# RF Circuit (what does capacitor do?)

I'm trying to work out why a 0.7pF capacitor was chosen. The circuit operates at 12GHz.

At 12GHz, the capacitor has an impedance of about $18\Omega$. Also at 12GHz, TL3 is $0.259\lambda$ (which is roughly a quarter of a wavelength. I worked out the characterstic impedance of TL3 to be $74\Omega$.

I thought that C1 was meant to be a DC blocking capacitor and prevent the 12GHz signal from going into the DC supply; however, I don't understand why an $18\Omega$ capacitor was chosen, as a quarter wavelength piece at $74\Omega$ does not transform $50\Omega$ to $18\Omega$...

Could someone point me in the right direction?

Thanks.

• Curiously, could TL3 have been chosen so that the very low impedance of the capacitor at resonance be presented at the TL1-TL2-TL3 junction? (So, say, the length be made to be either $\lambda/2$ or $\lambda$?) Would this have the same affect? (Because the RF signal would reflect off the low impedance like it does with the high impedance?) Apr 8, 2014 at 6:54