Design of Multi-band Mixed Signal Test System Based on DSP

1 Characteristics and test requirements of mixed signal testing

With the deepening of the digital wave, chips with mixed signal functions are increasingly appearing in people's lives. MODEM (such as ADSL), CODEC and rapidly developing mobile phone chips in the communication field, MPEG and DVD chips in the video processor field are all chips with mixed signal functions, which are characterized by high processing speed, wide frequency range, and increasingly shorter chip upgrade cycles. This requires the test system to have higher performance and wider frequency band, and a flexible architecture is required to cope with the ever-increasing chip test requirements, so as to effectively reduce the test cost of new devices. In addition, there are many types of mixed signal chips, and various chips with mixed signals have been widely used in various fields of production and life. Different application fields have different working frequencies and required accuracy. This requires that when testing mixed signals, we should grasp their commonalities to propose test solutions. One of the most basic commonalities of all mixed signal chips is that they have AD/DA inside, and some mixed signal chips also include PLL modules. The mixed signal test discussed in this article only involves ADC and DAC channels.

Generally, for ADC channels, the test system needs to generate appropriate excitation signals through waveform generators, and at the same time, collect the output signals of ADC through its own digital channels and perform operations to obtain test results; for DAC channels, the test system needs to generate appropriate excitation signals through digital channels, and at the same time, collect the output signals of DAC through its own waveform samplers and perform operations to obtain test results. However, whether for ADC or DAC testing, the system's analog modules, namely waveform generators and waveform samplers, are required to be synchronized with the system's digital channels to ensure the accuracy of the test. In addition, the system also needs to provide a wealth of built-in functions to quickly complete the operation of the collected signals in order to achieve the test of the static and dynamic characteristics of ADC/DAC. Therefore, in the process of testing mixed-signal chips, digital channels, waveform generators, waveform analyzers, DC power supplies, time slot analyzers, system clocks and other modules are generally required, among which waveform generators and waveform analyzers are the key links that determine the speed and accuracy of the system. This article makes a more detailed discussion on these two aspects.

2 Challenges of Traditional Testing Methods

Traditional pure software-based test methods have been unable to cope with the test requirements of new mixed-signal devices. For mixed-signal devices, it is usually necessary to test parameters such as intermodulation distortion (IMD) and multi-tone power ratio (MTPR) to obtain the nonlinear characteristics of the device. Testing these parameters requires the collection of a large amount of data and a large amount of calculations. Traditional test instruments are usually implemented in the host using software-based methods. The disadvantage is that on the one hand, the improvement of the calculation speed is limited; on the other hand, there is a large amount of data interaction between the host and the test bench, which easily causes bus conflicts, so the test efficiency is not high. In view of this situation, a practical solution is to establish a local instrument subsystem at the front end of the test system, and transplant the original software-based signal processing library to the local processor to complete it. The instrument subsystem integrates the functions of signal generation and acquisition, and uses high-speed DSP to perform a large number of mathematical operations to reduce the processing time of the host and eliminate bus conflicts. Moreover, the front-end subsystem and the device under test (DUT) are compactly structured, and a high clock frequency can be achieved, which facilitates the flexible handling of various test problems. For example, in traditional test systems, it is difficult to generate audio with the dynamic range required for ADSL testing. Now, using an arbitrary waveform generator based on DSP, this task becomes very easy. The newly developed BC3192V50 digital-analog hybrid integrated circuit test system is based on the VXI bus design. Its greatest advantage is that both the software and hardware have an open and standardized structure. This structure allows the development of new modules and subsystems to enhance the system's test capabilities to meet the test needs of the rapidly developing integrated circuit industry. In response to the test needs of multi-band mixed-signal ICs, a DSP-based test system is used to implement complex multi-band mixed-signal chip testing.

3 Current Status of Mixed Signal Testing at Home and Abroad

In recent years, the testing industry at home and abroad has kept pace with the development of IT technology, constantly researching testing methods for new devices, and developing and updating testing instruments to meet market needs.

Since foreign countries paid attention to IC testing earlier, the development scale of test instruments is large and the technology is mature. A typical example is Credence's ASL series of large test systems, which use digital signal processing technology to enable IC manufacturers to test all devices for consumer audio and video applications on a single platform. my country started relatively late in IC testing. Although some large test systems with mixed IC testing functions have been developed, mixed signal chip testing in multiple frequency ranges has not yet been put into practical use. In addition, large test instruments, especially imported test instruments, are very expensive, and the development of test programs and the maintenance and repair costs of instruments are very high. At present, the domestic test instrument market is mainly concentrated in medium and small test instruments. Therefore, it is very meaningful to realize the testing of multi-band mixed signal chips through instrument systems based on digital signal processing, and its market prospects are also very good. At present, the development direction of mixed signal test systems is to improve the testing capabilities and test efficiency of high-speed and multi-band ICs, and to reduce the testing costs as much as possible. Therefore, today's test systems must have a flexible architecture to meet the needs of multiple mixed signal tests. The mainstream practice in the industry is to further modularize the test system, use instrument subsystems to independently process signals and output test results, and integrate high-speed signal processing modules in instrument subsystems. In addition, in order to improve test efficiency and rationally utilize resources, using the same test system to perform multi-chip parallel testing is also a development trend.

