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In the following document, learn how instrument synchronization with the NI-TClk technology benefits applications such as baseband IQ generation; multiple input, multiple output (MIMO) systems; audio/video test; ADC characterization, and ultrasonic phase array imaging.
Table of Contents
Introduction
This tutorial is part of the National Instruments Signal Generator Fundamentals series. Each tutorial in this series will teach basic concepts about the architecture, features, or applications of signal generators.
Many modern automated test equipment (ATE) systems require the generation of multiple synchronized channels as a system stimulus. National Instruments signal generators are able to service these applications because of the timing and synchronization features available on the PXI platform. This white paper includes a brief overview of the PXI platform, highlights the technology that enables picosecond-level synchronization, and discusses several applications that can benefit from synchronizing multiple channels and multiple devices. These applications include but are not limited to baseband IQ generation; multiple input, multiple output (MIMO) systems; audio/video test; and ADC characterization.
Signal Generator Clocking Architecture
National Instruments signal generators use the same Synchronization and Memory Core (SMC) implemented on many NI modular instruments including arbitrary waveform generators, digitizers, and high-speed digital waveform generator/analyzers. This architecture provides a mechanism for multiple instruments to share timing and synchronization signals. In addition, the SMC offers a common input/output engine and trigger and event controllers. For more information on how this architecture enables modular instruments to achieve picosecond-level synchronization between instruments, view National Instruments Synchronization and Memory Core - A Modern Architecture for Mixed-Signal Test.
The SMC architecture enables signal generators to share external signals with other instruments to enable synchronization of multiple channels. A block diagram of a typical signal generator is shown below.
Figure 1. Signal Generator Block Diagram
As the block diagram illustrates, each signal generator utilizes common clocking signals to synchronize the output on multiple channels. In the following sections of this white paper, learn about the fundamental technologies that enable this capability and get an in-depth view into several applications that require tight synchronization between instruments. This includes applications that need multiple channels of signal generation with minimal channel-to-channel skew. In addition, explore applications that require synchronization between different types of instruments.
NI-TClk Overview
The common application programming interface (API) that provides the tightest level of synchronization between multiple instruments is the NI-TClk API. Synchronization is achieved by sharing two signals - a PXI reference clock and a signal called a trigger clock (TClk). Through a patent-pending routine, the NI-TClk driver is able to use common signals such as the PXI reference clock and a trigger clock to ensure that multiple instruments are tightly synchronized. With this API, a common reference clock guarantees that each instrument shares the same timebase. Moreover, a trigger clock enables each device to calculate the propagation delay of the PXI trigger lines to make sure that each module receives a start trigger at the same clock edge. As a result, the NI-TClk driver delivers an easy-to-use mechanism to achieve less than 1 ns of skew with three simple VIs. In addition, it is able to achieve within 20 ps of skew with an extra calibration step.
To learn more about how the NI-TClk API is able to achieve subnanosecond synchronization between multiple instruments, refer to National Instruments T-Clock Technology for Timing and Synchronization of Modular Instruments.
Software Interface
One of the distinct advantages of the NI-TClk technology is the simplicity of the programming interface. Figure 2 screenshot of a block diagram illustrates the use of three NI-TClk LabVIEW Vis to synchronize multiple signal generators..
Figure 2. NI-TClk API
As the block diagram above shows, you can configure each signal generator independently with the standard NI-FGEN API. In fact, the only programming step required to synchronize multiple instruments involves the three NI-TClk subroutines shown in blue. The last of these subroutines initiates generation of all signals. As a result of these simple routines, TClk synchronization enables the generation of multiple channels with 1 ns or less of skew. To guarantee even more precise synchronization, you can use the NI-TClk property node to manually tweak the delay for individual channels to account for factors such as cable length.
Synchronization Performance
The realized performance of the NI-TClk technique is limited by the PXI reference clock slot-to-slot skew and the accuracy of the circuitry which measures the propagation delay of the trigger lines. With an NI-TClk property node (Figure 3), it is possible to eliminate slot-to-slot skew almost completely. Thus, the level of synchronization can be as tight as 20 ps when the system is calibrated with this property node
Figure 3. NI-TClk Property Node
To illustrate the level of synchronization possible using NI signal generators, we have conducted a test where two NI PXI-5421 modules are configured to generate a 5 MHz sine wave at 100 MS/s. In addition, the sample clock delay on one of the signal generators has been manually calibrated to account for the slot-to-slot skew in the PXI system. The result is that the skew between the signal generators is within 20 ps. This is illustrated in Figure 4, which shows a histogram of 1,000 skew measurements.
