Oscilloscope probe principals

[Goodver]

As far as I understand the problem is, because of inner capacitance of an oscilloscope, if we apply a rectangular signal on the screen we will see semi-sinusoidal one, and we want to handle this capacitance, that the scope shows correct signal. Right?

When we use a probe the circuit looks like a voltage devide:

http://www.elexp.com/tips/P_SCHPRB.GIF

By this voltage devider, as far as I can see, we reduce input voltage to oscilloscope by the factor of 10 (consider X10 probe).

My problems:

1. How this reduction of voltage removes the fluctuation on the 1 pic?
2. The devision of voltage by the factor of 10 will occur, as far as I understand, only at 0 frequency and infinite frequency, because in this case we can neglate resistors or capacitors, right? But if say frequency something like in the middle, then we have to consider all resistors and capacitors, which, I assume, will not give an exact devision by 10.

 

[analogdesign]

The answer to both questions will be clear when you see that Cp in the probe can be adjusted. Note that Rp and Cp constitute a zero, and the scope input a pole. The idea is the Cp in the probe is adjusted so its time constant (RpCp) equals the time constant of the scope input (RinCin). In this case, the pole and the zero can cancel and the probe gives you "infinite bandwidth". So the capacitors cancel out and the division is exactly 10 across the range.

This removes the fluctuation in the pic because the bandwidth is higher. If we just connect a resistive probe then the signal sees a low-pass filter which rounds the edges like in the picture.

"infinite bandwidth" is in quotes because of course parasitics will limit the performance of this system.

 

[berkeman]

Good answer by analogdesign. You may also find the Textronix 'scope probe primer document useful:

http://www.tek.com/learning/probes-tutorial

 

[Goodver]

Thank you! So the idea is to achieve equal time constant that the ratios between impedances of the scope and the probe stays the same, thus output voltage does not change

 

[alphy]

Thank you for your question. I can provide some clarification on the principals of oscilloscope probes.

Firstly, let's address the issue of the inner capacitance of an oscilloscope. This capacitance is present in all oscilloscopes and is a result of the construction of the instrument. When a rectangular signal is applied to the screen, the inner capacitance causes the signal to appear as a semi-sinusoidal one. This is because the capacitance acts as a low-pass filter, allowing only the lower frequency components of the signal to pass through. To handle this capacitance and ensure that the scope shows the correct signal, we use an oscilloscope probe.

An oscilloscope probe works as a voltage divider, as you correctly stated. This means that the input voltage to the oscilloscope is reduced by a factor of 10 (for a X10 probe). This reduction in voltage helps to compensate for the inner capacitance of the oscilloscope, as it reduces the amplitude of the signal that is being measured. This allows for a more accurate representation of the signal on the screen.

To address your first problem, the reduction of voltage by a factor of 10 does not necessarily remove the fluctuations on the 1 pic. However, it does reduce the amplitude of the signal, making the fluctuations less noticeable. This is because the fluctuations are now a smaller percentage of the overall signal.

As for your second problem, the division of voltage by a factor of 10 will occur at all frequencies, not just at 0 and infinite frequency. This is because the resistors and capacitors in the probe are designed to have a consistent effect on the signal at all frequencies. While there may be some slight variations, the overall effect is a reduction of voltage by a factor of 10.

In summary, oscilloscope probes play a crucial role in compensating for the inner capacitance of oscilloscopes and providing accurate representations of signals on the screen. The voltage division of the probe helps to reduce the amplitude of the signal and make fluctuations less noticeable. While there may be some variations in the division at different frequencies, the overall effect is a consistent reduction of voltage by the specified factor.