10073D如何计算探头带宽？

有没有想过在调整无源探头的低频补偿时会有什么调整？
首次连接到示波器通道时，需要调整所有无源探头。
此过程涉及将示波器尖端连接到示波器上的“Probe Comp”或“Cal”端子，并调整探头中的微调装置以使步进响应变平。
在探针手册中，它通常表明这与探针电容与示波器输入电容相匹配。
对第一个答复中所示的电容补偿电阻分压器的分析将揭示DC或较低频率衰减由两个电阻器的比率决定，高频衰减由两个电容器的比率决定。
为使整个频率响应保持平坦，必须匹配电阻器和电容器的比率。
在10：1探头中，当R2 / R1 = 9且C1 / C2 = 9时，LF和HF衰减相等且响应平坦。
在典型的探头中，R2和C2是固定值分量。
R1完全由示波器输入电阻组成，或者由示波器输入电阻与探头内部的固定电阻并联组成。
C1由探头电缆电容，示波器输入电容和可变电容器组成。
正是这种可变电容器在低频补偿过程中被调节。
通过使用可变电容器，当探头连接到具有不同输入电容的示波器（或示波器通道）时，C1 / C2可以等于R2 / R1。
计算montaigne7j确实使用探头输入电阻和电容是低频和高频衰减之间的交叉频率的良好近似。



     以下为原文

  Ever wonder what is being adjusted when adjusting the low frequency compensation of a passive probe? 

All passive probes need to be adjusted when they are first connected to a scope channel. This process involves connecting the scope tip to a “Probe Comp” or “Cal” terminal on the scope and adjusting a trim
device in the probe to flatten the step response. In probe manuals, it typically states that this matches the probe capacitance to the scope input capacitance.

An analysis of the capacitively compensated resistor divider, shown in the first reply, will reveal that the DC or lower frequency attenuation is determined by the ratio of the two resistors and the high frequency attenuation is determined by the ratio of the two capacitors. For the overall frequency response to be flat, the ratio of the resistors and capacitors must be matched. In a 10:1 probe, when R2/R1=9 and C1/C2=9 the LF and HF attenuations are equal and the response is flat.

In the typical probe, R2 and C2 are fixed value components. R1 is either completely made up of the scope input resistance or is made up of the scope input resistance in parallel with a fixed resistor inside the probe. And C1 is made up of capacitance from the probe cable capacitance, the scope input capacitance and a variable capacitor. It is this variable capacitor that is being adjusted during the low frequency compensation process.  Through the use of the variable capacitor, C1/C2 can be made equal to R2/R1 when the probe is connected to scopes (or scope channels) that have different input capacitances. 

The calculation montaigne7j did using the probes input resistance and capacitance is a good approximation of the crossover frequency between the low frequency and high frequency attenuations.

关于示波器输入电容的一个词。
由于必须调整无源探头以补偿示波器的输入电容，因此示波器的标准做法是指定输入电容，探头指定可以补偿的示波器输入电容范围。
这些规格通常印在示波器前面板和探头主体上，或者可以在手册或数据表中找到。
利用该信息，用户可以确定示波器和探针是否彼此兼容。
许多示波器的输入有两种输入阻抗模式，50ohm和1megohm。
重要的是要了解输入电容规格仅适用于1megohm输入阻抗模式。
绝不应该使用此数字来近似示波器的输入电路带宽。
例如，DSOX3054A的输入电容规格为15pf。
如果示波器输入模式为50欧姆，则由50欧姆源1 /（2 * pi * 25 * 15pf）= 425MHz驱动。
这似乎与此型号示波器的500MHz带宽规范相冲突。
这背后的解释是，通向输入放大器的大部分输入结构由50欧姆传输线组成。
最明显的证据就是输入BNC。
在与探头LF补偿相关的频率（几KHz）下，传输线看起来像电容。
因此，包含在输入电容规范中。
但是，当选择50欧姆输入模式并使用50欧姆同轴电缆驱动输入时，此示波器输入传输线只是外部同轴电缆的扩展。
在这些条件下，输入传输线电容不再相关。



     以下为原文

  A word about scope input capacitance.

Since a passive probe must be adjusted to compensate for the scope’s input capacitance, it is standard practice for scopes to specify the input capacitance and for probes to specify the range of scope input capacitance they can compensate. These specification are typically found printed on the scope front panel and probe body or can be found in manuals or data sheets. With this information a user can determine if a scope and probe are compatible with each other.

The input of many scopes have two input impedances modes, 50ohm and 1megohm. Its important to understand that the input capacitance specification only applies to the 1megohm input impedance mode. This number should never be used to approximate the scope’s input circuit bandwidth. For example, the DSOX3054A has an input capacitance specification of 15pf. If the scope input mode is 50ohm and it is driven from a 50ohm source 1/(2*pi*25*15pf) = 425MHz. This appears to be in conflict with the 500MHz bandwidth specification of this model scope.

The explanation behind this is in the fact that much of the input structure leading to the input amplifier is made up of 50ohm transmission line. The most visible evidence of this is the input BNC. At the frequencies relevant to the probe LF compensation (several KHz), the transmission line looks like capacitance. And, thus, is included in the input capacitance specification.  However, when the 50ohm input mode is selected and the input is driven with a 50ohm coax cable, this scope input transmission line simply becomes an extension of the external coax. Under these conditions, the input transmission line capacitance is no longer relevant.

没有迹象表明探头调整不正确。
无源探头是将信号连接到示波器的最常用方法。
这是有充分理由的。
它们价格低廉，可广泛使用，相对耐用，几乎可与任何范围兼容（牢记LF补偿范围）并且非常易于使用。
它们非常适合在电路中从一点到另一点快速移动，并且可以很好地了解信号的作用。
但是，它们确实有其局限性。
在处理探头传输线上的反射之后，仍然存在很少的自由度来定制探针滚动以符合标准滤波器响应（Butterworth，Gaussian，Chebyshev等）。
您拥有的频率响应是非常典型的无源探头。
通带中会有波纹，很快就会滚动。
当一个人想要在时域和频域中获得最大保真度和准确度时，他们应该考虑使用50欧姆探测或有源探测器。



     以下为原文

  There is no indication that the probe is incorrectly adjusted.

Passive probes are the most common method of connecting a signal to a scope. And there is good reason for that. They are inexpensive, widely available, relatively durable, compatible with just about any scope (keeping in mind the LF compensation range) and very easy to use. They are great for quickly moving from point to point in a circuit and getting a very good picture of what the signal there is doing. However, they do have their limitations.

After dealing with the reflections on the probe’s transmission line, there are very few degrees of freedom remaining to tailor the probes roll off to conform to a standard filter response (Butterworth, Gaussian, Chebyshev, etc.). The frequency response you have is very typical of passive probes. There will be ripple in the pass band and it rolls off quickly. 

When a person wants the maximum fidelity and accuracy in the time and frequency domain, they should consider using 50ohm probing or active probes.

谢谢



     以下为原文

  Thank you