AC analysis of a Bias Tee

Question

I am designing a bias tee here and wondered whether it actually works or not? Hence I conducted an AC analysis instead of S-parameters (after verifying that s parameters are okay). The bias tee has been designed for 900 MHz assuming 50 ohm impedances. So the circuit is as below:

And the resulting currents are as below:

Now, my questions are:

1. Why DC current is frequency dependent, it should have been a constant of about \$i=\frac{1 V}{50 \ \Omega}=0.02\ \mathrm A \$?
2. Why the output (RFDC) current is smaller than the RF (input) current ? It should have been the addition of both currents, just shifted up by 0.2 A?

Answer 1

Why DC current is frequency dependent,

Because at low frequecies the inductor (L1) doesn't do a good job of blocking signals from the AC source

it should have been a constant of about \$i=\frac{1 V}{50 \ \Omega}=0.02\ \mathrm A \$?

The DC source (SRC4) doesn't have any effect on the AC simulation. AC simulations only calculate the response to the AC sources in the model.

Why the output (RFDC) current is smaller than the RF (input) current ?

Some of the current is going through the DC arm, because the inductor is an imperfect blocker at the frequencies being simulated.