus to other standard data buses, you observe that it provides both the highest throughput and the lowest data latency.

 

 

Figure 6. Bandwidth versus Latency of Popular Instrument Buses

The ability to stream data to disk provides substantial benefits for many applications. Two common applications examined here are signal intelligence/spectrum monitoring and digital video test. 

Signal Intelligence: Intermediate Frequency Stream-to-Disk

Modern military surveillance, satellite communications, and spectrum monitoring applications require the ability to stream large portions of data to hard disk for extended periods of time. In the past, these applications required custom hardware that was expensive to build and maintain. However, you can now develop applications for streaming waveforms to disk for signal intelligence with commercial off-the-shelf (COTS) PXI and PXI Express instrumentation. 

 

 

Figure 7. Stream-to-Disk Configuration for Communications System Test

To capture RF signals, use a high-speed digitizer to acquire an intermediate frequency (IF) from a downconverter. The downconverter operates at the RF frequency and uses one or more mixers to translate RF signals, which you can capture with a high-speed digital-to-analog converter, to a frequency range. Using two channels of an NI PXIe-5122 high-speed digitizer sampling data at 100 MS/s, you can acquire two IF signals with 50 MHz of bandwidth on each channel. Because of this, you can acquire a total RF bandwidth of 100 MHz.

With signal intelligence applications, a portion of the spectrum is typically streamed to disk for minutes or even hours of time. Once saved, this data can be post-processed in software with a power spectrum or time-frequency spectrogram. In some instances, captured spectrum data is also generated back with an arbitrary waveform generator to simulate a real-world environment.

Consumer Electronics: Digital Video Test

Another application that requires long acquisition of test waveforms is digital video test. The DVI standard supports LCD monitors and flat-panel plasma displays. As new technology demands higher clock rates, the generation and acquisition of moving DVI display patterns require even longer waveforms. 

The ability to generate or acquire digital video patterns for long periods of time is crucial for the accurate testing of modern set-top boxes with DVI output. As an example, testing today’s set-top box image decompression and decoding algorithms require moving test patterns. Because pixilation occurs only with moving pictures, you must acquire the digital transmission for several seconds or even minutes at a time to detect these bit errors. In the Figure 8, you can see the effects of pixilation on a digital image. 

 

 

 

Figure 8. Pixilation Occurs as a Result of Transmission Error

 

With PXI Express, you can acquire DVI images continuously for up to several minutes or even hours using off-the-shelf RAID hard-drive configurations. For example, you can configure the NI PXIe-6537 high-speed digital I/O module to stream to disk at speeds up to 200 MB/s for several hours (2.5 hours = 1.8 TB). As a result, you can use off-the-shelf PXI Express instruments to perform accurate digital video pixilation tests.

Conclusion

With the PXI platform, you can achieve:

• Flexible, software-defined measurements
• Integration of modular instruments
• High data throughput

 

Because of these benefits, PXI instrumentation provides significant advantages to a variety of applications including RF and communications measurements, mixed-signal ASIC characterization, signal intelligence, and digital video test. Moreover, though each of these benefits are available in the PXI platform, PXI Express greatly improves the platform by offering significantly greater throughput with its use of the PCI Express bus. As a result, you can create highly accurate automated test systems with lower test times and greater test capabilities than ever before.

 

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