How to set the termination resistance of a GPS LNA power supply?

[tony_lth]

I am estimating the power supply EMC issue of a GPS LNA, SKY65709.
I can simulated with software to estimate the decoupling effect of shunting C and seriesing L of the power supply.
But I don't know how to set the termination resistance in software.
The DC supply is 2.7V and 5mA, so DC resistance is 2.7V/5mA=540 ohms.
So could I set the termination to 540 ohms when simulating? I guess the RF impedance should be different from DC resistance, so how to set? My simulated freq band is from 100MHz to 10GHz.

 

[jiripolivka]

Away from a software you should separate the DC power and RF circuits. What you need is a "bias Tee" which uses an inductor to pass DC current to the LNA without affecting the RF signal. And the RF signal passes via a capacitor to RF output (connector), so DC is not affected.

 

[BigBoss]

You can measure this impedance with a VNA by using a bias Tee..
Then you will have enough information what order of RF impedance should be terminated.

 

[tony_lth]

Hi, Jiripolivka, thanks. My case is using MMIC LNA, so there is some bias Tee inside the MMIC.
My purpose is estimating the coupling factor of external RF signals (from 100KHz to 3GHz) to the MMIC powe supply pin.
I know the isolation between the RF pin and power supply pin of a MMIC, from its datasheet.

Hi, Bigboss, good idea. I will try later.
But does anyone can provide a typical value for simulation?

 

[BigBoss]

Hi, Bigboss, good idea. I will try later.
But does anyone can provide a typical value for simulation?

At VDD input of the IC, there is probably an internal regulator,a ESD protection diodes, some PAD-nitride capacitances,bonding wire inductances, intrinsic regulator capacitances etc.
So, input impedance can be considered being as a Resistor and an inductor ( 540 Ohm+1-2 nH) ) and a cumulated capacitance at each side ( maybe few hundred fF).
I mean a simple C(parallel)-RL(series)-C(Parallel) circuit can be a starting point for simulations.
But in order to find the exact circuit, a simple measurement is necessary..

 

[tony_lth]

Thanks, Bigboss, I finally got sense simulation results.