blocking capacitor values

I am doing 12 GHz LNA using microstrip and GaAs PHEMT FET. i am using voltage supply Vdd=5V and Vgs=-0.2V creating Id=10mA.

My problem is:
1)what is the suitable values of the DC Blocking capacitor for the RF input and output?( i know that the SRF must be higher than operating frequency but i dont know the way to identify the SRF of the capacitor)

2)would 0.01uF or 20pF be more suitable?(these are values that i currently have)

can anyone pls give a detailed answer.thanks

For 12GHz just a few pF would be enough.
Check the type (I mean manufacturer datasheet) if it is suitable for high frequency.

I would get a cap with SRF = 12GHz. This is when a cap appears as a short so it is not there to the 12GHz signal, infinite impedance to DC.

A murata 0.3pF 0402 size cap should have a SRF near 12 GHz.

C

The general problem is that higher value capacitors use very high dielectric constant materials. Since the εr is so high, the body of the capacitor can actually form a rectangular waveguide resonator. And resonators have nasty habits, like looking like open circuits at certain frequencies!

So, in general the smaller the capacitance value, and the smaller the physical size (like an 0402) work better at higher frequency.

That said, I have seen 0.1 uF caps up to 40 GHz as DC bypass caps, with good results.

Go figure!

But if you really worry about SRF, use single layer caps,
http://www.dilabs.com/products/products.aspx