基于单片机和蓝牙的电机转速测量系统设计 - 图文

陕西理工学院毕业设计

and analyzed individually, it is the understanding of how they interact that allows the ATE designer not only flexibility, but also measurement accuracy. As in any design there are trade-offs that will be made in order to get both flexibility and high measurement accuracy. For the high speed digitizing oscilloscope, these trade-offs typically come in the form of increased data and processing time.

The trade-offs interact, and impact on measurement capability and flexibility will be elaborated on with examples in the next section. Ⅳ.OPTIMIZING MEASUREMENTS

Once all the “components” that are part of the oscilloscope are understood, making accurate, flexible measurements are a matter of creative application. Ⅴ.Flexibility and Accuracy

While many tools can be utilized in unintended ways, the key to allowing the oscilloscope to become the most flexible system component is utilizing the measurement system in ways that optimize the performance as it relates to a particular measurement. For example, in the Introduction, the differences between the measurements of a multi-meter and a spectrum analyzer were discussed. These differences will be utilized in order to highlight the concept of optimizing the oscilloscope's measurements.

Ⅵ.DC Measurement Example

In the case of using the oscilloscope for a DC measurement, simply connecting the signal to the input channel and making the measurement would not necessarily net acceptable results. The first issue is voltage range. The preamplifier/attenuators need to be set in order to optimize the digitization of the DC signal. In this example, if the DC voltage were 5 volts, a setting of 5 volts per division would be inappropriate because only a small percentage (one division out of 8 0r 10) of the full range of the A-D would be utilized. The input range should be optimized (not clipping the signal) using as much of the screen (translating into A-D range) as possible.

The next consideration is the bandwidth of the measurement. The noise figure is specified in units of dBm/Hz, therefore, the noise of the system is equal to the system bandwidth times the noise figure. A system designed with less bandwidth will have a lower noise than a system with more bandwidth; given both have the same noise figure. In this example, a DC measurement is being made which is notably lower in bandwidth than the capabilities of the oscilloscope. Therefore, it will be beneficial from a measurement noise perspective to limit the bandwidth of the oscilloscope.

How the bandwidth is limited is critical to the noise floor. If, for example, a hardware low-pass filter were placed in the line of the input of the oscilloscope, it would limit the frequency content making it into the input of the oscilloscope. But the system bandwidth of the oscilloscope remains the same; therefore, the system noise remains the same. Another way of saying this is that the noise contribution from the measurement system remains the same even though the outside measurement noise has been reduced.

An alternate method is to use some form of mathematical filtering which acts upon the data after it is digitized. This method provides for a measurement noise floor improvement by limiting the system noise. Averaging is one form of filtering, often utilized, which requires the same signal to be acquired multiple times. The benefit of this type of averaging over other signal filtering is that the bandwidth remains the same. The negative is that many signal captures are unique; therefore, this method of reducing noise cannot be applied. For a DC signal, a low-pass filter could also be utilized; this method provides very low noise, without multiple acquisitions. If a 12 GHz bandwidth oscilloscope had a noise figure of－148 dBm/Hz on a particular volts per division scale or range,

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陕西理工学院毕业设计

the noise at full bandwidth would be approximately 1 mVrms (power per hertz converted to volts per hertz times the scope bandwidth). If that same oscilloscope digitized the full bandwidth and then a 187 MHz low-pass filter were applied to the digitized signal, the resulting system noise would be 0.125 mV. Simply put, one 64th the bandwidth would reduce the noise power by one 64th which is one 8th the RMS voltage. The key in this second method is that only one acquisition need be analyzed, with filtering providing the measurement noise floor improvement. Ⅶ. RF Measurement Example

The contrasting measurement discussed in the introduction was that of a spectrum analyzer. In this case the same 12 GHz oscilloscope was being utilized but now instead of a DC measurement, pulse power of a 500 MHz wide radar chirp centered at 10 GHz was being measured. While very different than a DC measurement, some of the considerations that drive measurement accuracy are surprisingly similar.

The discussion of scaling is once again important, ensuring that the A-D digitizing range is maximized. This brings in another important consideration for guaranteeing the most accurate measurements. Characterizing the oscilloscope as a “system” at all scaling ranges will allow for the greatest flexibility once the system is deployed. Because the oscilloscope can accept a wide range of signal levels, the performance at each range may vary slightly because of the various combinations of attenuators or preamplifiers that are in place for each range. A good practice is to characterize the noise, phase, and magnitude response at all the respective ranges as insurance of measurement accuracy, when input ranges other than the original deployed ranges are needed.

Once the radar chirp is optimally scaled, it can be digitized. In this case, the signal can be directly digitized. The assumption is that this oscilloscope is what is considered a “real-time” oscilloscope, and the A-D speed is adequate to insure an instantaneous capture at the full bandwidth of the oscilloscope; in this case 12 GHz is being utilized. While a power measurement could be made on the captured signal as originally digitized, this would not necessitate the most accurate measurement. This is because there is also wideband system noise also captured along with the chirp. The signal of interest in this example is only 500 MHz wide, which has been captured along with all the spectra between DC and 12 GHz. Much like the example of the DC signal, filtering out unwanted frequency content will result in a more accurate measurement. Once again, a filter outside of the oscilloscope could be utilized, but it only affects noise from the environment, not the noise of the measurement system.

