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I'm using an Arduino board to generate a 125KHz square wave signal to feed into this curcuit:

schematic

simulate this circuit – Schematic created using CircuitLab

So this is what I'm receiving from my scope. The output was connected to the channel-1 upper, and input is connected to channel-2 lower.

Any idea why above channel-1 output does not reach 0V when the pulse happens?

How do I make it to hit 0V?

enter image description here

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You are asking too much of an old, slow, wimpy part. The CD4011 has a propagation time of 60ns, an output impedance of around 1k\$\Omega\$ if you're powering it from 5V (I inferred this number by looking at the current drive capability of the part), and significantly less voltage gain in its linear region than 74HCxx and later CMOS parts.

Use a resistor value that the gate can actually pull high when the cap is discharged -- the chip can supply about 500\$\mu\$A at 5V, which translates into a resistance of about 10k\$\Omega\$. Then use a cap that'll give you a time constant that's at least as long as the chip's propagation delay (125-250ns at 5V). Then expect the result to have a slow rise time because of the limited gain of the part in its linear region.

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Your RC time constant is too low. Try increasing the resistor value by a factor of 10.

The RC time constant needs to be longer than the propagation delay through a CD4011 NAND gate. These gates are pretty slow at 5V.

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