The thing is that if you have a capacitor with 5V across it, like when the DTR is zero and the reset is pulled up to 5V and you take that end that has a zero and make it 5V, in that instant the end that was 5V now becomes 10V because the charge on the capacitor can not change instantly. This is the basis of of voltage doublers.
The diode shorts out that 10V to the 5V rail and so quickly discharges the capacitor.
An effective method for filtering high frequency power supply noise and cleanly sharing similar voltage supply rails (that is, analog and digital rails for mixed-signal ICs) while preserving high frequency isolation between the shared rails is the use of ferrite beads. A ferrite bead is a passive device that filters high frequency noise energy over a broad frequency range. It becomes resistive over its intended frequency range and dissipates the noise energy in the form of heat. The ferrite bead is connected in series with the power supply rail and is often combined with capacitors to ground on either side of the bead. This forms a low-pass filter network, further reducing the high frequency power supply noise.
You could often seen it in your Mobile Phone Data Cables,Computer Charger Cables and so and so on.
Ferrite Beads are also used in Electronics Circuits for the effective filtration of high frequency noises. The image shown is the smd version of Ferrite beads used for filtration in Electronics circuits.