4 DSP-based mixed signal test solution

In view of the current mixed signal testing needs and the actual situation of similar domestic products, a low-cost solution for mixed signal testing is proposed. That is, it draws on the architecture of the current mainstream mixed signal test system in the world, closely combines the characteristics of small and medium-sized test instruments, makes full use of the openness and standardized structure of the BC3192 test system, and adopts a new high-speed DSP to enhance the test capability of multi-band mixed signal chips.

The implementation scheme in this paper is to use a combination of an arbitrary waveform generator (AWG) and an audio/video digitizer (AVD) to deal with mixed signal chip testing in multiple frequency bands. The combination of AVD and AWG can meet the needs of waveform generation, operation and analysis. The internal integration of high-performance DSP can provide complete DSP library functions for testing and analysis, thus meeting the various testing needs of mixed signals.

As shown in Figure 1, the arbitrary waveform generator can work in single-ended and differential modes, has complete waveform generation capabilities, can generate modifiable complex waveforms, and has the timing function required to stimulate the device under test. As a module of the VXI instrument system, AWG communicates with the resource controller through a standardized interface module. When it is necessary to generate an excitation waveform, the resource controller sends commands and parameters to the AWG module. The AWG module uses a high-speed DSP chip as the main processor. The DSP, together with the waveform timer and control logic, completes the generation of arbitrary waveforms, thereby providing the excitation waveform required by the device under test (DUT). Due to the wide variety of chips under test and the wide frequency band range, the timers and control logic used for different chips under test are not the same. Moreover, with the development of electronic technology, frequency synthesis technology has been widely used in the field of test and measurement. The use of digital frequency synthesis (DDS) dedicated chips makes the circuit more concise and reliable. Therefore, the design of the arbitrary waveform generator is also easier, and its anti-interference ability and accuracy are easier to guarantee.

As shown in Figure 2, the audio/video digitizer is mainly composed of signal conversion and conditioning circuits, waveform memory, DSP, control logic and interface circuits. AVD can design multiple channels, each channel has independent hardware resources, and can capture the measured signals of various frequency bands. Similarly, AVD is also a standardized VXI instrument module, and it also completes the test task together with AWG and other modules under the control of the resource controller. The AVD module contains a high-speed DSP chip as the core processor. On the one hand, a large number of signal processing library functions are implanted in the DSP, and tasks such as FFT and digital filter can be completed in the DSP; on the other hand, some test algorithms can also be transplanted to the DSP. Therefore, some intermediate results of test data can be obtained inside the AVD module, which greatly reduces the amount of data transmitted to the host, and the data on the VXI bus is largely diverted. At the same time, the host's calculation amount is also shared by the DSP chip, which plays an important role in shortening the test time. When testing multiple large-scale mixed-signal chips, the parallel structure of multiple DSP chips is adopted to complete a large number of signal processing and test algorithms inside the AVD module, which can significantly improve the test efficiency.

This solution adopts standardized and modular methods in the design of hardware and software, so the architecture is flexible and easy to upgrade. First, the multi-band mixed signal instrument system composed of arbitrary waveform generator (AWG) and audio/video digitizer (AVD) as the main modules has relatively complete functions and can independently complete most of the work of mixed signal testing. For mixed signal chip testing in different frequency bands, different AWG and AVD combinations can be used. Secondly, due to the compact structure, signal acquisition and processing can be completed locally on the test bench, which improves the signal processing capability and eliminates bus conflicts. Thirdly, this flexible architecture makes it convenient for users to configure the test system with each module as an option, effectively reducing the test cost. In addition, this solution realizes the transplantation and optimization of the digital signal processing library, improves the signal processing capability, and provides conditions for multi-chip parallel testing.

5 Conclusion

The development of IC technology, on the one hand, provides a large number of powerful chips for system designers to choose from. The current cost-effective DSP chips not only cater to the current wave of digitalization, but also provide designers with a variety of low-cost options; on the other hand, the emergence of a large number of new chips has brought new challenges to chip testing. As one of the core digitalization of electronic products, mixed signal chips have become a new hot spot in the field of chip testing. In view of the actual situation of the domestic test equipment manufacturing industry, this paper proposes a low-cost solution for mixed signal testing. This solution makes full use of existing conditions, follows the design ideas of standardization and modularization, and solves the problem of mixed signal testing in multiple frequency bands. It is an exploration of the domestic test equipment manufacturing industry under actual conditions. I believe it will continue to develop and improve with the emergence of new technologies.