Figure 4. The phase difference between the two channels appears as a Gaussian distribution.
As Figure 4 shows, the phase difference between the two channels appears as a Gaussian distribution. In addition, you can calculate the average phase difference in degrees as well with the following equation:
Thus, for this given test case, the signal frequency is 5 MHz and the maximum skew is 20 ps. This translates to a maximum phase difference of 0.18 degrees, which is shown below.
As you will see in a later section, certain applications need tight synchronization between signal generators. For example, using two arbitrary waveform generators to create an IQ modulator requires tight synchronization between channels.
Multichannel Signal Generation Applications
Many applications benefit from the tight synchronization between multiple signal generators. These application areas include baseband IQ generation for radio frequency integrated circuits (RFICs); multiple input, multiple output (MIMO) test systems; audio/video signal generation; ADC characterization; and phase-array ultrasound systems. Typical test scenarios for each of these applications are described below.
Baseband IQ Generation
One of the most important requirements of a baseband IQ generator is precise synchronization between each channel. Ideally, I and Q baseband signals are perfectly synchchronized to prevent quadrature skew. In fact, even small phase differences between I and Q can result in unwanted spectral images. Thus, the NI-TClk driver provides an ideal solution to synchronizing channels for both I and Q. In addition onboard signal processing (OSP) available on the NI PXI-5441 arbitrary waveform generator can apply pulse-shape filtering and system impairments for a complete test solution. For more information about OSP features on the PXI-5441 signal generator, view Onboard Signal Processing (OSP) on National Instruments Signal Generators.
Figure 5 illustrates a test configuration for a radio frequency integrated circuit (RFIC). As the diagram shows, you can synchronize two PXI-5441 arbitrary waveform generators to generate I and Q outputs that are exactly 90 degrees out of phase.
Figure 5. Test Configuration for a Radio Frequency Integrated Circuit
Note that for baseband IQ generation, it is recommended that you use the NI-TClk property node to calibrate the signal generators to a level of synchronization within 20 ps. In addition, you can find a more thorough analysis of the benefits of OSP and NI-TClk synchronization for baseband generation in the Benefits of SMC-Based Arbitrary Waveform Generators for I/Q Signal Generation white paper.
Many RFICs use differential I and Q inputs. In these applications, four arbitrary waveform generators are synchronized and connected to the appropriate channels. A block diagram of this system is shown below.
Figure 6. Test Configuration for a Differential Direct Upconverter
As Figure 6 illustrates, an RFIC using differential I and Q signals requires four DACs to generate the test signal. Again, you can synchronize all signal generators with T-Clk synchronization to within 1 ns of skew without calibration. Also, you can use calibration to synchronize each channel to within 20 ps of skew. For a 10 MHz baseband signal, this translates to exactly 0.05 percent of skew. You can reduce the resulting quadrature skew to within 0.18 degrees of the ideal 90 degrees phase shift between I and Q.
Multiple Input, Multiple Output (MIMO) Communications Systems
Synchronization of multiple signal generators is also crucial when testing multiple input, multiple output (MIMO) communications systems. MIMO, an increasingly popular communications system technique, has become increasingly popular in a growing number of communications systems. It uses multiple antennae to increase spectral efficiency and reduce multipath interference.
Because MIMO systems require multiple channels of generation, synchronization between those channels is extremely important. In a typical MIMO system, anywhere from two to eight signal generators are synchronized with their output going to individual upconverters sharing a common local oscillator (LO). When using direct (homodyne) upconversion, you can pair signal generators to generate baseband IQ signals for each upconverter. However, heterodyne upconverters require an intermediate frequency (IF) signal instead. Because you are able to digitally upconvert baseband signals to IF frequencies of up to 43 MHz using the PXI-5441, you need only one signal generator for each heterodyne upconverter. A typical system setup is shown in Figure 7.
Figure 7. MIMO Signal Generation Configuration
In some MIMO systems, it is important that the RF output of each upconverter is synchronous but not necessarily in phase. In these instances, it is possible to generate MIMO signals with the multiple NI PXI-5671 modules. However, some MIMO systems require the antenna output to be both synchronous and in phase. In these systems, each upconverter must also share a local oscillator. Thus, as illustrated in Figure 7, a master upconverter shares a common local oscillator with three slave modules. As a result, the phase relationship between each antenna is preserved at the RF output.
Audio/Video ATE System Test
For systems requiring mixed audio/video signals, synchronization between multiple arbitrary waveform generators enables the synchronization between these channels. For example, when testing AV systems, it is necessary to synchronize both the audio and video channels. With National Instruments signal generators, out-of-the-box synchronization to 1 ns easily meets the synchronization requirement. A block diagram of a typical test system is shown in Figure 8.