The better approach is to digitally filter the unwanted signal content from the captured signal. Removing noise coming from both outside the measurement system as well as Gaussian noise of the measurement system itself. Because the 500 MHz chirp was captured with a full 12 GHz of instantaneous bandwidth, a band-pass filter would be setup in either the math function of the oscilloscope or in another data analysis software environment such as MATLAB@. In this case, a 750 MHz filter centered at 10 GHz is chosen, resulting in an approximate 12 dB improvement in the measurement noise floor.

SNR = 6.02N + 176 dB + l0og (fs/ (2fa)) Fs = sampling rate,

Fa = analog (filtered) bandwidth, and N = number of bits. Ⅷ.CONCLUSIONS

While most would agree that a high bandwidth digitizing oscilloscope is a flexible tool that can be used to capture a wide variety of signals, there would not be as much agreement on the usefulness of the resulting measurements, especially when compared to specialized instrumentation.

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陕西理工学院毕业设计

While this is a valid concern, it does not negate the potential benefit provided, especially when the oscilloscope as a “system” is well understood and then optimized for the particular measurement being made.

REFERENCES

[l] Agilent website, spectrum analyzer datasheet, 2009, [Online], Available: . [2] Rohde & Schwarz website, spectrum analyzer datasheet, 2009, [Online], Available: . [3] Agilent website, multi-meter datasheet. 2009,

[Online], Available: . [4] Agilent website, oscilloscope datasheet, 2009,

[Online], Available: . [5] LeCroy website oscilloscope datasheet. 2009,[Online],

Available: . [6] Tektronix website, oscilloscope datasheet, 2009,[Online],

Available: . [7] Robert A. Witte topic, Electronic Test Instruments: Analog and Digital Measurements, Second Edition, PTR Prentice Hall, 2002, pp. 31-36.

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陕西理工学院毕业设计

附录C

外文翻译：

最灵活的系统组件：高速数字示波器

Mark R Lombardi Agilent Technologies, Inc

[摘要] 在ATE系统中，示波器往往用做调试工具，但高速捕捉功能可以扩展其应用到众多预期的和未预料的测试需求。这延伸了读者对这个重要系统组件所提供的固有测量可能性的理解。 1.引言

示波器通常在ATE系统中作为调试工具。同时提供的“机架”调试是一个重要功能，现在示波器包括的功能，使他们能够涵盖众多看到的和无法预料的ATE测试需求。这些强大的表现能力，反过来，可以有足够的灵活性，迅速为新兴的关键任务要求配置。

有许多测试和测量设备为他们的专业测量效率和性能故意设计成一个ATE系统。例如，频谱分析仪可以高速，高动态范围地在万用表精确测量直流时进行频率测量，但也可能重新配置进行其他的测量。只有现代数字示波器在测量时具有灵活性。特别是，在ATE系统中这个工具可以用伴随着权衡和限制的灵活性进行分析。研究现代高速数字示波器的架构，同时了解如何权衡各种参数，允许ATE设计师在系统灵活性方面设计另一个维度。 2.数字示波器的结构

2.1 数字示波器的“系统”

有人可能会争辩说，能做多任务的工具，可能不会做好每一个任务。然而，当讨论部署ATE系统时，测量能力简直就是主要问题。一个现代的数字化示波器可以被认为是一个独立的系统，其本身含有多个高速AD转换器，前置放大器，衰减器，采集存储器，专门测量协处理器，个人电脑，和显示。这些“系统”包括校准程序，保证业绩可一直追溯到测量终点。

在介绍时用例子DC测量的RF测量进行说明，频谱分析仪不能进行直流测量，因为它是

[1，2]

设计来优化其射频性能包括混频器，通常不是DC耦合。万用表高频射频测量的情况下，

[3]

它是专为直流耦合低带宽的测量设计的，从而提高预期的测量精度。比较的关键在于懂得这两种测量如何用示波器能有效实现。

在频谱分析仪和万用表的例子中，是利用专门的硬件测量预定的信号以及信号的类型。在高带宽的数字示波器的情况下，它被设计用来捕捉宽范围的信号提供最终的从脉冲参数到协议解码的各种测量范围。示波器的唯一挑战是捕获在垂直（电压）和水平（定时）上具有足够分辨率和精度的原始信号，为获取需要的测量需要对信号进行后处理。 系统组件

接下来的问题是：如何做好示波器在重要电子任务中对不断变化信号的捕获？在讨论仪器类别时，有多千兆赫的瞬时带宽和几十千兆样本的采样率示波器，其很多进步大大提高了

，，

信号采集保真度[456]。通过本节分解并详细讨论这些进步。 1- 带宽

这一类的仪器，最典型的是找到滤波器校正幅度和相位响应，使在完全指定的仪器带宽内保持平稳。在示波器中，将这种与砖墙反应相近的转化为从直流到示波器规定的带宽的任何频率下对真正信号幅度和相位捕获的信心。它曾经是不可能达到±3分贝，现在在带宽中十分之一分贝的线性是可以预见的。这提供了在测量上更高的精度，例如占用和瞬时功率的测量。一个宽带宽的平坦度也将转换成任何被扩大至数百兆赫带宽信号更精确的表征，如雷达，超宽带，和频率捷变信号。 2- 模数转换器

紧密耦合示波器的带宽是模数转换器的性能。本文所涵盖的工具类，采样速度通常足够高，可以实时获得示波器指定的带宽指（单采集）。换言之，用于这些高性能示波器的模数转

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