impedance matching for saw sensors ()

[jouka]

hi everybody,

i'm new in RF design and I want to transfer the maximum power for generator (50 ohm source) to saw sensors so i want to create a board with Mstrip line for matching circuit. So if we suppose that's the saw is a pure capacitive load of 0.58Pf (measured) I think to add a inductance to recompance it but I don't get a good result when I do the schematic in ADS there's reflection when plottted S11.
I will add the image of the ADS schematic if you want.

also I think to used a stub to muched the capacitance but i now that the stub is usually used to eliminate the imaginary part of load and I don't now if we can use it here because our load is supposed capacitive.... i do an exemple of load of z=2+j1.5 and I get a good result for 2Ghz in ADS.

I need a help please
Than you
jouka

 

[BigBoss]

If this capacitor is series you should add an inductor in parallel.If it's parallel, you should add an inductor in series.
Depends on capacitance configuration.
But I'm not sure that the input impedance becomes purely capacitive.Because SAW filters generally have an input impedance with a real part by their nature.
There might be a measurement errors..

 

[jouka]

thank you for your response,

the capacitance is shunt so imaked the inductance in series and for frequence 2.7 Ghz Zc should be equal to Zl no??
So I calculate it L=5.6 nH and i added the mstrip line with this substrat characteristique er=3.55, H=1.524, conducteur thikness=35um ,condu=5.8e7
and the load is the capacite=0.58Pf but I get a reflection.

Tomorrow I will add the image

it's possible to used stub in our case????

Thanks for ur Help

 

[BigBoss]

Listen..
You can not do matching without real part of input impedance.If you have 0.58pF capacitance in parallel and then you place 5.6nH @2.7GHz.All you will see a simply short circuit.

1/2*PI*SQRT(0.58E-12*5.6E-9)=2.7926 GHz.
SAW filter must have a input impedance with real part.You can not match any purely reactive impedance to 50 Ohm or whatsoever...

 

[jouka]

Yes I understand...Thank you for your Help

Cheers,