When you're simulating circuits at these frequencies you need to include
the parasitic inductance of the capacitors as it can have a significant
effect - for the typical ATC100B type it's around 0.5nH, probably
several times that for a trimmer cap depending on the type.
To check or adjust matching with transistor amps using a VNA the usual
method is to put 50ohm terminations on the in/out and measure the
impedance at the transistor end. At UHF and above it can be tricky as
you're probably working at <10ohms impedance and positioning probes to
avoid stray inductance needs a lot of care, fractions of a mm matter.
You mentioned the 20W power you were getting out of your amp. It's very
helpful to also give the voltage/current as that gives useful
information about how the amp is working.
73, Steve G8GSQ
On 29/03/2025 12:13, Lukasz wrote:
I've been trying to simulate the 930MHz circuit from the datasheet as a
starting basis and I had to increase the trimmer by 5pf - otherwise numbers
don't work.. Then it seems to work? Can someone tell if this is correct or
not?
https://ibb.co/DgrFS77z
https://ibb.co/h1shgvYj
Also, with some help of (hopefully not hallucinating) AI I've managed to
establish the input likely has 1.3nH of parasitic inductance in addition to
177pF of gate capacity (this gives the 2.5+7j at 945MHz) and these values
calculated at 440MHz give 2.5+1.55j. Not sure how far I'll get with that.
73, Lukasz
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