Figure 8. Audio/Video Test Configuration
Note that for video generation in typical test systems, many engineers use the deep onboard memory of National Instruments signal generators. For a more in-depth tutorial on the types of video signals you can generate, view Video Signal Measurement and Generation Fundamentals.
Multi-Instrument Test Applications
As described above, you can tightly synchronize National Instruments signal generators with each other for applications requiring multiple stimulus channels. In addition, the same SMC architecture that enables synchronization between multiple signal generators also provides synchronization between other instruments. In particular, you can use the same NI-TClk LabVIEW programming interface to synchronize one or more signal generators with high-speed digital waveform generator/analyzers and high-speed digitizers. Later in this white paper, learn about several applications that benefit from the synchronization capabilities among multiple PXI instruments.
Channel Crosstalk Test on Multichannel ADCs
One type of measurement that requires the ability to synchronize signal generators with other hardware is a crosstalk measurement on a multichannel ADC application-specific integrated circuit (ASIC). With this measurement, you can use the NI-TClk API to synchronize the generation of an analog sinusoid with the acquisition of a digital ADC output. In particular, this test is most common on ASICs that contains multiple ADCs.
While channel crosstalk can be reduced by careful IC design, it cannot be completely eliminated. Thus, it is important to characterize of crosstalk on a multichannel ADC ASICs. To measure the effect of channel crosstalk, drive one or more ADCs to maximum input range while measuring the response on the adjacent channel. Figure 9 illustrates this test setup.
Figure 9. ADC Channel Crosstalk Characterization
As Figure 9 shows, you can use two NI 5406 arbitrary function generators to supply sine-wave signals of different frequencies and at the maximum input range to two ADCs (ADC0 and ADC1). Meanwhile, ADC2 is not driven with an input signal and is left floating. The PXI-6552 high-speed digital I/O analyzer, which can clock 16 lines of digital data at up to 100 MHz, supplies the clock signal for the ASIC and captures response of each of the idle ADC. Finally, this waveform can be analyzed in the National Instruments LabVIEW graphical development environment to fully characterize the response of the idle ADC.
As one might expect, a channel crosstalk characterization test requires the synchronization of multiple instruments. As the example above describes, you can NI-TClk to synchronize two channels of signal generation with fourteen channels of digital waveform acquisition. Moreover, these instruments are able to achieve less than 1ns of channel-to-channel skew by adding three simple LabVIEW VIs.
Ultrasonic Imaging Phase Array
Ultrasonic imaging in for nondestructive test also requires tight synchronization between stimulus and response. In general, ultrasonic signals range in frequency from 20 kHz to 25 MHz and beyond. The fundamental idea behind ultrasonic testing is to stimulate a system with an initial pulse and then characterize any reflections that it returns. A typical test can be implemented with an arbitrary waveform generator used to supply the stimulus and a high-speed digitizer to capture the response. The responses are then characterized according to the pulse delay and amplitude. Thus, when measuring the pulse delay, one fundamental requirement of an ultrasonic imaging system is tight synchronization between the signal generator and the high-speed digitizer. Figure 10 illustrates this concept by showing both the initial pulse and the reflections that are produced.
Figure 10. Pulse Characterization in Ultrasonic Testing
You can create an ultrasonic imaging phase array using an arbitrary waveform generator with an NI PXI-5105 digitizer. By synchronizing the generation of a single pulse with simultaneous acquisition on eight channels of the digitizer, an ultrasonic imaging phase array is able to reduce the test time in nondestructive ultrasonic testing. A block diagram of this system is shown below.
Figure 11. System Diagram of Ultrasonic Imaging Phase Array
As Figure 11 illustrates, you can use a single arbitrary waveform generator to stimulate multiple portions of the unit under test. Defects in the product produce reflections that the PXI-5105 captures. For more information on this product, view the PXI-5105 Digitizer 7-Minute Demo.
Conclusion
Synchronization between signal generators and other instruments is a crucial aspect of developing next-generation automated test systems. In addition, many NI modular instruments use patent-pending TClk technology to achieve picosecond-level synchronization. As a result, you can service applications requiring multiple inputs and multiple instruments within a single PXI system.
Related Links
Related Products
PXI-5441 Baseband IQ/IF Arbitrary Waveform Generator
PXI-5422 200 Ms/s Arbitrary Waveform Generator
PXI-5406 40 MHz Function Generator
PXI-6552 100 MHz Digital Waveform Generator / Analyzer
PXI-5105 8-Channel High Speed Digitizer
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