125兆赫谐波下的DSOX3024A输入阻抗设置问题

这些只是采样器的刺激。
采样时钟中固有的（少量）抖动意味着对捕获的数据添加了小的谐波。
当你在FFT上看它们时，它们似乎会弹出。
您将在任何范围内看到这一点。
根据我的看法，它们非常小。
例如，在spur2图像中，它们看起来处于〜-60dBm，即~225uV rms。
在5mV /格，这是不可见的。
我无法从你的图片或设置中看出范围的垂直设置是什么，但它必须非常小。
人



     以下为原文

  These are just spurs from the samplers. The inherent (small amount of) jitter in the sampling clock means that there are small harmonics added to the captured data. When you look at it on an FFT they appear to pop out. You will see this on any scope.

Based on what I see, they are very small. In the spur2 picture, for example, they appear to be at ~-60dBm, which is ~225uV rms. At 5mV/division, this would be invisible. I can't tell from your picture or setup what the vertical setting of the scope is, but it must be pretty small.

Al

谢谢Al的回复。
通道2的垂直设置为5.00V /马刺不小，它们比噪声水平高20dB。
它们的振幅高于噪声似乎不受通道垂直衰减器设置的影响。
今晚（东部时间）晚些时候，我可以发布一些更好的描述，显示马刺以及它们如何出现在范围内的信号。
我没有在我的其他数字示波器（TEK TDS2022B）FFT功能上看到这一点。
我想知道您是否在您的示波器上看到这一点，或者是否有任何拥有X3000系列的人可以重现这一点，或者这是我的范围特有的。
埃德



     以下为原文

  Thanks for the reply Al.  The vertical setting on Channel 2 is 5.00V/    The spurs are not small they are 20dB above the noise level. Their amplitude above the noise appears to be unaffected by the channel vertical attenuator setting.  

Later this evening (Eastern Time), I can post some better tracings showing the spurs and how they appear with a signal on the scope.  

I do not see this on my other digital scope (TEK TDS2022B) FFT function.  I would like to know if you are seeing this on your scope or if anyone with an X3000 series can reproduce this or this is something peculiar to my scope.

Ed

艾德，我现在正在推出门，所以我没有时间给出完整答案，但是： - 我没有在我的MSO-X 3054A上看到那些马刺。
我看到一些非常小的，但它们来来去去 - 我正在运行V 2.00固件，你在运行什么？
- 请打开通道，以便可以看到设置，并添加2个测量值，Vpk-pk和Vrms（AC）Al



     以下为原文

  Ed,

I'm rolling out the door right now, so I don't have time for a full answer, but:

- I'm not seeing those spurs on my MSO-X 3054A. I see some very small ones, but they come and go
- I'm running V 2.00 firmware, what are you running?
- Please turn the channel on, so the settings can be seen, and add 2 measurements, Vpk-pk, and Vrms(AC)

Al

请返回原始帖子中的设置，没有任何输入连接到任何内容。
我想看看未连接通道上的噪音水平，这是最初的问题。
打开您感兴趣的频道。适当设置垂直。
添加FFT。
在频道上添加Vpk-pk和VRms。
确保它们显示实际值。
当你在它时，向FFT添加一些标记。
将Y1设置为125 MHz尖峰的近似峰值（我知道它会变化）。
将Y2设置为375MHz尖峰的近似峰值。
顺便说一句，3000这个型号是什么？
人



     以下为原文

  Please go back to the settings you had in the original post, with no inputs connected to anything. I'd like to see the noise level on the unconnected channel, which was the original question.

Turn on the channel you are interested in. Set the vertical appropriately. Add the FFT. Add Vpk-pk and VRms on the channel. Make sure they show actual values.

While you're at it, add some markers to the FFT. Set Y1 to the approximate peak (I know that it varies) of the 125 MHz spike. Set Y2 to the approximate peak of the 375MHz spike.

BTW, which model of 3000 is this?

Al

我问安捷伦，他们说你是对的Al。
它们是AD转换器的交错杂散（正如你所说的那样），它们距垂直范围的顶部约50 dB。
（编辑24APR2012）我最初使用了“规范”这个词是不正确的。
安捷伦表示，这种现象没有（保证或典型）规范。
我要求工程师允许将他的回复发给我，但他要到星期二才离开办公室。
我会在获得许可后发布。
再次感谢您对此的帮助。
EdEdited：KG4ARN于2012年4月24日上午5:35



     以下为原文

  I asked Agilent and they said that you are correct Al.  They are expected interleave spurs from the AD converters (as you said they were) and they are about 50 dB down from the top of the vertical range.  

(edited 24APR2012)  I originally used the word "specification" which is incorrect.  Agilent states that there are no (warranted or typical) specifications for this phenomenon.

I asked the engineer for permission to post his response to me, but he is not out of the office until Tuesday.  I'll post it when I get permission.  Thanks again for your help with this.  Ed

Edited by: KG4ARN on Apr 24, 2012 5:35 AM

我在3024上看到同样的事情但是当我将垂直每/ div降低到2mv时，谐波消失了。



     以下为原文

  I'm seeing the same thing on my 3024 but when I drop the vertical per/div to 2mv/ the harmonics are gone.

所以，这一切都已经弄明白了。
采样器“抖动”是数字示波器中固有的问题引起的。
- 范围越好（价格越高，性能越高），刺激越小 - 可以通过一点点数学来“掩盖”这一点。
有些范围根本没有显示出来。
他们中的一些人只是在FFT中使用一些陷波滤波来摆脱它们。
我看到一个没有在屏幕上显示它们的示波器，但如果你提取数据并运行自己的FFT，它们就会“神奇地”重新出现。
- 问题不在于它们位于本底噪声之上的程度，更重要的是它们在信号之下的位置。
SFDR（无杂散动态范围）的概念在这里发挥作用。
- 噪声基底以上的水平将根据衰减而变化。
正如Willco788所看到的那样，它们可能会在低衰减水平下“消失”，因为噪声基底足以隐藏它们。
- 所有这些尖峰以及它们之间的噪声都集成在一起，并在时域中显示为RMS噪声。
给定范围可能具有较少的总噪声，但在频率上显示出一些真正的峰值。
范围设计者试图避免这种情况，但它会发生。
周末愉快！
人



     以下为原文

  So, it's all figured out. The spurs are caused by sampler "jitter", an inherent issue in digital scopes.

- The better (higher price, higher performance) the scope, the smaller the spurs
- It is possible to 'mask' this with a little math. There are scopes that don't show them at all. Some of them just use some notch filtering in the FFT to get rid of them. I saw one scope that didn't show them on screen, but if you extracted the data and ran your own FFT, they 'magically' reappeared.
- The issue is not how much they sit above the noise floor, but more importantly how much below the signal they are. The concept of SFDR (Spurious Free Dynamic Range) comes into play here.
- The level above the noise floor will vary, based on attenuation. As Willco788 saw, they may 'disappear' at low attenuation levels because the noise floor comes up enough to hide them.
- All of those spikes, and the noise in between, get integrated together and show up as RMS noise in the time domain. A given scope could have less total noise, but show some real spikes in the frequency. Scope designers try to avoid that, but it happens.

Have a great weekend!
Al