Electronic – Why is the AC coupling capacitor always 0.1uF or 0.01uF in high speed transceivers

capacitorhigh speed

As I know, high speed transceivers always use a 0.1uF or 0.01uF capacitor for AC-coupling.

                end resistor

Capacitor and end resistor are a high-pass filter.

If end resistor is 50 ohm.

I calculate bandwidth with
f = \frac{1}{2 \pi R C}

if \$C = 0.1\mathrm{\mu F}\$, then \$f = 31.847\mathrm{k Hz}\$

if \$C = 0.01\mathrm{\mu F}\$, then \$f = 318.47\mathrm{k Hz}\$

But I always see 0.1uF and 0.01uF both used in the 1GHz, even 10GHz circuit.

As I calculated above, bandwidth should not enough.

I'm confused.

Best Answer

It's a HIGH-pass filter, not a low-pass filter. That cutoff frequency you calculated is the LOWEST that can get through, not the highest. Frequencies lower than that are blocked, and frequencies higher than that are passed.