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| Does a capacitor charges smooth, or in stairs? |
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| eti:
Capacitors are incredibly versatile; they can charge on stairs, in hallways... basically anywhere. |
| rhb:
Just for grins I tried a 750 ohm series resistor. The impedance mismatch between the source and the resistor became so large that it rings like mad between the source and the resistor. Bear in mind that this is a thru hole metal film resistor, so it also has significant inductance at 20 GHz. The first photo is the full step response: The second is a detail from the 2nd step showing the effect of the reflections during the RC constant: As you can see, there are a lot more steps visible because I increased the RC constant by 5x. As has been pointed out, the impedance of a capacitor is generally low, though not at all frequencies. This is why high gain stages need to have the electrolytic caps bypassed by a small ceramic to prevent oscillation. It would well nigh impossible to match a capacitor from DC to 20 GHz unless it was made from a very good piece of coaxial cable as in my examples. The next step for these two resistors is to examine them with the nanoVNA. In general, capacitors do not resemble transmission lines and therefore do not behave the same way. They can be accurately described in terms of transmission line theory, but they won't *look* like a transmission line capacitor. I'd like to add that these sorts of tests are wildly sensitive to the connection make up and often not easily repeatable. Have Fun! Reg |
| StillTrying:
--- Quote from: rhb on June 10, 2020, 09:04:35 pm --- --- Quote from: StillTrying on June 10, 2020, 07:35:06 pm ---Are you viewing the open circuit end of the cable rather than the driven end, I don't know why your first step is so high in Reply#49. --- End quote --- This is the reflected signal seen at the driven end. --- End quote --- If I simulate a 40cm 50R cable driven by 150R I still can't see where your very large first step comes from, it's about 4 times higher than what would fit anywhere. :-// |
| rhb:
Read an app note about sampling scopes and step responses. The open end produces a reflection of +1. So you see the initial step and then a 2nd step. A short produces a reflection of -1, so you see a step up and then down to form a single pulse. The latter was the way short pulses were created for many years using a coax stub and a mercury wetted relay. Reg |
| StillTrying:
Your very large first steps must be due to probing the wrong end of the 150R, Pulse_Out rather than Driven_End, of the cable. |
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