Hi Jason,

My suggestion would be to look at a smith chart representation of the
circuit, along with realistic models for the capacitors (/ resistors
etc. ?) - i.e. add series inductance according to the packages
involved.  For example, the 22pf cap will ave a reactive component of
-8johms as -is, but if you include a 1nH series inductance, this
changes to about -2.4jOhms.  R1 will have paralell capacitance as well
as series inductance. The smith chart will give you a bit of an idea
about what's actually going on.

RP

On 21 March 2016 at 08:07, Jason White
<whitewaterssoftwareinfo@gmail.com> wrote:
> Hello, I have been looking at the reference design [2] for the PD85004
> 900Mh RF power MOSFET [1]. In particular I have been trying to analyze th=
e
> input matching network used which uses a combination of capacitors and
> microstrip transmission lines to match the 50 ohm input to the FET's gate=
..
> The gate presents an impedance of around 2.6+j7 at 900MHz.
>
> I built up the schematic for the input matching network in LTSPICE using
> ideal transmission lines. However, simulation shows that while it provide=
s
> ~56 ohms input impedance, only about 10% of the input power is delivered =
to
> the FET. This clearly isn't the case in real life, so something has gone
> amiss in the simulation.
>
> If anyone had some knowledge of how how these matching networks work, I
> would be grateful if you could point me to some relevant resources. I
> cannot ascertain the function most of the input capacitors, I all know is
> that they are not filters and their impedance do not seem to be suitable =
to
> perform matching through the transmission lines.
>
> Attached is:
> * A picture of the PCB layout for the reference design
> * A picture with the input matching schematic and SPICE schematic
> * The LTSPICE simulation I built to simulate the input matching circuit
>
> PD85004 RF FET Datasheet (Includes info on evaluation board)
> [1]
> http://www.st.com/web/en/resource/technical/document/datasheet/CD00178461=
..pdf
>
> PD85004 RF FET Evaluation board datasheet
> [2]
> http://www.st.com/web/en/resource/technical/document/datasheet/CD00287652=
..pdf
>
> --
> Jason White
>
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