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?